Barrel Length vs Burn Rate.

[JmacD]

At the same pressures? That's the catch.

Ding ding ding! DITTO

 

[Charlie98]

If you want me to start, here I go--in my 15" barrels

I think the problem you have is the short barrel. Rifle powders in a (very) short barrel don't behave like they do in a longer barrel... my example of my 20" .348WCF with H4831 vs IMR3031 is like that. I don't think slower burn powders have the time to develop the proper pressure for a correct burn, a faster powder would have the upper hand on that.

 

[Varminterror]

I think the problem you have is the short barrel. Rifle powders in a (very) short barrel don't behave like they do in a longer barrel... my example of my 20" .348WCF with H4831 vs IMR3031 is like that. I don't think slower burn powders have the time to develop the proper pressure for a correct burn, a faster powder would have the upper hand on that.

That’s the opposite of the proven premise - a powder can be right on the edge of “too slow” for a given cartridge in a 26” barrel and then slip off of the cliff when the barrel gets shorter, but typically those won’t produce the highest muzzle velocity for that cartridge anyway - for example, H1000 is ok-ish in 6 creed in 26-28” barrels, but doesn’t make the muzzle velocities like H4350 even in the long barrels - but H1000 is abysmal in 15” barrels. BUT… I can’t use Varget in my 15” 6 creed to outrun H4350 either. I can reduce blast, but can’t get the same speed. The powders which produce the highest velocity in standard barrels also produce the fastest velocities in shorter barrels.

 

[Charlie98]

That’s the opposite of the proven premise

But I wonder... if there isn't a tipping point with very short (for cartridge) barrels and slower powders. I don't really have any experience with rifle barrels shorter than 16", so I'll have to defer.

 

[GeoDudeFlorida]

But I wonder... if there isn't a tipping point with very short (for cartridge) barrels and slower powders. I don't really have any experience with rifle barrels shorter than 16", so I'll have to defer.

The T/C Contender is the poster child for short barrels with rifle cartridges. The “tipping point” is around 10” - which is why the T/C was marketed as being for 10” barrels. Boxed-Shelf retail ammo works fine, but it’s unpleasant and wasteful. Custom hand loads are the way to go but it takes a balanced approach - burn time is only a small part of the overall equation.
What the Contender data published and tested decades ago showed is that the time in pressure is much more important than the burn rate of the powder.
Example:

This is Lyman’s data for .30-30. You’ll notice the highest velocities are from the slowest powders. W748 is on par with BL(C)2 and H335. The Super-14 and Super-16 barrels are not as common or as popular as the standard 10” barrels. Lyman’s editorial introduction makes clear that this is not specifically T/C as much as it is intended for a variety of handgun length barreled firearms intended for use with rifle length cartridges. It’s not watered down or gussied up prop wash put together for academic analysis; it’s for handgun hunting.
Look through the T/C data section for any load manual and you’ll see similar numbers. The slower powders deliver the highest velocities at the lowest pressures. When a faster powder is used - such as Harris’ “The Load” - it comes with high pressure warnings. You “could” use Nitro 100 in a 6.5CM load for a Super-14 barrel - but you won’t get close to Reloader 7 velocity before blowing the chamber walls.

 

[Varminterror]

But I wonder... if there isn't a tipping point with very short (for cartridge) barrels and slower powders. I don't really have any experience with rifle barrels shorter than 16", so I'll have to defer.

If there IS an inflection point, then it’s shorter than 7-9” for 2.3” mini-length cartridges, shorter than 10” for short action 2.8” cartridges, including short action magnum cases. Maybe at 5”, 4198 starts outrunning 748 with 50grn 5.56, but I know it doesn’t at 7”. And at the other end, it seems the barrels need to be well over 30” before they’d shift in utility of progressively slower powders. Maybe a 34” 223/5.56 barrel would run 77-80.5’s faster with Retumbo than with Varget or BL-C(2)c, but I know it doesn’t at 29”.

 

[Charlie98]

What the Contender data published and tested decades ago showed is that the time in pressure is much more important than the burn rate of the powder.

shorter than 10” for short action 2.8” cartridges, including short action magnum cases.

Well... and that's my theory, that slower powders in shorter barrels... very short barrels... don't have time to reach adequate pressure before the bullet leaves the barrel. As I say... I may be all wet on that, I don't have direct experience with it.

 

[Varminterror]

Well... and that's my theory, that slower powders in shorter barrels... very short barrels... don't have time to reach adequate pressure before the bullet leaves the barrel.

I like to do some pretty weird stuff, but like I mentioned, even shooting 300wsm from a 10” barrel, aka, massive case with a high degree of overbore, and shooting 243 from 10” barrels, again, a massively overbore cartridge, we don’t hit a significant inflection. We know the pressure peak happens somewhere typically between 1.5-2.5” for most bottleneck cartridges, so it seems we need to be REALLY close to that, again, south of 5” to make this inflection relevant… and even as a fan of the eccentric and nearly impractical, I’m really not certain what I would do with a 5” 300 WSM… it starts to feel like talking about the great gas mileage I get on my motorcycle (usually cruising around 35mpg), and trying to figure out how to make my pickup (which gets 17mpg) lighter and lighter to get to that mark… but I can bet it won’t do very well hauling a 6-horse trailer down the highway if I ever could get there, which is one of the tasks I ask of my pickup…

 

[GeoDudeFlorida]

Well... and that's my theory, that slower powders in shorter barrels... very short barrels... don't have time to reach adequate pressure before the bullet leaves the barrel. As I say... I may be all wet on that, I don't have direct experience with it.

