I'll agree, Topgun. I've read this post several times over the last few days, but I have no idea what is happening.
When you say you "want to check expansion of the round," I think about BULLET expansion, meaning shooting bullets into gelatin or at least into water. I suppose if a guy mic'd the base of the recovered slug, it could be a bit smaller diameter than the raw bullet before reloading - but that's not what was said on the page.
Alternatively, since you've said "magazine to chamber," my mind assumes that you're talking about bullet set back, meaning you need to check your feeding, as an AR shouldn't set back your bullets if properly built.
Little more detail on what is happening would be helpful!
