I was talking to a guy online today that was selling a rifle, (DPMS lower and a Bushmaster upper). Says he cant get the rear pin to push in and lock. He sent a  closeup pic and it appears that the receivers lack about 3/32 from mating up. Is this normal with this receiver combo?

Somebodys' CNC is slight out of spec. AR/M16 parts should all readily interchange as long as std dimensions were followed.  

I dunno. My DPMS lower takes my Rock River and Colt uppers just fine; but then DPMS uses parts from different suppliers, so it is possible that one of those pieces is out of spec.

I just put together a Bushmaster VMAX flattop upper receiver and a Rock River Lower. They didn't initially fit together in that the rear lug of the upper didn't seat all the awy into the lower. Rock River explained that although mil spec, there are variances that occur and a 1/1000th of an inch may cause the problem. RRA suggested that I tap the upper gently with a rubber mallet, and after doing so carefully and working the connection with oil, I have a lowe to upper fit that is flawless.
Note that my Bushmaster A2 upper fit in the RRA lower without any difficulty. There rae slight variations in machining. We aint building rockets, ya know.
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is it a preban lower (under SN10300 [no letters or prefixes]for dpms i think)?  i have a BM upper and of all my dpms lowers (3 of them) only the preban (cast) one does not fit.  you can tap it in with a punch (lightly so you don't mar it up) or a rubber mallet (what i use).  i think it has something to do with the gear they got from Essential Arms.  i've heard this is a common problem on thier prebans.  you can also dremel the pin so it goes in easier.  i wouldn't screw with the upper much as it would be harder to replace.