Grendel upper?

[pinzgauer]

I would be willing to bet that a .223 case necked up to 6.5mm would give about the same performance out of a 14" barrel as a 6.5 Grendel out of a 14" barrel.

I can tell you have not spent much time with rifle caliber handguns. This is something that you learn shooting IHMSA (metalic silhouette) early on.

You won't be able to match the velocity, the grendel has more case capacity. You may be able to have improved efficiency- similar velocity with much less powder & recoil.

With shorter barrels you shift to faster powders. You will not improve the velocity overall, but will burn more of it in the barrel while maintaining the velocity. And use less powder, which reduces recoil and muzzle flash.

Faster powder means less powder which means case capacity is less of an issue, etc. But then bullet weight also becomes a factor. Heavier bullet loads burn more of the powder in the barrel.

But all of this is an issue in the 10" range.... I can tell you side by side that for 16" barrels Grendels and 5.56 NATO have similar flash with my AA2520 loads and 120-123g bullets.