Optimizing fast powder for shorter barrels.

[cosmic creepers]

Optimizing velocity by using faster powders in short barrels is a myth.

I posted this in a .22 cal forum because it was originally .223 specific, but I have ran a test to validate what the pressure models say.

If anyone is interested in this, as I have, you can watch the entire video essay including the process and describing my findings here.


To summarize, it seems that the amount of optimization that can be achieved from these macro adjustments is very minimal, which leads me to conclude that the maximum velocity that can be achieved is determined by the cartridge, bullet and powder, but it may not matter how long of a barrel you put onto it.



I had a harder time wrapping my head around it before I got my test results, but what would appear to be the case is this: the term internal ballistics is perhaps a bit of a misnomer. There seems to be chamber or cartridge ballistics and then barrel ballistics, perhaps internal ballistics is subdivided into these two groups.



It would seem that as we walk the fine line between burn, explosion, and detonation, we are playing a game of millionths of seconds, or milli-milli-seconds. In this game of milli-milli-seconds it would seem that the maximum velocity attainable is achieved in the chamber before the bullet even passes beyond engraving, and so it would seem to not matter whether the charge is burnt up in six inches or sixteen inches of barrel, that the rate of gas expansion, and therefore, velocity of that expansion was already determined long before that powder completed burning.



Although the difference in velocity may be more pronounced in slower powders, the maximum achievable velocity for a shorter length barrel can not be a load which achieves less than the max velocity in the longest barrel tested.



**grain of salt** I think when it comes to extremes of cartridge volume or barrel length, this would be an exception. (Imagine you made a .223 case that was 6 inches long and then put that cartridge into a 60 inch barrel)



But anything short of the extreme or audacious examples, I would presumptuously assume that the results I got would be similar.



It just required me taking a look at the data from a different angle to trust the math.



Thanks for coming out.

 

[Homerange]

Slower/normal powders in short barrels increase muzzle blast and report.
Neither is nice for the shooter or his/her game.

When I got my Rem Model 7 with 18" bbl and wanting to use 50gr pills a faster powder than is normal was selected; ADI AR2207 that when loaded beyond what is seen in books has proven to be a fast and accurate load.
YMMV

 

[dellet]

Good beginning. Lots of testing still to do

First thing to realize, is that to almost any conclusion with Ballistics, some joker can come along and give you an exception.

Powder burn charts are a moving target. Depending which chart you use, 2460, 2495, xterminator will swap places. Some 2460 is the fastest of the three, some slowest. Based on that alone your results can flip flop.

Depending on how your predictions are set up, you may be setting yourself up for failure. I’m not familiar with Gordon’s, but am with Quickload which will allow you to pick a bullet and COL. Then sort loads by pressure and velocity. Fornwhat you are doing setting the pressure to a max of 55,000 psi will generate change weights and velocity for a given barrel length. Rarely will the fastest load change with barrel length input change.

What that won’t do is spit out an “optimized load”. It will just give you an idea of pressure and velocity based on a random chosen cartridge length.

Your two example loads of LT 32 and 2460 (which may or may not be the second slowest powder tested) are just random loads, based loosely on max pressure. Changing the seated depth of the bullet, will change the velocities of both powder charges. Again optimizing load density may change your results of which powder is capable the highest velocity.

You have work up the optimal loads separately and then compare velocities at differing barrel lengths. Easy to do with the graphs in the loading program.

Rarely is the fastest load the most accurate, and this is where things really get sticky. Your fastest load in long barrel, will generally be the fastest load in a short barrel. Cartridge efficiency really doesn’t change with barrel length, if you’re defining efficiency as highest rate of velocity produced. Optimal load, for most would be most consistently accurate within a certain velocity window. 1/4 MOA tighter getting to the target 50 fps slower, is most likely the load most would choose.

To a certain extent, muzzle velocity is like any other race for speed. Starting faster doesn’t always mean finishing first. At peak pressure both bullets have reached maximum acceleration, then at some point the bullet with faster burning fuel accelerates slower than the bullet pushed by the slower powder. At some barrel length, the slower powder will exit the muzzle in less time than faster powder. That does not mean that it will automatically exit with a higher velocity.

