bhn and burn rate range

[BJung]

Given the burn rate of various powders within a load table for a certain cast bullet load, have you noticed the speed of the burn rate change depending on the bhn?

[mehavey]

^^^^ ^^^^
^^ ^^

(That said....)

Given that "burn rates" as usually listed in the various rainbow charts are only the start-point rates...
... and those rates change with changing pressure rise...
it's the pressure-rise profile as the bullet goes down the barrel that usually changes...
... and softer bullets have generally lower friction ...
... therefore slower pressure rise than their harder cousins...
... but you'd have a hard time noticing it as a practical matter...

But softer seals the bore more readily...
leading to faster immediate pressure rise...
and usually faster burn rate increase...
balanced against lower friction...
and slower pressure rise...

So I eventually haven't got a clue.




Have I mentioned practical matter?

[MUSTANG]

Given the burn rate of various powders within a load table for a certain cast bullet load, have you noticed the speed of the burn rate change depending on the bhn?

Not in my experience. Have never measured to test velocity changes, and of course - have never seen any pressure changes between softer and harder lead boolits in my loads. Accuracy is obviously a different story as too soft a boolit does not fare well at increased velocity.

[Bigslug]

Seems like a metaphysical question for handloaders: How many angels can stand on the anvil of a primer while watching powder burn?

I'm guessing the way to test this would be hook a rifle up to one of the gizmos used to measure chamber pressure to see what difference you get between bullets made of diluted wheel-weight and linotype. But you're still going to have variables to account for:

*The softer bullet is going to weight a few grains more - slightly more resistance to getting started.

*The softer bullet will compress into the bore easier, but may be obturating more and sealing tighter.

*The harder bullet will be harder to swage to shape, but may have less ongoing resistance once it does.

Having ZERO knowledge of the pressure testing gizmos, I don't know if they only measure peak pressure, or if you can measure the buildup and decrease of pressure from ignition to bullet exit at the muzzle. THAT would be some truly ****** science to mix in with barrel harmonics, chronograph data, and ultimate accuracy.

They have a pretty good understanding of pressure at the gas ports of autoloading rifles so the ability to do this clearly exists. It's too deep-geek for my math-crippled brain.

[racepres]

There is a Difference between Conventional lubed Cast, and Jacketed...so, Lets throw PC in there Too!!!!
I ain't...but maybe someone will...Like driving a Stuck Bullet Out!!!! Great Fun!!!!

[Kennibear]

Along time ago the NRA lab did a test of oversize jacketed bullets effect on chamber pressure. The test barrel had an enlarged chamber neck to handle the cartridges with large bullets. Same bullet just different diameter. To their surprise there was not much difference. Reason: two inches down the barrel the bullet had been swaged down to bore diameter while the chamber pressure was relatively low as the rifle powder peaked the pressure curve much further down the barrel. I suspect that the lube on the bullet is much more of factor than the alloy on pressure and powder burning. Initial resistance at ignition does affect the burn rate as the pressure curve rises higher before the bullet moves so the firing cycle is further along the pressure curve. This is seen way more in pistols than rifles and is why some powders (think H110, 296 burn rate) benefit from a strong bullet crimp.
I have not seen bullet alloy or hardness affect the shot in any meaningful way and I have been chronographing all my loads since the first Chrony - the one before they had the Alpha, Beta, etc. The lube is way more important than the alloy IMHO.

KB

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[racepres]

Along time ago the NRA lab did a test of oversize jacketed bullets effect on chamber pressure. The test barrel had an enlarged chamber neck to handle the cartridges with large bullets. Same bullet just different diameter. To their surprise there was not much difference. Reason: two inches down the barrel the bullet had been swaged down to bore diameter while the chamber pressure was relatively low as the rifle powder peaked the pressure curve much further down the barrel. I suspect that the lube on the bullet is much more of factor than the alloy on pressure and powder burning. Initial resistance at ignition does affect the burn rate as the pressure curve rises higher before the bullet moves so the firing cycle is further along the pressure curve. This is seen way more in pistols than rifles and is why some powders (think H110, 296 burn rate) benefit from a strong bullet crimp.
I have not seen bullet alloy or hardness affect the shot in any meaningful way and I have been chronographing all my loads since the first Chrony - the one before they had the Alpha, Beta, etc. The lube is way more important than the alloy IMHO.

KB

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Cheating....Jam it in there with yer Mauser Bolt...and let her Fly...

