Lets discuss Barrel Time

[Mozella]

I think there are a few basic things on which most target shooters can agree, even though we may not agree on the exact mechanism causing what we observe. For example:

For a particular target gun, there is a certain “recipe” which gives the best performance for the job at hand; i.e. for 600 yd F-Class, 100yd BR, 1000yd Steel Shooting, etc. That recipe is a complicated combination of powder, bullet, case, and primer brand/type as well charge weight, seating depth, neck tension, lubrication, and brass prep. Can we agree that there are other factors like temperature, air density, and so-on which also come into play? Yes, is the obvious answer, even though we might disagree on the details.

I would also argue that most shooters believe our barrels vibrate in a particular way and that a “good recipe” acts in harmony with a particular barrel and how it vibrates while a bad load recipe one does not. I’m not going to go down the path of exactly how a barrel reacts when we fire a round because understanding that complex mechanism is not germane to this discussion. Of course, barrel length, bore diameter, rifling design, freebore, and other dimensions are important as well. Barrel temperature? Yep, that is bound to be a variable too. Your barrel will never match my barrel, but that doesn’t matter as long as each of us develop our own recipe which matches well with our respective barrels, right?

I think we all agree that charge weight is one of the most important variables when developing a good ammo recipe. Even those who are not real serious about load development will spend some time testing various charge weights.

Most folks interested in precision shooting also fret about how temperature might change the performance of our optimum recipe. Quick load predictions claim that even with the most stable powder brand/type, the muzzle velocity and barrel time will change with changes of temperature. Makes sense, no? A temperature change will, at least in theory, run the risk of putting your rifle “out of tune”, as they say.

Having said all that, I’ve been thinking that it might be possible to use Quickload to fine tune a load recipe developed at one temperature so that it will work well at a different temperature. That’s the idea anyway.

It would seem to me that if a good recipe works in harmony with favorable barrel vibrations, then barrel time would be a more important factor than muzzle velocity when it comes to finding a factor which should be held constant over a range of temperatures. I can’t measure barrel time directly, but I can do a pretty good job of measuring muzzle velocity. By working backwards, I can get Quickload to give me a theoretical barrel time for a particular set of circumstances which have been proven to produce good performance at the target. I chronograph nearly every round I fire using Labradar and I keep detailed records of my scanned/measured targets, temperature, and just about everything else you can imagine.

Having found the barrel time for a recipe which performs well in cool weather, it occurs to me that, with the help of Quickload, I could tweak the charge weight to produce an identical barrel time when the ambient temperature is much higher. In other words, without having to test in a wide range of temperatures, I should be able to adjust my good cool-weather load recipe to get good performance at some other temperature by tweaking the powder charge so as to keep the barrel time the same.

Does that make sense? It seems to me that it should make sense, but I also know enough about shooting to know that actually proving it will be difficult if not impossible.

At the moment I’m trying to fine tune Quickload to match the predicted muzzle velocity with my measured muzzle velocity because like most shooters, I can measure MV but not barrel time. I’m tweaking bullet weight, weighing factor, and burn rate factor in attempt to get more reliable predicted data from Quickload. This is easier said than done, I can say that for sure. But I would imagine that getting QL to match my real world measured results should be the first step before trying to extrapolate predicted data at other temperatures.

I have already tried making some preliminary tweaks to compensate for the temperature variations in the last couple of F-class matches. But I don’t know if the good results are because of or in spite of using this equal barrel time theory.

I know that there are shooters who concentrate on muzzle velocity as an important constant for tweaking loads, but I wonder how many shooters using QL pay attention to barrel time and use that as the foundation for tweaking charge weights?

 

[brantfredrickson]

When a shot is fired I believe there are two temperatures that effect the sweet spot, OBT, and that they are independent variables. The first is the temperature of the powder. The second is the temperature of the barrel. During load development I monitor the temperature of the barrel during a 50 shot string and observe that it changes the charge load where the sweat spot occurs. And I believe it is not temperature of the barrel alone that moves the sweet spot, but temperature/condition of the barrel that moves the sweet spot.

I can see where you could modify the vivacity in QL to compensate for a change in powder temperature, but I don't see a mechanism in QL for compensating for change in temperature/condition of the barrel.

This may be of interest to you. For a couple of bucks I bought a 5ml graduated cylinder of ebay and measured the bulk density of my powder and compared it to the figure QL uses. There was about a 3% difference. And when I plugged the actual powder bulk density into QL it changed the charge weight at the OBT a couple of tenths of a grain.