I can and have worked up loads that use same bullet, same charge weight, different powders(fast and slow) that have statistically the same muzzle velocity, in the same rifle. The pressure curves and exit time of the bullet are totally different. So what that means is that rate of acceleration and velocity crosses at a point at some distance. The bullet with the slower start, had more muzzle pressure on exit. With two inches more barrel it would have had the higher velocity. With two inches less barrel, the bullet with the faster start, would have had the higher velocity. It may not have been outside the margin of error due to SD numbers, but it was there. Might have needed a foot of barrel length to see it reliably on a chronograph.

You just stepped into a pretty deep hole. Enjoy the fall. Just remember conflicting information can still be true in the right conditions.

 

[Doom]

To understand how powder speed plays into velocity you have to understand the basic principles of physics that are in play.

The first is what we call burn rate. It is a function of how fast a powder burns and generates gas from the burning of the mass of powder in the cartridge. In this respect a fast powder converts the powder mass to high temperature gas very rapidly and a slower powder takes longer to convert that mass to a high temperature gas.

The second is volume which in this case is the the free gas space between the base of the cartridge to the base of the bullet. This volume changes with time as the bullet travels down the barrel to the muzzle. Initially it is very small (only in the cartridge) and increase as the bullet begins it travel.

The third is the pressure volume relationship of real gases, which can be considered an ideal gas for this discussion. For this discussion a given mass of gas in a given volume will have a given pressure. If the volume increases the pressure decreases. If the mass increase for a given volume the pressure increases.

The fourth item is bullet inertia. The tendency of an object at rest to remain at rest and an object in motion to remain in motion. To change the velocity (acceleration) of the bullet requires a force which in this case is the pressure time the bore cross sectional area at any point along it travel.

Basic Assumptions: In this analysis it is assumed that changing the burn rate doesn't affect the gas generation. In reality this may happen so that a slower powder may generate more gas per unit mass, It is also assumed that the powder behaves consistently. In reality a very slow a powder may not burn properly if the bullet move fast enough that the pressure doesn't increase properly. A powder that is too fast may detonate.

If an extremely fast powder is used with a heavy bullet in a given cartridge the gas generation is extremely quick and before the bullet can accelerate the pressure will rise extremely fast to a given limit (SAAMI Pressure) because the total volume isn't changing very fast due to inertia of the bullet and also the interaction with the lands (engraving force). In fact the gas generation may be so fast that the mass of powder must be limited to avoid exceeding the pressure limit. Limiting the powder limits the gas generation so that once the bullet begins to travel down the barrel the increase in volume causes the pressure to decay rapidly since no more mass is being generated. We have a situation where we have adequate initial pressure but insufficient gas volume to continue the acceleration of the bullet and early burnout of the powder. The result is low muzzle pressure and velocity.

If we can slow the gas generation down then pressure doesn't rise as fast and the bullet has time to begin moving increasing the effective volume and the pressure doesn't go as high for a given amount of powder. Now we can add additional powder and stay within the pressure limit. More powder means more gas and higher average and muzzle pressure in the barrel. The result is higher velocity. But because the powder takes longer to burn the burnout of the powder occurs further down the barrel.

We can continue to slow the burn rate and increase the ultimate velocity of the projectile by increasing the powder charge until we reach a limit. Ultimately there are two limits. The first is a physical limit to the amount of powder we can put in the case. The other is the fact that eventually the barrel length will limit the burnout of the powder and the total gas generation.

So for maximum velocity we would seek a powder that provides a maximum chamber pressure that doesn't exceed our pressure limit and provides the highest average pressure down the barrel. As for barrel length the powder and bullet have no knowledge of how long the barrel is. The barrel length doesn't affect the choice of powder so long as the powder is 100% burned before muzzle exit. At some point the loss of gas due to less than 100% burnout and the change in average pressure may result in a faster powder giving better velocity.

