velocity goes down with more powder

[daniel brothers]

it's a air volume density thing

 

[chkunz]

I will counter your theory with in your testing you were not pressurizing a confined space. or tube. so the pressure curve can change drastically.. by just changing bullet seating depth .010. . I am going to retest through a different chrony were it's at and .010 into the rifleing. I'm set up at just touching right now but i'm betting if I was .010 more in I would have seen more velocity and high pressure.
But hey just a theory. I really do not believe it to be a chrony error.

will be the weekend of 10/04 that I will have time to do it.

Great! I will offer one recommendation, give your test some thought to see if you can improve on it. The time spent in designing the test is small as compared to the time and effort spent in conducting the test. If possible, get other knowledgeable folks to help you with the design of the test. You may find that there is a more meaningful test. For example, I think that Eric pointed out that the change in volume for 0.010 difference in seating depth is not significant relative to case volume and oxygen content and I agree. Think thru what the possible results of your (our) test may be and what they will tell you. There is no use to hurry if you are on a wrong road. I wish I were more knowledgeable on chronographs because I am still concerned that the phenomenon we are focused on may be within the variability of the data we get from them. But I do not know. Others do know and can give us valuable information and with 25,000 forum members somebody knows. How do we separate out the folks who are knowledgeable from the forum bullies? One last point, my use of the rocket motor analogy is overly simplistic, combustion at sonic velocity is very complex and does not scale. Aerodynamics does scale and is very well behaved (think external ballistics) but internal ballistics (think combustion as in rockets and turbojets and our rifles) does not scale and is very complex. Anyway, your efforts could inspire other to participate and make a meaningful contribution. Two heads are better than one and we have 25,000 heads on this forum and if we work together we can move beyond the junk science and all benefit. No apology but I just got carried away a little. Take care, Clyde.

 

[chkunz]

I will counter your theory with in your testing you were not pressurizing a confined space. or tube. so the pressure curve can change drastically.. by just changing bullet seating depth .010. . I am going to retest through a different chrony were it's at and .010 into the rifleing. I'm set up at just touching right now but i'm betting if I was .010 more in I would have seen more velocity and high pressure.
But hey just a theory. I really do not believe it to be a chrony error.

will be the weekend of 10/04 that I will have time to do it.

T-REX thanks for your input. as to your question how accurate is a chronograph. the manufacture's claim 99.5% accuracy .
now IMO they are only as accurate as the guy who sets them up. if you want true reading's you need to measure out to it. shoot straight over the sensors and they seem to work quite well. now if you don't set it up correctly or shoot threw it on a angle you will get bad. sometimes light can throw them off or out of wack but you will usually notice it's not right. I have set up many of range cards for a lot of rifles that were spot on from the velocity readings I have got and also tuned loads and have seen the benefits at long range from that so I feel they work great and accurately most of the time.

It looks like we have identified our chronograph expert and that is you. You are definitely more qualified than I and since you will be our expert in this experiment it is on your head to be sure we are interpreting the velocity data correctly. As you point out the manufactures specs say all is well but how we use the instrument is critical to obtaining good data. Now we need to enlist an expert in internal ballistics to help us understand why the laws of physics say that an increase in powder charge will result in a decrease in velocity. Will our internal ballistics expert please step forward?

 

[BoydAllen]

As I have said, I do not have the answer to the question currently under consideration, but perhaps I can contribute something that may stimulate some thought.

We are used to seeing pressure time curves that have a certain shape, but they are not always that way. This link will take you to a page that has some examples that are decidedly nonstandard and undesirable. http://www.shootingsoftware.com/pressure.htm

Another thing that I would bring up is that although the ranges of charges for a particular caliber and bullet weight that are generally used fall on a relatively linear part of a chart of all possible results, that above and below certain pressures, these curves are known to loose their linearity.

A third point that I will contribute is that burn rate, is partly controlled by particle size and shape, and that with enough pressure, that these may be modified, altering burn characteristics.

