steve4102 said:
"All the powder that's going to burn, burns within the first few inches in front of the chamber. How much of it burns depends on the powder, pressure, etc. Rarely does it all burn, but 99+ percent can" John Barsness
MarkCO said:
"The quickness of a propellant is a relative term only, expressing its rate of burning compared with others. A quick propellant will burn more rapidly and produce a higher pressure in a given gun than a slow one." USAMC
I can see how these statements could lead to some confusion. Barseness is correct that that most powders never burn completely. The idea that they do is a persistent belief even among some in the industry, but you can prove to yourself a couple of ways. A simple one Mark Humphreyville pointed out is that you can ignite the sweepings from in front of the firing line at any indoor range. They burn and flare like smokeless powder does in open air because there's lot of unburned powder in that dust. Another is to compare a very quick powder to a very slow one in a highly overbore chambering like a 22-06. You quickly figure out that it takes perhaps three times as much slow powder as of fast pistol powder to achieve the same peak pressure, and that, based on energy content of the charges, for both powders to have burned completely by their respective pressure peaks, the slow powder would have made so much more gas that the peak would be happening more than half way to the muzzle and that the pressure then wouldn't drop enough at the muzzle to prevent it distorting the bullet. (Laramore pointed out long ago that bullets leaving the muzzle at near high power rifle peak pressure are upset by the gas and, I would add, that gas cuting of the base would also be apparent on their bases.)
Regarding relative burn rate, it is assumed by many to apply accross the board. It doesn't. Relative burn rate is a result for a standard test under standard conditions. Start varying pressures and temperatures and the burn rate order changes. Moreover, its costs money to do that testing, so most burn rate charts just approximate it based on known load data, which is why no two such charts seem to agree exactly. The powder companies know the burn rate specs they provide their manufacturing contractors, so each powder makers relative burn rates for their own powders should be correct. But changing order under pressure is a natural result of the differences in powder burn characteristics.
Twenty-some years ago John Milosovich did a good experiment to show this. He made up loads for .308 Win using IMR4895 and IMR4064 behind 80 grain bullets. Instead of loading to matching pressures, he adjusted loads to get matching velocities at 2200 fps, 2300 fps, 2400 fps, 2500 fps to see how much powder that required. To reach 2200 fps it took less 4064 than 4895, indicating the 4064 was the faster powder at the pressures involved. At 2300 fps the differnce was smaller, but 4064 still appeared to be the faster powder. At 2400 fps the charge weights were the same within the 0.1 grain resolution of typical powder scales. At 2500 fps it took more 4064 than 4895, indicating the 4064 was now the slower of the two powders.
Differences in grain length and perforation count and size were likely responsible for that behavior. The differences get bigger as the differences in powder type become greater, such as when going from flake to stick to spherical, single-base to double-base, and some change for other additives and additive quantities. The point is, saying which powder is actually faster than another depends on the conditions and they change as you work a load up.
As to short and long barrel velocities, the differences are easy to see in the QuickLOAD model, which is a pretty good one from what I am able to discern. Assuming you achieve the same peak pressure for all powders you try, the powders that are fastest in that pressure range with peak soonest. Since bullet acceleration is greatest at the pressure peak, bullets fired with fast power start out faster than those fired with slower powders, but the pressure drops off pretty fast as the bullet moves forward, so acceleration (the rate at which additional speed is addes) slows quickly, too. Because of the greater amount of total gas the slow powder produces, pressure and acceleration drop off more slowly after the peak. As a result, even though the fast powder's bullet is going faster at first, past its own peak, the slow powder's bullet starts to catch up. Typically, it will pass the velocity of the fast powder a few inches past the peak.
To make an extreme example, QuickLOAD's output for the bottom plot in the graph below, compares the super fast Hodgdon Clays with and and impossibly compressed load of IMR4350 in 308 Winchester with a 150 grain bullet in a 24" barrel. You'll note the traces plot bullet base travel in inches starting from its position in the loaded cartridge. In the bottom trace the Clays velocity (yellow trace) is higher right up until the bullet has moved about 2 inches, at which point the IMR 4350 velocity surpasses it. So, for any barrel longer than 2 inches, the slower 4350 load would produce more velocity, and you would have to cut the barrel down to below that length for the Clays to give you more velocity. You would then suffer Naramore's problem with your bullets.
But, that example is pretty extreme. The same bullet comparing IMR4198 and IMR4350 has the slower powder's velocity surpass that of the faster powder at about 8 inches, so, assuming the model prediction is accurate, you would have to cut a barrel down to eight inches or less to get faster 150 grain bullet velocities from 4198 in that model. Comparing 4895 to 4350, the powders are enough more similar that the numbers is swinging back the other way, with 4350 surpassing 4895 velocity at about 4 inches down the bore.
The computer model won't be dead on exact, but it shows the principles, and the actual barrel cutdown tests linked to earlier tend to verify it. Some people may not get the same result, but most don't cut barrels down to ensure the same chamber is involved with each shot and even if they did, it's likely most do not have the pressure testing equipment to adjust peak pressure to a actual match. Pressure signs don't work well for this as the appearance of them happens at different pressures depending on the speed of the firing event and how that influences the dynamics of the forces creating their appearance.