I got another 6920 in a trade, the guy threw in a 10rd PMag and a 30rd Troy mag, both plastic (obviously). When I got home I tried to put a Brownells mag in, and it won't go, like, can't even force it in. I then tried a surplus USGI mag, it goes in, but it's hard. All my other mags are Lancers and PMAGs, so I don't have any others to try, but the Brownells is like new. Is this common?

No,  not common.  Older Bushmaster had tight magwells.  Mine had problems with Orolite mags.  I would try the mags in another AR.  I there is isn't a problem with another rifle I would contact Colt.