I like Spikes Tactical
Kingfisher,
I don't believe you can use a 5.56 upper on a dedicated .22 lower. I may be wrong. I have a .223 RRA upper on a Stag Arms Lower. When I wanted to go the .22 LR route for the cheaper shooting. I purchased another Stag Arms lower, and a Spikes Tactical dedicated .22 upper. This combination runs just fine, and at any point I can take off the .22 upper and swap in a .223 or whatever suitable caliber upper. I hope I made some sense. Good luck with your decision.
regards,
Luis Leon