I was recently gifted a .300 Blackout AR. It has a 16" barrel w/carbine length gas system. I understand that this setup is less than ideal for this barrel length. Would there be any issues having the barrel cut to a shorter length and paired with a pistol-marked lower so that I could still use it with the carbine gas system while avoiding an extortion stamp? Is this something that a gunsmith could do?

Alternatively, I have a new Aero Precision complete lower that I need to pair with something. My other option would be to repurpose the upper and rail with a .223 barrel, new gas system, and BCG (assuming I can find one).

I'm just spitballing and trying to explore my options.

I feel like cost would be pretty high. 300blk performs just fine out of a 16" barrel. Unless you want something short, I would leave it alone. If you are wanting a pistol, buy a 10" barrel with a pistol gas length port and build it from scratch or reuse the upper.

It will run just fine, it's just a longer barrel than needed for 300blk. Agree with BCS, if you want to reduce barrel length to "ideal" (8.5" or so) just buy a new barrel (with corresponding gas system and handguard) and add pistol brace. Sell the barrel if you can and repurpose the rest for your new build on the other lower.

Nothing is wrong with a 16 inch barrel on 300blk. The only advantage of cutting it is if you are running a suppressor and the gun feels too long.

If you are shooting supers you are getting substantially more velocity with the long barrel

You may have a little dwell time issue with the 16"/carbine system, but that barrel length is fine. Nothing wrong with it. Maybe use a heavier buffer if recoil is an issue.

Most accurate AR I've ever seen was a buddy's Noveske 16" 300 BLK. That thing would cut the first hole for 10 rds at 100 yards.