SAVAGE SAYS DO NOT ATTEPT TO REPLACE BARREL AND UPPER RECEIVER PARTS DUE TO PROPRIETARY PARTS. Lowers were mi spec in the MSR15, but uppers we re proprietary. Barrel nuts ARE PROPRIETARY. I just got off the phone with them. Their suggestion was to buy another MSR 15 upper in 223 which they do just happen to have in stock. They said their AR10 was 100% proprietary! Today is 4/12/23.