I wanted an AR, but I'm not too enamored of .223/5.56, so I decided on 300 Blackout for several reasons:
1-everything on the rifle is the same as a .223 except for the barrel, so a generic build is easy using milspec parts
2- the 300BLK is a versatile round with great ballistic performance with a very wide variety of bullet weights, and lends itself exceptionally well to suppressed, subsonic loads.
3- I primarily reload .30 caliber, so it simplified what I need to keep on hand.
I chose parts based on 1- Milspec, for compatibility between brands and makers, 2- quality, I wanted upper-shelf parts and was willing to pay the extra for them, 3- availability, but without having to compromise on the quality significantly. I decided on a carbine length, to avoid the $200 fee for SBR, but still have the option later of paying the fee for a suppressor, even though the resulting rifle would not be quite as compact as a suppressed SBR. The carbine also made sense as 85% of my use would be supersonic.
My parts list is as follows:
Mega A3 upper and lower receiver,
CMMG parts except for trigger group
Geissele DMR trigger group
AIM NiB BCG and bolt
Spike's Tactical VLTOR charging handle
Rainier Match 16" barrel, 1 in 8 twist
AAC flash suppressor, for compatibility with future AAC suppressor
Wilson Combat adjustable gas block and gas tube
Geissele Super Modular 13" rail MkII Mod 1 and barrel nut
Magpul ACS buttstock and buffer tube
Magpul BAD lever (battery assist device)
Hogue pistol grip
Redfield Counterstrike red/green dot with laser sight.
I've since added a 200 lumen light.
It shoots pretty well, first is sandbagged at 100 yards and taking my time, second is standing freehand, rapid fire (30 rounds in 2 minutes) at 100 yards
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