[FrankenMauser]

I have a 7.5" 5.56 barrel. Since I am perfectly content with the velocities that I see (which are right in line with the link provided above), I'd also be more than happy about the velocities achieved in the longer barrels. As a reloader, I can also carefully select what bullet I am going to use, and can make sure the selected bullet will be appropriate for the calculated impact velocity.

I am also more of a fan of barrels at 9.5" to 10.5" than 12.5" (not that there's much difference), so I'd be quite happy at 10.5".

But, of course, some one shooting factory ammo might need to be a bit more picky.


Concussion won't be much different between 10.5" and 12.5".
I really don't think it's bad at all out of even a 7.5" barrel (nor does anyone that has shot mine or stood near someone firing it). Yes, it's worse than a 20" barrel, but still light-years from the blast that bystanders get from a .308 with a good break on it (let alone something like a .300 Wby).

Ah, heck... The blast and noise from my 10.5" 300 Blk are worse than the blast and noise from my 7.5" 5.56. Why? I don't know. Part of it may be the YHM 5C2 flash hider on the 5.56 redirecting a bit of it, while the .300 Blk has no muzzle device (unless suppressed). Or it may have something to do with the rate at which pressure builds behind the different projectiles (mass and bearing surface come to mind here). I really don't know, because predicted muzzle pressure for my go-to .223 load is five times that of my sub-sonic .300 Blk load, at those barrel lengths.
It's all subjective.

But I'm fairly confident in saying that you would have to spend a lot of time comparing side-by-side, before you'd be able to notice the difference in blast and noise between 10.5" and 12.5".