So the old adage that the highest velocity powder in a given length barrel will be the best in a shorter barrel is actually valid, to a point. What that point is will be a function of a given powder and cartridge and barrel length.

 

[Loudenboomer]

Interesting for me to understand would be how the ratio of bore diameter of a given charge affects a short barrel. Does total bore volume have as much to do with burn efficiency as just barrel length? Given a the same medium burn rate powder in both the .243 Win and the .358 Win. wouldn't the .243 require a longer barrel to reach 100% burn efficiency? It would seem a bore volume factor could be valuable. Perhaps someone with a much better physics back round could explain.
Merry Christmas All

 

[Doom]

The bore diameter affects the rate of flow of gas/powder out of the case and also affects the rate at which the total volume is changing. Consider a straight bore cartridge like the 45-70. There is no restriction to the flow out of the case and the case/bore are essentially the same diameter from case base to muzzle. For bottleneck cartridges the bore is smaller that the case so the shoulder/neck act as a nozzle and tend to restrict the flow. Also a small bore results in less total volume for a given barrel length.

Consider the 308 win family of cartridges. The 308 can use a certain range of powders efficiently (4064 range) but reduce the bullet to a 7mm and the powder needs to burn a little more slowly because the flow from the case is more restricted and the volume is changing a little less for each inch of bullet travel. Drop to the 243 and you see the real change as the effective powders are now likely to be in the 4350 range.

So the tendency is that the burnout will require a longer barrel for the slower powder. If the case fill limits the pressure then the powder will take even longer to burn out.

 

[cosmic creepers]

Good beginning. Lots of testing still to do

First thing to realize, is that to almost any conclusion with Ballistics, some joker can come along and give you an exception.

Powder burn charts are a moving target. Depending which chart you use, 2460, 2495, xterminator will swap places. Some 2460 is the fastest of the three, some slowest. Based on that alone your results can flip flop.

Depending on how your predictions are set up, you may be setting yourself up for failure. I’m not familiar with Gordon’s, but am with Quickload which will allow you to pick a bullet and COL. Then sort loads by pressure and velocity. Fornwhat you are doing setting the pressure to a max of 55,000 psi will generate change weights and velocity for a given barrel length. Rarely will the fastest load change with barrel length input change.

What that won’t do is spit out an “optimized load”. It will just give you an idea of pressure and velocity based on a random chosen cartridge length.

Your two example loads of LT 32 and 2460 (which may or may not be the second slowest powder tested) are just random loads, based loosely on max pressure. Changing the seated depth of the bullet, will change the velocities of both powder charges. Again optimizing load density may change your results of which powder is capable the highest velocity.

You have work up the optimal loads separately and then compare velocities at differing barrel lengths. Easy to do with the graphs in the loading program.

Rarely is the fastest load the most accurate, and this is where things really get sticky. Your fastest load in long barrel, will generally be the fastest load in a short barrel. Cartridge efficiency really doesn’t change with barrel length, if you’re defining efficiency as highest rate of velocity produced. Optimal load, for most would be most consistently accurate within a certain velocity window. 1/4 MOA tighter getting to the target 50 fps slower, is most likely the load most would choose.

To a certain extent, muzzle velocity is like any other race for speed. Starting faster doesn’t always mean finishing first. At peak pressure both bullets have reached maximum acceleration, then at some point the bullet with faster burning fuel accelerates slower than the bullet pushed by the slower powder. At some barrel length, the slower powder will exit the muzzle in less time than faster powder. That does not mean that it will automatically exit with a higher velocity.

I can and have worked up loads that use same bullet, same charge weight, different powders(fast and slow) that have statistically the same muzzle velocity, in the same rifle. The pressure curves and exit time of the bullet are totally different. So what that means is that rate of acceleration and velocity crosses at a point at some distance. The bullet with the slower start, had more muzzle pressure on exit. With two inches more barrel it would have had the higher velocity. With two inches less barrel, the bullet with the faster start, would have had the higher velocity. It may not have been outside the margin of error due to SD numbers, but it was there. Might have needed a foot of barrel length to see it reliably on a chronograph.