For now the best I can do with QL is identify a charge weight for a high node and the next lower node. And then in my load development make sure to extend the range of charge weights to include both nodes. Perhaps there is more possible. Best of luck in you endeavor! Brant

 

[Bart B.]

I’m not going to go down the path of exactly how a barrel reacts when we fire a round because understanding that complex mechanism is not germane to this discussion.

I think it's germane because the bore axis at the muzzle is vibrating vertically. The frequency and direction is very repeatable from shot to shot if the barrel is free floating. Barrel time for all bullets varies a little because pressure curves vary a little. If the load makes all bullets leave in the best place in the muzzle axis cycle, bullet departure angles will compensate for muzzle velocity spread.

Tuners on barrels adjust to change the barrel vibration frequencies so bullets leave at the right place for a given load. Adjusting powder charges and cartridge dimensions can do the same thing.

I've not observed any accuracy change as barrel temperatures go from ambient to skin burning hot.

 

[Mozella]

I think it's germane because the bore axis at the muzzle is vibrating vertically. The frequency and direction is very repeatable from shot to shot if the barrel is free floating. Barrel time for all bullets varies a little because pressure curves vary a little. If the load makes all bullets leave in the best place in the muzzle axis cycle, bullet departure angles will compensate for muzzle velocity spread.

Tuners on barrels adjust to change the barrel vibration frequencies so bullets leave at the right place for a given load. Adjusting powder charges and cartridge dimensions can do the same thing.

I've not observed any accuracy change as barrel temperatures go from ambient to skin burning hot.

I don't disagree with what you say and plenty of people agree with you about vertical barrel vibrations being important. Others, particularly those who believe in the OBT theory, say that longitudinal vibrations are what should concern us. But making those arguments is a subject for a different post.

My question is a narrow one and has to do with the importance of barrel time. Specifically, if you discover a good recipe in cool weather, can you count on good performance in the hot summer time if you tweak the charge weight with the help of QL so as to maintain the same barrel time that works so well for you in the winter time. That is the question.

It is of interest to me because I can change my charge weight to give me a particular barrel time. Absent a tuner, I can't do much about how and why my barrel vibrates in a particular way.

In other words, is consistent barrel time a good way to fine tune a load when the weather changes, or is that approach simply chasing one's tail.

If barrel time is meaningless, what do people suggest as a way to fine tune a load when you haven't tested at the temperature forecast at an upcoming match?

 

[Mozella]

When a shot is fired I believe there are two temperatures that effect the sweet spot, OBT, and that they are independent variables. The first is the temperature of the powder. The second is the temperature of the barrel. During load development I monitor the temperature of the barrel during a 50 shot string and observe that it changes the charge load where the sweat spot occurs. And I believe it is not temperature of the barrel alone that moves the sweet spot, but temperature/condition of the barrel that moves the sweet spot.
........ snip...........

Have you collected data on how barrel temperature effects performance? If so, would you be kind enough to post it. This is of interest to me because it's not immediately obvious to me how tracking barrel temperature vs charge load can be tested without firing a large number of rounds.

 

[GLZ]

I've not observed any accuracy change as barrel temperatures go from ambient to skin burning hot.[/QUOTE]
I sure have, with that big blister on my trigger finger my groups go to Hell.

 

[oldroper]

[QUOTE="Bart B., post: 37142331, member: 1284499"

Tuners on barrels adjust to change the barrel vibration frequencies so bullets leave at the right place for a given load. Adjusting powder charges and cartridge dimensions can do the same thing.

.[/QUOTE]

Bart, could you tel everyone what tuners you are using?

 

[Bart B.]

My question is a narrow one and has to do with the importance of barrel time. Specifically, if you discover a good recipe in cool weather, can you count on good performance in the hot summer time if you tweak the charge weight with the help of QL so as to maintain the same barrel time that works so well for you in the winter time. That is the question.

It is of interest to me because I can change my charge weight to give me a particular barrel time.

I do not think any load change one makes based on QL data will make their ammo change as QL calculates. Too many variables in each contributing part; both yours and that of QL data. You and your stuff will produce different velocity numbers with the same component suite as QL used to get theirs. Even if you shot the same stuff, velocity numbers will be different.

Never saw any significant difference in accuracy across several hand held rifles with the same load in ambient temperatures from 20 to near 100 degrees F. If you can shoot your stuff no worse than 2/10ths MOA at any range, you might see a difference.

