[Joe Brower]

Rifle cartridges like the .223 Rem need that 16" of barrel length to attain proper velocity. ARs (and other designs) chambered for pistol cartridges like 9mm, or subsonic rounds like 300 BLK running 200+ grain bullets -- all that extra barrel doesn't do anything.

In fact, from what I understand, 300 BLK was designed to attain even max supersonic velocities in 9" of barrel. At which point, getting the shorter barrel and then putting a can on it makes good sense.

[bigfootbob]

Before the boating accident, my 300 blackout could hit a steel gong at 200 yards with every shot. It had a 7.5” stainless steel barrel with a 1.8 twist. I really miss that gun..

[Myrddin]

My Form 1 SBR for 300BLK was built with a 10.5 inch barrel for that exact reason. The Form 4 9mm has an 8.5 inch barrel. I intentionally avoided pistol braces in anticipation of waffling by ATF. It may be cheaper to convert pistol barrels to 16.1 inch and change buffer tubes to use standard collapsible stocks.