You just stepped into a pretty deep hole. Enjoy the fall. Just remember conflicting information can still be true in the right conditions.

You touched on a lot of good points.

The premise of this particular test was only to prove cartridge efficiency is primary over barrel length, which you also seem to have touched on.

See the test barrels in case we're 18 inch and 11.5 inch.

Accuracy was not a factor, because the 11.5 inch barrel shoots everything I put in it more than "good enough" but maximizing velocity with faster powder was a wives tale that made me chase my tail. Thanks so much for the insight.

 

[cosmic creepers]

Interesting for me to understand would be how the ratio of bore diameter of a given charge affects a short barrel. Does total bore volume have as much to do with burn efficiency as just barrel length? Given a the same medium burn rate powder in both the .243 Win and the .358 Win. wouldn't the .243 require a longer barrel to reach 100% burn efficiency? It would seem a bore volume factor could be valuable. Perhaps someone with a much better physics back round could explain.
Merry Christmas All

Well, the interesting thing in my test, if you watched the video is that in two of my tests, the powder should have been burnt long before the muzzle exit, but the powders that were not burnt completely yet, still yielded higher velocity for the same predicted pressure.

 

[cosmic creepers]

The bore diameter affects the rate of flow of gas/powder out of the case and also affects the rate at which the total volume is changing. Consider a straight bore cartridge like the 45-70. There is no restriction to the flow out of the case and the case/bore are essentially the same diameter from case base to muzzle. For bottleneck cartridges the bore is smaller that the case so the shoulder/neck act as a nozzle and tend to restrict the flow. Also a small bore results in less total volume for a given barrel length.

Consider the 308 win family of cartridges. The 308 can use a certain range of powders efficiently (4064 range) but reduce the bullet to a 7mm and the powder needs to burn a little more slowly because the flow from the case is more restricted and the volume is changing a little less for each inch of bullet travel. Drop to the 243 and you see the real change as the effective powders are now likely to be in the 4350 range.

So the tendency is that the burnout will require a longer barrel for the slower powder. If the case fill limits the pressure then the powder will take even longer to burn out.

Yes, yes, yes, and yes, but my point is, that when you take .243 and build a load striving for nothing but Maximum velocity out of a 26 inch barrel, and built another load that attempts to optimize for a 16 inch barrel, the load that yields the highest velocity out of the 26 inch barrel will still be the fastest load out of the 16 in spite of the percent burn in the 16 inch barrel.

Try it out one time. It still makes me scratch my head a little

 

[dellet]

You touched on a lot of good points.

The premise of this particular test was only to prove cartridge efficiency is primary over barrel length, which you also seem to have touched on.

See the test barrels in case we're 18 inch and 11.5 inch.

Accuracy was not a factor, because the 11.5 inch barrel shoots everything I put in it more than "good enough" but maximizing velocity with faster powder was a wives tale that made me chase my tail. Thanks so much for the insight.

Maximizing velocity with faster powders is not exactly a wives tail. The problem is that you’re talking numbers and statistics. Numbers and statistics can be manipulated.

Basically you set up your experiment to prove your point, and it did.

Velocity is determined by pressure over time, @Doom gave a very nice explanation.

You have a fixed max pressure, although your loads varied from that. The time is tightly controlled by barrel length. So the only real variable is how fast the load reaches max pressure, and how much pressure drops at the muzzle. Commonly termed as burn rate. The only way to change velocity, is vary the pressure curve.

You mentioned cartridge efficiency being more important than barrel length. Cartridge efficiency is manipulated by load density, and varies with chamber volume. By using a fixed COL for all powders tested, you did not maximize the efficiency of any of the test cartridges.

Is it possible that you minimized the efficiency of the LT32 load, and maximized the 2460 load?
Would maximizing the efficiency of both loads change the outcome?

When you get this all figured out, move to sub-sonic loads. The load software doesn’t really recognize friction. The longer the barrel the faster the bullet velocity. The problem with sub-sonic loads, or pistol carbine loads, is depending on powder used and load developed in a short barrel, burn rate, will determine if you will need to add or subtract powder to maintain 1000fps in a long barrel.

