My current collection of ARs: 22LR, 5.56x45mm, 300 AAC Blackout, 30 Remington AR, 450 Bushmaster.
Tough firmly in my cheek. You could go to the opposite extreme and buy an old 30 Remington AR upper. Nearly everything in the upper is proprietary. The upper itself, the bolt, the barrel, barrel extension, barrel nut, handguard, and the magazine is down right funky.
30 Remington AR 4-rd Factory (only feeds from the left side), 450 Bushmaster 5-rd factory, NHMTG 5,56x45mm straight 20rd.
30 Remington AR (its a modified DPMS LR-308 bolt lug pattern with a body that allows use in a standard AR-15 bolt carrier, 450 Bushmaster bolt, Standard 5.56 bolt. (yes they are all dirty)
30 Remington AR brings close, but not quiet, 308 Win performance to the AR-15 platform when using medium to light 30 caliber bullets.
Killed several armadillos and a nice buck last year with it.
Any yes this is a completely unrealistic post for this thread, 30 Remington AR uppers and rifles were only made in low numbers for 2-3 years over a decade ago and ammo is getting hard to find. With the demise of Remington we will probably never see new factory guns, parts or ammo again. There is a small group that is trying to revive it but they are going slow.
-rambling