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That sucks having a lower so far out of spec even after it had been worked on..
I did file down the area between the front pin on my lower (mgi, not RR)
Im curious what was the actual issue? If it was years ago i would wonder if the upper was crap, but idk if you tried multiple units.
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When a previous owner bought it, it would only fit the original Oly upper. He carefully filed down the top deck of the receiver a slight bit, but there was still an issue with the takedown pins: The front one was drilled slightly too far to the rear. It didn't help that the crappy original takedown pins were slightly oversize, and squared off at the leading edge.
I partially solved that for him by taking a new set of Colt mil spec takedown pins and test-fitting them. The were better, but still not enough, so I removed and beveled the leading edges of the Colt pins, and reinstalled them. That worked fine for all his uppers and for my Colt uppers: Sometimes for some uppers you had to tap the pins into place, and out of place, with a little brass hammer, but it worked and was a very nice, tight lockup.
My early (IIRC, 2nd-gen) Shrike was just slightly too long between the TD pin holes. Put in the front pin, and it wouldn't close; put in the rear pin first and you could see how misaligned the front TD pin hole was.
I'm sure M60joe could have welded up/redrilled/installed bushings that would fix it permanently. But by then I had also had some trigger time with my Shrike on another host, and it just didn't do it for me. With short barrels, it did not balance the way my M4 uppers did; with long barrels, I still preferred the accuracy of my LMG. Plus I was able to get my money back through resale of the Shrike and of the Old/PAWS.
In defense of Olympic, they were building semi AR15s at a price point significantly below Colt. That meant looser quality control to keep costs down, and it didn't matter that the factory staff might have to try a couple of uppers when assembling them.
Remember that when the hosts were made in the early 1980's, there was no huge aftermarket of AR15 Barbie Doll accessories. Extra uppers were just not out there, so 95% of AR15 owners only had the upper that came with their rifle, and they were just fine with that.
And then Al Gore invented the internet, and the rest is history....