Lastly, I would point out the recent technology that has produced powders that yield more than usual velocities by elongating the pressure curve, increasing the area under it, without creating unsafe peak pressure. I mention this because I think that it may be possible that the opposite can happen, and that some of the graphs in the page that I furnished a link to showed this.

All of these factors might be considered in searching for an answer to the question.

 

[JohnKielly]

Dang, I hate it when I see a thread & don't read it. Recently, on one of the forums I am a member of (might even have been this one) was a thread on whether/how the burning rate of powder changed after the load was compressed.

I don't know whether it might have relevance to this subject, bu I wish I had read it now.

 

[gunsandgunsmithing]

Dang, I hate it when I see a thread & don't read it. Recently, on one of the forums I am a member of (might even have been this one) was a thread on whether/how the burning rate of powder changed after the load was compressed.

I don't know whether it might have relevance to this subject, bu I wish I had read it now.

This goes along with my theory.
Powder burn rates are controlled by coatings, shapes and sizes of powder kernels. A tightly compressed powder charge will "act" more like one big kernel, than say, a light fluffy stack of flake powder with holes in the middle of each flake. They use those holes and shapes in some powders to expose more surface area of the flake/kernel to the flame, changing it's burn rate to a faster one.
--Mike Ezell

 

[Syncrowave]

smokeless powder's do not need oxygen to burn. but there is just enough oxygen inside the case to help the burn or speed up the burn.

My hunch is that they put in enough oxidizer such that the stoichiometric ratio is at least 1:1. In other words, there is enough oxidizer to completely convert the powder to gas with 100% combusion, even in the total absence of atmospheric oxygen.

 

[Joe Salt]

Well I'm not into this Rocket Science like you guys but I would guess that that particular rifle with the powder used plus the length of the barrel, you have exceeded the amount of that powder that can be burned in that rifle. Try a hotter powder or primer and see what happens.

Joe Salt

 

[chkunz]

I will counter your theory with in your testing you were not pressurizing a confined space. or tube. so the pressure curve can change drastically.. by just changing bullet seating depth .010. . I am going to retest through a different chrony were it's at and .010 into the rifleing. I'm set up at just touching right now but i'm betting if I was .010 more in I would have seen more velocity and high pressure.
But hey just a theory. I really do not believe it to be a chrony error.

will be the weekend of 10/04 that I will have time to do it.

T-REX thanks for your input. as to your question how accurate is a chronograph. the manufacture's claim 99.5% accuracy .
now IMO they are only as accurate as the guy who sets them up. if you want true reading's you need to measure out to it. shoot straight over the sensors and they seem to work quite well. now if you don't set it up correctly or shoot threw it on a angle you will get bad. sometimes light can throw them off or out of wack but you will usually notice it's not right. I have set up many of range cards for a lot of rifles that were spot on from the velocity readings I have got and also tuned loads and have seen the benefits at long range from that so I feel they work great and accurately most of the time.

Just for reference the manufacturers stated accuracy of 99.5% would result in +/- 15 fps at 3000 fps. This would be without any of the set up problems that you refer to. So in looking at your data the variability may be significant. This does not say that the phenomenon did not occur but it does say we are pressing the limits of the instrument. Maybe the next test should be to fire ten rounds in the range we are looking at to see what the variability of the data is.

 

[chkunz]

As I have said, I do not have the answer to the question currently under consideration, but perhaps I can contribute something that may stimulate some thought.

We are used to seeing pressure time curves that have a certain shape, but they are not always that way. This link will take you to a page that has some examples that are decidedly nonstandard and undesirable. http://www.shootingsoftware.com/pressure.htm

Another thing that I would bring up is that although the ranges of charges for a particular caliber and bullet weight that are generally used fall on a relatively linear part of a chart of all possible results, that above and below certain pressures, these curves are known to loose their linearity.

A third point that I will contribute is that burn rate, is partly controlled by particle size and shape, and that with enough pressure, that these may be modified, altering burn characteristics.