In all honesty, you really don’t even know if the loads had anywhere near the same pressure. So it’s all conjecture.

This is why I say you have just scratched the surface of your question.

 

[Doom]

Yes, yes, yes, and yes, but my point is, that when you take .243 and build a load striving for nothing but Maximum velocity out of a 26 inch barrel, and built another load that attempts to optimize for a 16 inch barrel, the load that yields the highest velocity out of the 26 inch barrel will still be the fastest load out of the 16 in spite of the percent burn in the 16 inch barrel.

Try it out one time. It still makes me scratch my head a little

To get the highest velocity at any length starting with a fixed peak pressure (usually with the first 4 inches of barrel) the maximum velocity is obtained by having the highest integrated pressure throughout the entire length of the barrel. Cutting the barrel length does nothing to change what is happening before the muzzle. The pressure profile from chamber to muzzle doesn't change with a shorter barrel up to the muzzle.

 

[Krogen]

The bore diameter affects the rate of flow of gas/powder out of the case and also affects the rate at which the total volume is changing. Consider a straight bore cartridge like the 45-70. There is no restriction to the flow out of the case and the case/bore are essentially the same diameter from case base to muzzle. For bottleneck cartridges the bore is smaller that the case so the shoulder/neck act as a nozzle and tend to restrict the flow. Also a small bore results in less total volume for a given barrel length.

Consider the 308 win family of cartridges. The 308 can use a certain range of powders efficiently (4064 range) but reduce the bullet to a 7mm and the powder needs to burn a little more slowly because the flow from the case is more restricted and the volume is changing a little less for each inch of bullet travel. Drop to the 243 and you see the real change as the effective powders are now likely to be in the 4350 range.

So the tendency is that the burnout will require a longer barrel for the slower powder. If the case fill limits the pressure then the powder will take even longer to burn out.

What you describe is known as expansion ratio. It's the ratio of bore volume to effective cartridge volume (available powder space). High expansion ratios, like the 45-70 do better with faster powders while low expansion ratios (e.g. 300 Weatherby) prefer slower powders. A good way to think of this is to consider blowing on a large diameter soda straw vs. a small diameter WD-40 "straw." WIth the soda straw you have to blow a lot of volume quickly to get any back pressure, the WD-40 straw not so much. Of course the analogy only goes so far. There's no bullet friction or inertia in the analogy, but it's still instructive.

 

[Homerange]

What you describe is known as expansion ratio. It's the ratio of bore volume to effective cartridge volume (available powder space). High expansion ratios, like the 45-70 do better with faster powders while low expansion ratios (e.g. 300 Weatherby) prefer slower powders. A good way to think of this is to consider blowing on a large diameter soda straw vs. a small diameter WD-40 "straw." WIth the soda straw you have to blow a lot of volume quickly to get any back pressure, the WD-40 straw not so much. Of course the analogy only goes so far. There's no bullet friction or inertia in the analogy, but it's still instructive.

Yup, this ^.
270/30-06/35 Whelen are fine examples of what a bigger bore and same parent case can offer.....especially with the heavier pills.

 

[cosmic creepers]

Maximizing velocity with faster powders is not exactly a wives tail. The problem is that you’re talking numbers and statistics. Numbers and statistics can be manipulated.

Basically you set up your experiment to prove your point, and it did.

Velocity is determined by pressure over time, @Doom gave a very nice explanation.

You have a fixed max pressure, although your loads varied from that. The time is tightly controlled by barrel length. So the only real variable is how fast the load reaches max pressure, and how much pressure drops at the muzzle. Commonly termed as burn rate. The only way to change velocity, is vary the pressure curve.

You mentioned cartridge efficiency being more important than barrel length. Cartridge efficiency is manipulated by load density, and varies with chamber volume. By using a fixed COL for all powders tested, you did not maximize the efficiency of any of the test cartridges.

Is it possible that you minimized the efficiency of the LT32 load, and maximized the 2460 load?
Would maximizing the efficiency of both loads change the outcome?

