06-17-2023, 04:38 PM
When I bought a Grendel upper from a fellow here, I wanted to use the 5.56 lower I already had. Having the same questions you do, I discussed it with the local and knowledgeable gun shop folks who deal primarily in ARs. They suggested that the 5.56 lower/spring/buffer should work fine, so I tried it. It worked fine. Not exactly a technical answer, but just an experience. I used the Grendel like that till I had a new Grendel lower assembled.