Lastly, I would point out the recent technology that has produced powders that yield more than usual velocities by elongating the pressure curve, increasing the area under it, without creating unsafe peak pressure. I mention this because I think that it may be possible that the opposite can happen, and that some of the graphs in the page that I furnished a link to showed this.

All of these factors might be considered in searching for an answer to the question.

Boyd, I took a look at the reference and it was focused on measuring chamber pressure. It did say that there is a point where an increase in powder charge results in little increase in velocity and it showed one case where the velocity had flattened out. But there was no information on the uncertainty of the instrumentation being used. Anyway this was a sales pitch on an instrument for measuring chamber pressure and there was not much to help explain why velocity goes down with an increase in powder charge.

 

[Milo 2.0]

I noticed years ago, that while working up a load with progressively higher charges, that when you got to a certain point the velocity would not go up as much as the previous weight did. It always seemed to be right at the point of to much powder, and approaching max weights for that cartridge...
Some folks say it don't happen, but it did to many times to not pay attention to it...

I've witnessed this too on many occasions, your velocity increase per charge weight increase just levels off or drops. Keep adding powder and sooner or later you'll get a stiff bolt lift or ejector mark. You may get a velocity spike when this occurs.
IMO when the velocities level off, you've reached the maximum efficiency of your components, powder, case, bullet and rifle, and you need to develop your load at a charge weight under where it started to level off to maintain a safe load. If you're not happy with the speed, you need to change something?

 

[mikecr]

The tricky part in this, and probably the clue, is that continued increase beyond drop in velocity causes pressure spiking.
Between the pressure traces Boyd pointed to and with Joes notion about exceeding the amount burned INSIDE the barrel, there may lie the answer.

Just a theory,, Load density reaches a point of detriment to powder burn. We end up with a near solid slug of powder pushed with the bullet, and no longer contributing to velocity. In fact, lowering velocity because to powder slug mass is adding to the bullet mass.
At the muzzle the condition is released, all is burned and we get a spiking muzzle pressure peak that extends all the way back to the chamber(as shown by an RSI pressure trace). This does not contribute to increased velocity because the bullet is free by then, but contributing to pressure signs seen at the case.

I think you would need a very bad mismatch in the shooting system to see this. Like a 30-06 in 24" barrel and powder that's just too slow, packed in their beyond rational.

 

[CatShooter]

My theory is. you guy's are going to laugh and make fun of me. it is a slow powder for the case, smokeless powder's do not need oxygen to burn. but there is just enough oxygen inside the case to help the burn or speed up the burn. now more powder filled the case so there was no oxygen which slowed the burn or burn speed down.
hence the slower velocity. or may have just slowed the pressure curve down to where the bullet exited the barrel as pressure peeked, so as you had high chamber pressure but that pressure never had a chance to push the bullet. now I feel you could change anything that could change the pressure, as the list is long. and not see it happen. even internal cases volume. like a different lot of cases. as I said the list is long.

I can address this one directly - a member of my gun club got into a bloody food fight about the air in a case is needed to "burn" the powder...

... it reached the level of interfering with club activities.

So we loaded 10 30-06 cases with an appropriate load of IMR 4350 and 180gr bullets normally (with "air).

Then we loaded 10 cases with the same load, and during the process, we keep the cases filled with CO2 from a fire extinguisher as they were loaded.

There was no difference in velocity.

The reason is that smokeless powder does not burn like wood or paper, it decomposes (deflagerates) in a two stage process (first an interim transitional gas, then a flame), and does not, and CONNOT, use any oxygen from other sources. It does not consume oxygen, it generates oxygen. The formula of cellulose hexa-nitrate (what we use) (C12H14[NO3]6O10) is lousy with oxygen.

Bets were paid and there was peace on the firing line.

