It's not the mag compatibility it's the bolt compatibility. But yes on both as the bolt needs to work with the type of mag being used. If the upper is like an ar15 then there are uppers you can get to work. Some offer colt compatibilities. I think Brownells offers them, but unsure? There are custom barrels that can be made to work with colts as the feed ramps to mag to bolt ALL is required. Glock and other brand won't work with colts on feed ramps. SA really doesn't know and just give a marketing answer., unless the upper isn't an AR-15 compatibility.