How many microseconds spread in barrel time for a given load will you accept?

If this barrel time thing has to make bullets exit the muzzle when it's not enlarged by shock waves at the same time (Long's OBT theory), would the presense of copper fouling in the last inch of the bore prove that shock wave was somewhere else when bullets exit? And if there was no fouling, that shock wave was there at bullet exit?

 

[Keith Glasscock]

Just one thing to note. If you read the documentation that is part of QL, you will notice that the temperature function is a fixed percentage change based on a study. I essence, the resultant change in burn rate (percentage basis) will be the same for a very temperature sensitive powder (like RE-15) and a relatively insensitive one (like Varget).

 

[Bart B.]

I agree, hotter temperature powders shoot bullets faster than colder ones. At long ranges, I come down a click for every 20 to 30 seconds of chamber time while powder cooks.

 

[CharlieNC]

For a given powder and load, I would be surprised if you did not find that there is a perfect correlation between barrel time and velocity within the range you are considering. This is what I have seen when running a ladder test and comparing the results to QL, that is the BT and velocity correlate over that charge range. So tweaking one uniquely tweaks the other. I'm not familiar with how to change the temperature sensitivity within QL.

 

[mikecr]

I feel like there are too many abstracts to hang hopes on any one of them.
There are vibrations traveling breech to muzzle, back & forth, that open/close the bore.
Barrel droop that leads to whip.
Barrel torque. Barrel temps.
Then you have the same with bedding/stock/rest combinations.
Then every component behind a load changes from lot to lot, leading to a requirement to re-calibrate QL for any different combination(to the finest detail).
You have a powder node, a seating node, primer striking node, optimum charge, optimum viable pressures(peak & muzzle), all combining to setup system timing that happens to be good enough. And ALL temperatures help or hurt.
With all this, nobody today can predict a build/load tune.

One time I wanted to believe OBT. I drank the cool-aid for a bit. But then I figured out that I could tweak/cause pretty much ANY load to seem OBT, whether it actually shot for damn or not.. Of course I figured this out when OBT loads sucked for me!

 

[gunsandgunsmithing]

I feel like there are too many abstracts to hang hopes on any one of them.
There are vibrations traveling breech to muzzle, back & forth, that open/close the bore.
Barrel droop that leads to whip.
Barrel torque. Barrel temps.
Then you have the same with bedding/stock/rest combinations.
Then every component behind a load changes from lot to lot, leading to a requirement to re-calibrate QL for any different combination(to the finest detail).
You have a powder node, a seating node, primer striking node, optimum charge, optimum viable pressures(peak & muzzle), all combining to setup system timing that happens to be good enough. And ALL temperatures help or hurt.
With all this, nobody today can predict a build/load tune.

One time I wanted to believe OBT. I drank the cool-aid for a bit. But then I figured out that I could could tweak/cause pretty much ANY load to seem OBT, whether it actually shot for damn or not.. Of course I figured this out when OBT loads sucked for me!

I understand. Much like, I can set a tuner on a given barrel for temp alone, IIRC 72% of the time. It was and is not 100%. I quit trying due to the fact that tuners are so easy to make work, the temp/humidity/density altitude stuff all went away for me. I simply check tune on the sighter and adjust IF necessary. IOW, the target is all that matters.

I do understand long range shooters and others that either don't have or can't see sighters, wanting a better way. It works by those factors at a good rate but not always.

The days that are the hardest are times before or during a very low pressure weather condition, such as a strong storm coming along with high humidity.

There are a gazillion factors at play. Most days, I hardly or not t ever, touch the tuner. Other days, it can be significantly out of tune.

Most if not all of this discussion is very relevant to tuning with powder charge/seating depth, as well as a tuner.

We're accomplishing the same thing, just by different methods. The point is to time bullet exit with optimal muzzle position.

 

[brantfredrickson]

"Have you collected data on how barrel temperature effects performance? If so, would you be kind enough to post it. This is of interest to me because it's not immediately obvious to me how tracking barrel temperature vs charge load can be tested without firing a large number of rounds."


Above is a series of data shot at 39.6, 39.8,40.0,40.2,and 40.4 grains charge weight. The first five shots are single shots, the second group of five is 4 shots each, and the third group of shots is 5 shots each. The barrel/action area was measured with an IR gun before the first shot at 75F, after the 25th shot at 85F, and the 50th shot at 92F. The first group show a sweet spot between 40.2 and 40.4 charge weight, the second group shows a sweet spot between 40.0 and 40.2, and the third group show a sweet spot between 39.8 and 40.0.
I've interpreted the sweet spot as moving as the barrel/action joint heats up. Others may see it differently.

