So I got a 45ACP AR lower sitting at the FFL and trying to figure out what would be a good barrel length for this thing when I start slapping it together. Would a 16" be over kill ?

16 would be fine, didn’t know it was legal to go shorter

If your doing. It as a AR pistol you can go as low as you want . BCA has them down to 5"

You’re not going to get that much more out of the .45ACP with a 230gr, keep it agile with the shortest barrel you can go.

I’d go with the barrel it was created for. 5”

http://ballisticsbytheinch.com/45auto.html Quick glance looks like you gain a little up to 10 or 11 inch , then start losing.

Can you purchase a round intended for a 16” barrel…

I bought a Ruger PC carbine, 9mm, and it performs nicely with standard Blazers …

Go short and if you ever want to suppress it it won't be ridiculously long.

No clue honestly not even really a .45 acp fan... Well not against it by any means just never really even thought about it since I'm no much of a pistol guy at all

Mabye I'm reading it wrong but seems to me everything it topping out around the 16" mark

Wow that's a terrible screen shot

Hadn't looked it up but I would've guessed about 12 inches max before it ran out of steam. Good to know my grey stuff still works, most of the time.

I'd try looking up what the Thompson did ballistically. I believe they were close to that in barrel length.

16”unsuppressed just because you said carbine. 9” with a can would be awesome. That’d be the same sized package as a lot of 300blk pistols. Just shooting subs.

You could shoot subs out of a 300 as well. Hornady makes some 190gs that they claim expand or just shoot 220s and roll the dice on it they destabilize or just pen hole.

45acp tops out for velocity around the 12 inch mark , fast burning powder and not that much of it

the original Thompsons were all 12 inch barrels

dude you have Maritime creds , no way you don't pass NFA background check , form 1 it

also all 45acp is almost always Sub Sonic from any barrel.

a 9" with a can and SBR it it is a whole new world of experimentation for you. no ear protection needed to shoot , your wife won't even know you are shooting from inside the house.

Pete,
The original barrels were 10.5", with comps were 12". The work-around to get the minimum 16" for rifle had at least three manufactures making 13.5" and 14" threaded barrels with longer comps. BATF still considered threaded and pinned as "detachable", so they welded the comps on.

I never had one.
I knew a guy who had one he said 12 inches but I would guess he was measuring to the comp. he had to let it set in NFA jail a while waiting for his tax stamp to be processed.

the weld necessary on a pin & weld Job is very minimal to make it legal but they do require it

I owned an auto ordinance Thompson back about 40 years ago. It had the finned barrel and front pistol grip. I'm pretty sure the barrel was 16" or 18". The ww2 cheaper military models were shorter.
It had a 50 round drum and weighed about 10# loaded.