Which is why I provided tested, published, proven data. Comparing the same load for each powder with 24” barrel tests, the 14” barrels lose about 20fps/inch of barrel. Ditto for 10” barrels. Once you go under 10” the situation changes and the loss is dramatic. Compare the results published by handgun hunters and the manuals with T/C-specific tests. Then DON’T do what I did - which was to go out and buy a T/C frame, a dozen barrels and LER scopes for hunting critters that can be killed with a really cheap bow and arrow.

 

[Charlie98]

Once you go under 10” the situation changes and the loss is dramatic.

Understandably.

 

[GeoDudeFlorida]

Understandably.

The view is what gets really dramatic

By “dramatic” I mean 50fps/inch of barrel vs 20fps/in. I’ve shot 10” T/C’s with shelf-stock Winchester hunting ammo in .30-06. The recoil and report are punishing. But the accuracy and range were surprisingly close to what I expected from a standard-length rifle. Using faster powders to shift the curve doesn’t give a faster or more accurate load; but it does give a much less punishing load. That’s why they’re used - to reduce the report and muzzle blast. It’s also an okay way to shift the recoil to a sharper blow with shorter duration as opposed to the longer, harder push of the slower powders.

 

[CraigC]

No. This is an old “gun counter fool’s myth,” which truly needs to die.

Amen! I think a lot of it is rooted in the basic misunderstanding of how long powder "burns". I honestly see and hear it more in relation to revolvers than rifle cartridges but the same applies. People think that powder "burns" all the way down the barrel. It doesn't. It burns very quickly and produces gases that expand over time. They think that muzzle flash is burning powder. It is not. People think that if you're shooting a 4" revolver, you need to use faster burning powders to yield the most velocity for that barrel length. False. Out of all the powders appropriate to that cartridge, the same slow burning powders will yield the highest velocities, whether the barrel is 4" or 20". Even out of a 3" barrel, H110 outruns Unique by 180fps.

 

[Varminterror]

I’m not terribly certain it’s true to say “the loss is dramatic” for barrels less than 10”.

Here’s a quick montage of various cartridges, from itty bitty 300blk with its great big bore capacity to 338 Lapua. We can see we have to get a LONG ways shorter than 10” with these to see anything “dramatic” happening. The slopes of the blue and red curves, bore pressure and bullet speed, aren’t “dramatically” changing just below 10”, and we’re getting down in that 5” ballpark for most of these before we see the true inflection points (greatest incremental change in slope of either line). Only the 30-06 load below seems to still not have reached a NEARLY linear expansion and NEARLY linear acceleration by 5” - which is an interesting simulation in itself that they have nearly identical P & V curves for both powders depicted there, and IMR4895 and H4350 are really pretty far apart in burn rate.

So expanding on the above statement of comparing barrel length losses of 50fps per inch vs. 20 for a given cartridge - seeing that change in loss within a given cartridge is REALLY difficult. We’re cutting ~16” off of barrels to make that happen, in my experience, and we can’t have very big cases - so we really don’t have many practical applications where that occurs. My personal references of relatively valid applications at both extremes for a given cartridge being to compare a 9” AR pistol against a 29” Highpower Match Rifle, and a 10” 300wsm Savage Striker against a 28” bench rifle - which, I’ll concede, most folks don’t find much sensibility or utility in a 10” 300 WSM. In these scenarios below, 50 vs. 20 would be visually represented by the slope of the blue line somewhere near the left end being 2.5x greater than the slope somewhere in the right end of the curve, and that’s looking to be comparing something north of 20” to something around 5-7” for most of these (300blk doesn’t go out past 20”, and it’s obviously a very fast powder with a tiny, tiny overbore ratio) - and again, we’re not talking about the same vehicle class if we start taking 24” vs. 7” barrels, so the difference is pretty moot. Wanna talk velocity loss in a 7” vs. 12” barrel for a PDW? Sure, let’s talk. Or maybe discuss the benefits of 28” vs. 29” vs. 31” in a Match rifle? Again, let’s talk. But talking about different barrels where we’re talking comparative losses of 50fps/“ vs 20fps/“ in the same cartridge, ~24” vs. 8”, is comparing apples and goldfish…

 

[hdwhit]

As of the time I retired (2018) there were no industry-wide standards on determining a powder's burn-rate.

Some testers burn the powder in open air, others burn it in various test enclosures which may (or may not) represent the conditions inside your cartridge's case.

Manufacturers caution AGAINST using burn rate comparisons in load development for a reason.

 

[Walkalong]

I thought all burn rates were from "closed bomb" tests. They burn poorly/extremely slow in the open air, they need pressure.

 

[mcb]

Burn rate is not constant either. Many of the new rifle powders have retardant treatments to the outer layers of the powder grains. This slows the burn rate initially and thus slows the pressure rise a bit especially useful in big magnum cartridge that are burning a lot of powder.

Also the grain shape has a measurable effect. It does not change the actual burn rate but the change is exposed area does have an effect on the effective burn rate. If you take the exact same propellant formula and extrude it as sticks versus flake vs ball will have measurable effects on the apparent burn rate of the powder.