I am thinking of building up a RRA 9mm. Would I be able to do this with a current Colt lower. I ask because I am told the the FCG pin size is larger than normal, hence the RRA 9mm trigger wouldn't fit with those pins? I could also wait until the AWB ends, and get a Colt 6450 (I think?), but that doesn't seem to have a fully adjustable rear sight, or the option of a flattop. Thanks!