When you get this all figured out, move to sub-sonic loads. The load software doesn’t really recognize friction. The longer the barrel the faster the bullet velocity. The problem with sub-sonic loads, or pistol carbine loads, is depending on powder used and load developed in a short barrel, burn rate, will determine if you will need to add or subtract powder to maintain 1000fps in a long barrel.

In all honesty, you really don’t even know if the loads had anywhere near the same pressure. So it’s all conjecture.

This is why I say you have just scratched the surface of your question.

Of course, I do not have a drilled, tapped chamber port, and so my conjecture, which is, in fact, a little more than conjecture, as I based my anticipated loads off of three different platforms, and attempted to max out pressure somewhere between 55,000 and 57,000 psi based off of those imprecise reports.

However, the anticipated result was actually opposite of the actual result.

I anticipated to get the fastest velocity from LT-32, on down to slowest at sta-ball.

What I actually got was not that.

So it would be fair to assume that yes, load optimization can be achieved as a function of cartridge volume, bore diameter, and burn rate, and, to an extent, the length of the barrel will not be possible to optimize for velocity due to the fact that what happens in the chamber is way more important than what is happening down the barrel.

Maybe you could run some tests with your drilled tapped chambers and give us some real pressure data and perfect my redneck methods so we have some more accurate and precise data to go off of other than thw testing I did.

As for the subsonic loads, I do agree that when the extremes are reached, such as very voluminous cartridges with long barrels, very voluminous bore diameters per case volume, and very fast powders [I include subsonic loads here] the theory breaks down, as I don't think anyone can argue that you are maximizing velocity with titegroup in a .223.

But, I would posit that with said line of reasoning, I would predict that if you were able to maximize velocity in a 8 inch .44 magnum for example, the powder which yielded the highest velocity in an 8 inch barrel would probably still be the powder which yields the highest velocity in a 4 inch barrel.

Try it one time and get back to us.

Happy fireballs.

 

[dellet]

I anticipated to get the fastest velocity from LT-32, on down to slowest at sta-ball.

What I actually got was not that.

Try it one time and get back to us.

Happy fireballs.

I have tried it, that’s how I know that I can give models and show results to argue this both ways and be “right”.

What I’m trying to point out is the flaws in your testing.

The first is quoting Hodgdon for max loads, them modifying those loads. Hodgdon does not have data for LT32 and a 75 grain bullet, it appears you used 73 grain bullet data . Then interpreting that data incorrectly. The Hodgdon data shows that LT32 consistently has the slowest velocity, while being the fastest rated powder. This data was tested in a 24” barrel.

Since you don’t have live fire data for your loads in an 18” barrel, or even the 24” test barrel, we have trust a secondary data source. Or use Hodgdon data against yours.

Since your test did exactly that, let’s look at the whole data. Hodgdon data for a 73 grain bullet, note that LT32, has the slowest velocity in a 24” barrel.

Edit to add.
Below is the 77 grain data because it had all five powders listed in the op’s video, the 73 grain data omitted Exterminator. No Matter LT32 was still the lowest velocity.



Based on your own live fire results in an 11” barrel, the faster powder, LT32, yielded a faster velocity 65% of the time.

Or am I confused and these were not your velocity results in your short test barrel?



I can only reference Quickload as I am not familiar enough to say for Gordon’s. With Quickload if you select a bullet, COL, and barrel length and a powder charge, the program can generate suggested load data for a fixed pressure. Then sort those results multiple ways. Sorting by velocity yields interesting results for different barrel lengths. Generally the top 20 powders are the same, order is generally different based on barrel length. Often the fastest powder is one no one uses, because it’s only quality is velocity.

Again your results are interesting, but possibly misleading. What I see is that going to a faster powder can be beneficial, but LT32 might be about the point of diminishing returns, where max pressure is hit before max velocity.

Finally this is an example of one cartridge, max pressure and bullet weight. Your results will vary as any of those factors change.

I needed to try it more than on time to convince myself that I can present live fire data to prove that faster powders work better in shorter barrel, or not. As you said, it’s really not about barrel length. It’s how you manipulate the data.

