300 Blackout Accuracy Problem

[Beckett]

I recently aquired a 300 Blackout upper from CMMG (16 inch barrel, 1:8 twist) and ordered a bunch of the Remington 220 grain subsonic Ammo. Took it to the range, sighted it in and am getting 1 inch groups at 50 yards and 4 - 5 inch groups at 100 yards. I have put about 40 rounds down the barrel.

Im trying to figure out if this is normal or not. Should I try other ammo, be patient till the barrel is broken in better or what? Anybody have similar problems or any suggestions?