 

[scpaul]

I suggest that the next series of tests are done with one of the chrono's with the 3rd screen that confirms the first set of screens measurement. Paul

 

[Joe Salt]

OK guys one more time! This isn't my first day at the range, years ago I use to watch guys try and get more velocity out of there rifle with slow burning powder like H-570 most of you never had the chance to try that. But they would fill the cases of the lone version of the 30-378 and in the daylight you could watch a three foot flame come out of the muzzle. Unburned powder! But they were getting better velocity with using the old H-4831. Just because you put more of one powder in a case than another doesn't mean more velocity. A man needs to know his limitations! (Clint Eastwood) And that is what is happening when you start losing out on velocity.

Joe Salt

 

[chkunz]

smokeless powder's do not need oxygen to burn. but there is just enough oxygen inside the case to help the burn or speed up the burn.

My hunch is that they put in enough oxidizer such that the stoichiometric ratio is at least 1:1. In other words, there is enough oxidizer to completely convert the powder to gas with 100% combusion, even in the total absence of atmospheric oxygen.

For what it is worth, stoichiometric ratio is the ratio of the oxidizer to the fuel that results in complete combustion. That ratio is different for different fuels and oxidizers. For example oxygen and hydrogen have a stoichiometric ratio of 8. Hydrocarbons have stoichiometric ratios between ten and twenty depending on the specific compound. Normally you would expect gunpowder not to be stoichiometric and the combustion products to be fuel rich. So theoretically the oxygen in the air in the case does have an opportunity to contribute to the combustion process but for our example cited by the OP if you calculated the weight of the oxidizer in the gunpowder and compared it to the weight of the oxygen in the air in the case you would find the oxygen in the air in the case to be insignificant. Anyway what we are talking about in this thread is why would an increase in powder charge in this specific case result in a decrease in velocity and with all due respect your point does not appear to me to be relevant but I could be wrong.

 

[chkunz]

Dang, I hate it when I see a thread & don't read it. Recently, on one of the forums I am a member of (might even have been this one) was a thread on whether/how the burning rate of powder changed after the load was compressed.

I don't know whether it might have relevance to this subject, bu I wish I had read it now.

This goes along with my theory.
Powder burn rates are controlled by coatings, shapes and sizes of powder kernels. A tightly compressed powder charge will "act" more like one big kernel, than say, a light fluffy stack of flake powder with holes in the middle of each flake. They use those holes and shapes in some powders to expose more surface area of the flake/kernel to the flame, changing it's burn rate to a faster one.
--Mike Ezell

Mike, the Op did not describe his load as "a tightly compressed powder charge" and he was increasing in increments of 0.5 grains. Maybe he will comment on this and tell us if his load would be considered a "tightly compressed powder charge". But even if it is how does this result in a lower velocity for an increase in powder charge for the condition he cited?

 

[chkunz]

Well I'm not into this Rocket Science like you guys but I would guess that that particular rifle with the powder used plus the length of the barrel, you have exceeded the amount of that powder that can be burned in that rifle. Try a hotter powder or primer and see what happens.

Joe Salt

Joe, the question that we are discussing is why did the velocity go down with an increase in powder for the configuration that the OP cited. Others have said that they have seen it in other situations but I do not think we have yet had anyone answer that question.

 

[mikecr]

Did you read #31?

 

[chkunz]

I noticed years ago, that while working up a load with progressively higher charges, that when you got to a certain point the velocity would not go up as much as the previous weight did. It always seemed to be right at the point of to much powder, and approaching max weights for that cartridge...
Some folks say it don't happen, but it did to many times to not pay attention to it...

I've witnessed this too on many occasions, your velocity increase per charge weight increase just levels off or drops. Keep adding powder and sooner or later you'll get a stiff bolt lift or ejector mark. You may get a velocity spike when this occurs.
IMO when the velocities level off, you've reached the maximum efficiency of your components, powder, case, bullet and rifle, and you need to develop your load at a charge weight under where it started to level off to maintain a safe load. If you're not happy with the speed, you need to change something?

Preacher, with all due respects we are not talking about a leveling off of velocity with increased powder charge but an actual drop in velocity with an increase in powder charge for the configuration cited by the OP. I think we would all agree with your observation of a leveling off and that is completely understandable but what we have not yet explained is why would the velocity actually drop with an increase in powder charge,