 

[Bart B.]

When I got software that calculates the resonant frequency a barrel vibrates at, I asked why there was no barrel temperature input. The mechanical engineering vibration specialist said it doesn't matter in the temperature range target rifle barrels are shot at. Machine gun barrels, yes it does.

So I took 15 rounds each of 30-338 ammo with different bullet weights and shot them alternately at 1000 yards. Two separate sighters to get zeroed, then let the barrel cool down to ambient temperature in the 60's. Each test round's chamber time was about 10 seconds before firing. Shots fired about 35 seconds apart.

Both load's 15-shot groups just under 5 inches. Their centers under 2 inches apart. Barrel was pretty darned hot after 30 shots fired that fast.

I don't think 3-shot groups are statistically significant to represent any number the load produces. It's the same as testing 30 pairs of dice with 3 rolls each to see which pair produces the smallest average number in the 2 to 12 range. Better yet, for more resolution, 30 twin pairs of dice producing a number in the 4 to 24 range.

 

[Mozella]

........ snip..............Each test round's time in chamber was about 10 seconds. Shots fired about 35 seconds apart.

........ snip........

I'm afraid I didn't phrase my original question very well. When I use the term "barrel time", I didn't mean what happens when you let a round sit in a hot chamber for a while before you fire it.

My question has to do with the time QL calculates from when the round fires until it exits the barrel, measured in ms not seconds. In theory at least, this time in the barrel as the bullet travels from chamber to muzzle should match with how a particular barrel vibrates so as to produce good performance at one particular charge weight, temperature, bullet weight. etc etc. etc.

I would hope that when a match comes up with a much different forecast temperature than the temperature at which I tested, I could tweak my charge weight to compensate for the increase (or decrease) in the weather forecast. I would also hope that by trying to match the barrel time at various temperatures, the performance would remain good over a wide range of weather conditions.

Of course letting a round sit in a hot chamber should change the performance too, or at least one would think that is the case because that should increase the temperature of the powder. But that is a different question for a different thread.

 

[Bart B.]

I'm afraid I didn't phrase my original question very well. When I use the term "barrel time", I didn't mean what happens when you let a round sit in a hot chamber for a while before you fire it.

My question has to do with the time QL calculates from when the round fires until it exits the barrel, measured in ms not seconds.

Thanks for your comments.

My use of "barrel time" has been the same as stated in SAAMI web site, the elapsed time from the contact of a firing-pin with a cartridge primer to the emergence of the projectile(s) from the muzzle of the firearm.

I understand how "test round's time in chamber was about 10 seconds" might be construed as barrel time. So I changed it to "chamber time was about 10 seconds before firing."

 

[Mozella]

IMO you seem to be confused between internal and external ballistics. OBT is all about internal ballistics. All the data calculated by QuickLoad has to do with internal ballistics. QuickLoad does have the means for you to change the ambient temperature and that is useful too. But once the bullet leaves the muzzle then the external ballistics take over, that's what JBM Ballistics and other ballistics calculators are for.


Joe

Thank you for your help.

 

[243Mendoza]

Above is a series of data shot at 39.6, 39.8,40.0,40.2,and 40.4 grains charge weight. The first five shots are single shots, the second group of five is 4 shots each, and the third group of shots is 5 shots each. The barrel/action area was measured with an IR gun before the first shot at 75F, after the 25th shot at 85F, and the 50th shot at 92F. The first group show a sweet spot between 40.2 and 40.4 charge weight, the second group shows a sweet spot between 40.0 and 40.2, and the third group show a sweet spot between 39.8 and 40.0.
I've interpreted the sweet spot as moving as the barrel/action joint heats up. Others may see it differently.

I think that Vaughn of "Rifle Accuracy" may agree with you!

 

[gunsandgunsmithing]

I think that Vaughn of "Rifle Accuracy" may agree with you!

Perhaps, but I think his writings in this area were about unloading of the bbl/recvr joint when fired. I may be wrong. Just going from memory here. Just about every bbl I've ever seen got tighter, which would seem to concur with the joint unloading, to a point, when fired. Logically, a rh twist barrel will tighten itself to the point where this unloading stops.