I will say the Cross locks up nice and tight to my MEGA billet lower. Zero wiggle. Fit and finish is very nice.
I like Cross a lot, as well as SanTan.............w/o doubt my 2 favorites.
My experience with the CMT fit is the same as yours, SanTan, slightly tighter. Neither close to BCM.
I've used a lot of BCM, probably more than anything else, mainly because of cost, but the fit & finish of the CMT & SanTan is just much nice.
I really wish Vltor would tighten up the barrel fit on their MUR uppers as I like everything about them but the barrel fit.
MM