This is my 1st AR15, so maybe Im missing somthing. I decided to build it, to save some$$$. I bought a "delaware machinery" stripped lower, from a company called JRS supply, and a DPMS upper assembly. Today I finally got the lower parts in, and I installed the front pin, but the 2 receiver halves, do not go together tight, and the rear pin will not go in these 2 parts clearly do not fit. I was under the impression that all AR15 parts were interchangeable???

Help me