[Larry Gibson]

Along time ago the NRA lab did a test of oversize jacketed bullets effect on chamber pressure. The test barrel had an enlarged chamber neck to handle the cartridges with large bullets. Same bullet just different diameter. To their surprise there was not much difference. Reason: two inches down the barrel the bullet had been swaged down to bore diameter while the chamber pressure was relatively low as the rifle powder peaked the pressure curve much further down the barrel..........

KB

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Such generalizations should not be taken as 100% factual. I would really like to see the parameters of that test as it does not agree with my pressure testing of such loads. As an example, I have substituted bullets of the same diameter, same construction (cup and core) and the same weight in M80 LC 7.62 arsenal loads (often referred to as "mexican match") and found the measured psi to vary as much as 11,000 psi. The bullets with the longer bearing surface giving the higher psi's.

Another test was with a load of 43 gr IMR 4895 under 150 gr Sierra and Speer SPs. A ten shot test of that load with five of each bullets mixed still gave very uniform internal and external ballistics. The average velocity for the 10 shots was 2717 fps with an ES of 35 fps and an SD of 12 fps.
The average psi was 50,600 psi with an ES of 1,900 psi and an SD of 700 psi. When a Hornady .312 diameter 150 gr SP was substituted the velocity increased to 2805 fps and the psi increased to 62,100 psi.....an increase of 11,500 psi.

However, when an Albanian 147 gr FMJBT having a soft steel jacket and a steel core was substituted the psi was 73,500 psi for the first shot and 72,500 for the second shot. I only shot two shots for that test , the reason being obvious.

As to "oversize" cast bullets I cast some group buy C312-291 #2 alloyed bullets, GC with Hornady GCs, lubed with Javelina and sized them at .308, .309, .310, .311, .312, .314 and "as cast" at .316. I pressure tested them in the 308W with a 28 gr load of IMR 4895 and a dacron filler. The bullets are sized down to throat then bore/groove diameters within one bearing length of travel through the throat and into the bore. That occurs, as mentioned, very early in the rise to peak pressure curve and with all the bullets from .308 up through .312 there was no discernable increase in the measured pressure. However. at .314 and at .316 diameters there was a slight increase in pressure noted on the pressure curve and in the "rise" data. My assumption was that increase was caused by the bullet diameter being larger than the throat entrance.

[JonB_in_Glencoe]

Given the burn rate of various powders within a load table for a certain cast bullet load, have you noticed the speed of the burn rate change depending on the bhn?

nope.
.
speed of boolit change...yep.

[Kennibear]

Larry Gibson:
This is from about 40 years ago, same caliber as you though, and the diameter was .307" to, as I recall, .312". The bullets were of a commercial source and bumped up and down in dies so that bullet construction stayed the same. They may have stopped at 0.311" as that may have been the original diameter. 40 year old memory from a 69 year old mind. The bearing length was measured but was not greatly different big to small.
They also did a test in 30-06 with a bunch of 150gr bullets including Mil FMJBT ball and it was a confirmation of your tests. Bearing length and bullet construction were measurable factors with the short, fat roundnose bullets that had the longest bearing length being the highest and the Milspec FMJBT with the lowest engraved length being the least pressure. This has always bothered me to see the loading manuals list all of their same weight/caliber bullets on the same loading table. Surely the thin jacketed sleek profiled target bullets are much lower pressure than the 3%Sb lead core thick jacket roundnose bigandtough game bullets in the same table?
The answer I gave is for the same bullets dropped from the same mold sized down a few thousandths as opposed to being shot as cast. My own tests were in 357 Magnum with the Lyman 358156 & 358429. No change in the chronograph readings from 0.357" to as cast 0.359". The 358156 did shed it's gascheck at 0.359" though.
The monometal lead free bullets required in some areas are grooved to lower the engraving friction and resistance so they have a more favorable pressure/velocity ratio compared to cup and core bullets. In 357Mag I found that the 150gr 358477 can be driven faster with more powder than any cup and core commercial bullet I have loaded. If there is an increase in pressure switching from a jacketed bullet to a cast bullet (+P cast) it is probably related to reduced powder space in a pistol caliber. Pistol caliber pressure climbs very fast if you seat the bullet deeper. Not so in rifle calibers as a general rule. In pistol calibers the listed C.O.A.L. is God's law not to be trespassed by us unworthy mortals.
At least I think so.....

KB


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