6.5 Grendel Barrel Length Comparison

[lazyengineer]

This is a great graphic, and really points out the remarkable aspect of the 6.5 Grendel in shorter length BBL's. In my own usage and assessments, I run 6.5 Grendel in AR15's. And with that, 20" is about as long as typical. And when looking at the velocity loss from a 20" down to a 14.5" BBL, it's remarkably less than you might think. Dropping only about 100 FPS. Which no doubt is why 12.5" is so popular, because that's probably right about where the inflection point is, where further cuts start getting expensive. And with that, Grendel impact energy out of a 14.5 (or 12") is remarkably good out to very practical distances. It's one of the cooler aspects of this round.

From a logic standpoint, this makes more sense the more I learn. The best way I can figure it: after about 3" of travel, most of the powder is burned, and only a little more additional gas is generated. That peak pressure and volume in the casing and little bit of barrel, is basically the "bottle" of high pressure gas that's pushing the bullet the rest of the way down the barrel. with just a little extra gas generation from additional burn.

In 6.5 Creedmoor, it's a big reservoir bottle, so can keep pushing down a longer barrel. In 6.5 Grendel, it's a shorter little casing, and so a smaller little bottle. Meaning additional barrel length doesn't buy you as much, as there's less gas in a bottle to keep pushing it. In the most extreme case, I recently discovered, of .22LR, where the gas bottle is a straight walled little .22 Casing of quite small volume, additional barrel buys you very little. I recently SBR'd an 18" .22LR BBL down to just 13", with ZERO velocity loss with 40 gr bullets (within stnd dev.), due to this. Anyway maybe everyone else recognizes that, but to me, it helps explain while shortening the barrel in a 6.5 Grendel isn't all that expensive; compared to .223, which is higher pressure and has a large bottle volume ratio compared to the bullet and barrel diameter.

Grendel is a very cool little round.