6.5 Grendel barrel Length?????

[lazyengineer]

Pistols and revolvers especially are not the best firearms designs to compare or analyze in this respect, since revolvers have the barrel cylinder gap, and that is mentioned specifically in the American Chemical Society document you linked. Pistol and revolver cartridges typically use the fastest-burning flake powders due to bore volume and propellant volume in the smaller cases, pushing bullets with the same diameter as the case (straight-walled). With a straight-walled case, powder conversion is not as efficient as it is with a bottle-necked case, where the combustion reflects off of the shoulder back into the propellant column.

Revolvers are notorious in a forensic analysis because of the barrel cylinder gap. For example, there was a very high profile "suicide" of a certain attorney 26 years ago. He had significant propellant burns and GSR on both webs of his hands between index and thumbs, attributed to a revolver that was later left at the scene.

With a rifle, as I stated, you still will have some particulate residue that went through the combustion process back in the case and chamber, but the maximum conversion opportunity has passed back in the chamber, whether it was fired from a 10.5" Grendel, or a 24" Grendel.

The claim that powder is still burning with a function of producing velocity increases between a 12" and 24" rifle barrel is a misnomer or misunderstood aspect of internal ballistics that is often repeated, but doesn't hold up well to the science. The contained wave of expanding gas that was released back in the chamber area is what continues to propel the bullet.

I had this discussion with Bill A. many years ago, and he explained that conversion happens much closer to the chamber area than most would think in a rifle.

As to QuickLoad, yes, QuickLoad can model a theoretical calculation of what powders are optimum for your cartridge and the bullet, vs ones that would have poor conversions and blown propellant. For example, if I used RL17 or RL22 with a 90gr TNT in the Grendel, I'm going to get unburnt powder blowing out of the muzzle, no matter what barrel length I shoot, because the powder wasn't converted back in the chamber area.

We deal with the same phenomenon in explosives where explosive volume needs certain burn rates or multiple points of initiation in order to get a complete detonation of the charge. A lower explosive compound that has a slower detonation rate can blow away much of its explosive mass without detonating that mass, because the mass was too large for the burn rate.

Thanks for the reply. I've been casually researching this some tonight, and there is indeed an interesting phenomenae, where once the pressure falls, the burn rate falls precipitously as well. Which makes sense, since high density (i.e. high pressure) and hot (i.e. high pressure; PV=nRT) gas will transfer heat more agressively into the powder to cause it to burn faster. And vice versa, and if the pressure gets low enough, it essentially quenches the burn. Fascinating science.

Here, I'm just tagging these here for future read, which perhaps dive into the topic more:

https://apps.dtic.mil/dtic/tr/fulltext/u2/462060.pdf

https://apps.dtic.mil/dtic/tr/fulltext/u2/676246.pdf