I build match AR's.
For NRA Highpower Rifle Competition - which are the same rules used for military EIC matches, all State, Regional, and National Matches - only a full sized AR-15 with 20" heavy match barrel in Service Match conditon can stand up against the M1a (308) and M1 Garrand (30-06) in 200, 300 and 600 yard competition using open sights - no scope. I have had a dedicated AR for this type of competition since 1990.
However, a 20" service match is heavy, and, not much fun plinking. That, and the match barrel only stays in match condition for a limited number of thousands of rounds.
When I add an 8" suppressor to that Service Match AR - it is wwaaaayyyyy to long to do much with!
I have several 16" carbines. I have found even the lightweight carbine barrels are just as accurate as the heavy bolt varmint rifles when it comes to winter varmint shooting. Especially the faster twist barrels. That, and buying 1:8 and 1:7 barrels in carbine length are a heck of allot more affordable than longer lengths.
I have allot of AR's and a 223 suppressor. When I am off plinking with the guys, or, heading out to do some hunting - it's the carbines I take.
Although my 20" Heavy Barrel Olympic Service Match rifle has become a safe queen - it does get dusted off every spring and summer for competition.
Bottom line. Buy a nice 20" "Service Match" rifle today - then keep your eyes open for a good used carbine upper. A Service Match rifle will have a NRA legal trigger, nice crisp trigger, and will make the best lower you can mate with other uppers.
Hope this helps