No adapter that I know off. I had a PSA 10.5" 5.56 that I hated. Dropped in a .22LR conversion kit, removed the gas block/tube and reinstalled a gas block backwards (to block the gas port). It is now the gun I shoot the most. At some point will get a true .22LR upper, but for now, I shoot that since 300blk ammo isn't super cheap.