For a few years we reloaders postulated that short barrels needed fast powders, slower powders would not burn fast enough and just blow out the muzzle. I believed it. We all did.
Today we have some tools that help us understand internal ballistics. QuickLoad is very interesting, it is not used to determine load data, but it is an excellent learning tool to gain an understanding of our cartridges and their components. Good use is to compare components, to evaluate them holding other variables constant. Let's run some data for SlowFuse's 10.5 inch barrel and compare fast, medium and slow powders.
Run 223 Rem (SAAMI) with ordinary 55fmj-bt at 2.260" OAL using 26.0 grains Varget in a 10.5 inch barrel. Max pressure 52,666 psi is achieved at 0.31 milliseconds. (Not 0.3 seconds, but 0.3 thousandths of a second). Pressure falls in half at 0.5 milliseconds. Then down to 20,000 psi at 0.6 milliseconds. The powder is burned. Pressure is dropping dramatically. At 0.66 milliseconds the bullet leaves the muzzle of a 10.5" barrel at calculated 2552 fps.
Varget is one of the slowest powders we normally use in 223 Rem. Just for kicks, run a slower powder, VV N140. 26.0 grains. Max pressure 54,063 psi at 0.32 milliseconds. Yep, a slower powder only burns 0.01 milliseconds slower. Once again pressure drops to 20,000 psi at 0.6 milliseconds and the bullet leaves the barrel at 0.65 milliseconds at calculated 2604 fps.
OK, let's try a faster powder. Benchmark. 24.5 grains. Pressure peaks out at 54,608 psi at... 0.30 milliseconds. Only 0.01 milliseconds faster. Just like the others, drops below 20,000psi at 0.6 milliseconds and exits the muzzle at 0.65 milliseconds at a calculated 2576 fps.
In inches, peak pressure is about half way down the 10.5" barrel. It still carries nearly 20,000 psi when bullet leaves the barrel, if the barrel is longer that residual pressure still pushes the bullet (20k psi is nothing to dismiss). That's why longer barrels get more velocity. They make use of residual pressure after the powder is completely burned in the first few inches.
So short barrels don't necessarily 'require' fast powders. And long barrels don't necessarily 'require' slow powders. Your barrel will tell you what it likes. Some like it fast, some like it slow. That's what handloading is all about. Each gun is its own platform in your hands, seek the best load for the best purpose in your gun using best components available.
[So why the big flame and muzzle flash from short barrels? They tell us it is hot carbon still red hot from deflagration, but that I cannot confirm. Industry tells us a longer barrel allows the carbon to cool off and we get less muzzle flash. That requires more investigation. If powder burns in the first few inches, it would be wise to investigate further when we get a big muzzle flash. It would not be the powder.]
Note: If we use a very slow powder (especially in a pistol but also in rifle) at very low charge, we get unburned kernels. Those kernels never ignited. They blew out the muzzle because they never got their chance to ignite due to the retarder that made them 'slow' powders. I've seen other members here on THR cover that subject before. It isn't related to powder still burning past the muzzle, but poor choice of burn rate and powder charge.
Now, I don't expect you to believe any of this. I expect you to rip me apart, and rip QuickLoad, and rip my story telling, and rip the horse I rode in on. I expect you to deny this post vehemously. I expect you to dismiss me as an idiot. I expect you to claim I made it all up. I'm OK with that, as long as you go out and endeavor to educate yourself. Dig up all the technical data you can find, not just internet forum jabber but real technical reports from the industry. Don't believe me. Go find the best industry technical reports you can find, because that's the science you really should believe.
