Originally, I was inspired by pics of the Israeli CAR 15’s that had been partially up-dated with M4 bbls and the USAF GAU M4s. I carried a Colt Commando back in the mid to late 80s and feel comfortable with the old 2 position buttstock and A1 sights.
Here’s me and the Colt Commando,
My KISS Carbine build, Colt A1 upper, DTI Chrome Lined 14.5" bbl with Phantom 5C2 [perm attached], DPMS Lower with Colt Bolt, Carrier and SP1 CAR buttstock.
I like it. Makes me want to take my RO635 9mm upper apart and put a new M4 or similar barrel onto the A1 upper. The A1 is deflectorless and assist-less. It would also look right at home with an 11.5" or so barrel and nice short old fashioned FH.
The 9mm barrel could easily be installed on a new flattop and there are tons of possibilities for it at that point.
I’m just picturing this one upper being able to spawn two complete uppers that each have a unique feel to them…