Not knocking your results, just encouraging you to keep looking.

 

[Doom]

Interesting for me to understand would be how the ratio of bore diameter of a given charge affects a short barrel. Does total bore volume have as much to do with burn efficiency as just barrel length? Given a the same medium burn rate powder in both the .243 Win and the .358 Win. wouldn't the .243 require a longer barrel to reach 100% burn efficiency? It would seem a bore volume factor could be valuable. Perhaps someone with a much better physics back round could explain.
Merry Christmas All

You would not be able to use the same powder. In the two cartridges everything changed including the bullet weights. The powder bulk densities will be different. You cannot get enough IMR 4350 in a 358W case to get enough pressure to get a good burn. Part of the powder burn rate control is based on the physical properties of the powder. Slower burn rate powders kernels are typically bigger and/or may have more deterrent.

The burnout is a function of pressure and residence time in the barrel. The powder burn faster at higher pressure.

 

[DiffEQ]

Although the difference in velocity may be more pronounced in slower powders, the maximum achievable velocity for a shorter length barrel can not be a load which achieves less than the max velocity in the longest barrel tested.

Oh my, that's a word salad, and was a disincentive for watching the video.
By "optimizing" velocity, I read maximizing velocity.
As I have learned from decades of reloading and shooting, chasing speed is a fool's errand. Optimizing, to me, is finding that sweet spot marrying the most consistent MV and most accurate on target.
My understanding of what I read is that the ultimate intent of your experiment sought to determine if you could get the same velocity out of a shorter barrel by using a faster powder.
As @Doom wrote, burn rate is affected by pressure, and thankfully we (generally) don't have to worry about rifle powders detonating (runaway burn rate) once a certain pressure is reached.
Peak chamber pressures is the ultimate determinant of MV, and the completion of the burn as the bullet travels down the barrel will affect the rate of acceleration. The trade-off, as I believe @dellet mentioned, is higher pressure and report at the muzzle.
The higher the pressure behind the bullet as it exits the muzzle tests the consistency of your rifling and crown. There is a point where this has marked affects on your accuracy.
Coming back to your experiment, I hope you optimized powder charges for the same bullet in the same barrel, then repeated the optimization for the different powders using the same bullet in a shorter barrel.

 

[Doom]

Oh my, that's a word salad, and was a disincentive for watching the video.
By "optimizing" velocity, I read maximizing velocity.
As I have learned from decades of reloading and shooting, chasing speed is a fool's errand. Optimizing, to me, is finding that sweet spot marrying the most consistent MV and most accurate on target.
My understanding of what I read is that the ultimate intent of your experiment sought to determine if you could get the same velocity out of a shorter barrel by using a faster powder.
As @Doom wrote, burn rate is affected by pressure, and thankfully we (generally) don't have to worry about rifle powders detonating (runaway burn rate) once a certain pressure is reached.
Peak chamber pressures is the ultimate determinant of MV, and the completion of the burn as the bullet travels down the barrel will affect the rate of acceleration. The trade-off, as I believe @dellet mentioned, is higher pressure and report at the muzzle.
The higher the pressure behind the bullet as it exits the muzzle tests the consistency of your rifling and crown. There is a point where this has marked affects on your accuracy.
Coming back to your experiment, I hope you optimized powder charges for the same bullet in the same barrel, then repeated the optimization for the different powders using the same bullet in a shorter barrel.

A case in point is something I tried in my 20" 308. The normal load is about 43 gn of IMR 4064 and 168 SMK. When it came out Alliant 2000MR was touted to give more speed and more speed it did indeed give. To the tune of about 100 fps or more as I recall. But the muzzle flash became apparent in shaded bright sunlight. The report was deafening and the recoil became punishing. This was about 47.5 gn and the listed max for that powder is 49.5 gn. For me the rifle became unbearable to shoot.

Note that not only was the powder speed vastly different but IMR 4064 is a single based powder and 2000MR is a double based powder which is responsible for the speed increase.

