The first upper I made used a 16" YHM fluted barrel and upper receiver, both with m4 ramps, with a PWS FSC30 and a MGI bolt. It shoots awesome with everything it is fed. Most of the time it's just shooting 10 round ASC magazines. The 10 rounders are unmodified. I last replaced the RRA Operator A2 stock with a Magpul ACS-L stock.
You can read about it more
The second one I did was a bit simpler, for a friend. I used a barrel/bolt combo I found on GunBroker, and put them in an old Bushmaster receiver... so far, so good. It was just a simple M4 style carbine build.
My current build is going CRAZY... You can see that going and