Has anyone gone the route of starting with a Carbine Length upper while waiting for approval and then having the barrel cut down to size rather than starting with a pistol and adding the stock later?

It seems like a carbine would be more fun to use in the mean time. I would imagine that the only downside would be the low risk of it not being crowned correctly when cut.

and what would be the shortest barrel length reasonable in going the route of trimming the barrel of a carbine length gas system in a 9mm?

There is no gas system in a 9mm AR so I guess you could go with just about any length you wanted.  I've seen 9mm ARs with 5" barrels.  10.5 seems to be common.

aaah never even thought about that

<< Just starting to consider the 9mm option

I sorta did what you're thinking, and i used a 16" 7.62x39 barrel.  I built my pistol lower first, then had the barrel taken to 10.5 and I was glad I did that with a pistol because I function tested it and figured the gas port needed to grow a little.  The factory carbine gas port was 2mm, about .079 and it became .094 after researching a great deal.  Basically a half-millimeter larger to get it to run right.  It's been super-reliable since then, and I still have my pistol.  I'm on day 78 of my SBR stamp process so hopefully not much longer.   But in this way, I have a pistol lower to use anytime I want and I'll be getting my SBR stamp on a different lower.  I'll be transfering the pistol parts onto a different lower to function test it at a later date too before I SBR that lower.  Plans....$$$$.....

I started with a 16" 9mm AR and am waiting on the Form 1 to come back. Once it's approved, I'm putting a 5" barrel on it. It's really not a big deal to start with a rifle. It's even easier with a 9mm, because you more or less know that it will function with a shorter barrel, since there is no gas system to need tweaking. The benefit of starting with a pistol on a 5.56 SBR is that you can make sure it functions properly with your chosen barrel/buffer/gas system.

Barrel length really doesn't matter in the 9mm AR's. You can just pick whatever you prefer. I chose a 5" because that will keep most 147 grain ammo subsonic for use with a suppressor. The longer you go, the more limited your subsonic ammo choices will be, but you can still find stuff that's subsonic in a 16" barrel.

excellent info  thanks.
would like to see some accuracy range reports when you get it finished