 

[Loudenboomer]

You would not be able to use the same powder. In the two cartridges everything changed including the bullet weights. The powder bulk densities will be different. You cannot get enough IMR 4350 in a 358W case to get enough pressure to get a good burn. Part of the powder burn rate control is based on the physical properties of the powder. Slower burn rate powders kernels are typically bigger and/or may have more deterrent.

The burnout is a function of pressure and residence time in the barrel. The powder burn faster at higher pressure.

Understood. The question was the same medium speed powder ( half way to optimum for both cals) and corresponding barrel length for optimum combustion in both calibers. The second part of the question was given the pressure drops off faster in the larger bore and wondering if a mathematical bore ratio bore pressure curve could be applied. I may not have stated the question properly. Probably too many variables but this was intended just for fun.

 

[cosmic creepers]

I have tried it, that’s how I know that I can give models and show results to argue this both ways and be “right”.

What I’m trying to point out is the flaws in your testing.

The first is quoting Hodgdon for max loads, them modifying those loads. Hodgdon does not have data for LT32 and a 75 grain bullet, it appears you used 73 grain bullet data . Then interpreting that data incorrectly. The Hodgdon data shows that LT32 consistently has the slowest velocity, while being the fastest rated powder. This data was tested in a 24” barrel.

Since you don’t have live fire data for your loads in an 18” barrel, or even the 24” test barrel, we have trust a secondary data source. Or use Hodgdon data against yours.

Since your test did exactly that, let’s look at the whole data. Hodgdon data for a 73 grain bullet, note that LT32, has the slowest velocity in a 24” barrel.

Edit to add.
Below is the 77 grain data because it had all five powders listed in the op’s video, the 73 grain data omitted Exterminator. No Matter LT32 was still the lowest velocity.

View attachment 1617933

Based on your own live fire results in an 11” barrel, the faster powder, LT32, yielded a faster velocity 65% of the time.

Or am I confused and these were not your velocity results in your short test barrel?

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I can only reference Quickload as I am not familiar enough to say for Gordon’s. With Quickload if you select a bullet, COL, and barrel length and a powder charge, the program can generate suggested load data for a fixed pressure. Then sort those results multiple ways. Sorting by velocity yields interesting results for different barrel lengths. Generally the top 20 powders are the same, order is generally different based on barrel length. Often the fastest powder is one no one uses, because it’s only quality is velocity.

Again your results are interesting, but possibly misleading. What I see is that going to a faster powder can be beneficial, but LT32 might be about the point of diminishing returns, where max pressure is hit before max velocity.

Finally this is an example of one cartridge, max pressure and bullet weight. Your results will vary as any of those factors change.

I needed to try it more than on time to convince myself that I can present live fire data to prove that faster powders work better in shorter barrel, or not. As you said, it’s really not about barrel length. It’s how you manipulate the data.

Not knocking your results, just encouraging you to keep looking.

Oh I'm definitely going to continue the testing as we go on here, and yes I took presumptions and then ran the numbers in Gordon's [a friend ran them for me] and P-max, which I ran myself, and both had me slightly over max pressure. So yes, it was a pressure range we were at.

I'd love you to understand, I really enjoy getting you guys perspective on stuff like this, I'm not getting heated, just exploring the angle. On anything highly differential in result.

What we are going to do today is shoot the LT-32 out of both the rifle length 18 inch gun and the 11½ inch gun because the groups were very nice, and record velocity data and group size.

And you are correct these are 75 grain RMR. Not 73 grain ELD. So I've made changes to the "max charges, but the pressure models should not have changed much by the programs I ran.

And I've got velocity data from all of these loads except for the xterminator and the LT-32 out of a 18 inch barrel and their 24 inch test barrel when making the handloading book was also handy because i am then not required to shoot most similar loads out of that barrel length again, as I don't think Hodgton is lying to us.

We will see. I've upped the charge weight with the LT-32 a dangerous amount. I've got 21.3 grains where my former test was ran with 21.0 if that doesn't yield me some flat primers or some brass flow on my ejector orifice, I'll be surprised and we will see if it matches or defeats velocity from some slower powders.

Thanks again for your input.