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Posted: 9/12/2008 9:56:55 AM EDT
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I have an Anvil Arms lower receiver I want to mate to a RRA upper. However, with the receivers fully contacting each other, the rear take down pin does not want to go all the way through. It goes through the upper just fine, but then hits the lower on the opposite side. It appears this is because the hole in the RRA upper is slightly "high" on the bottom causing the pin to hit high on the opposite side of the lower. The front OR rear pin will go in by themselves, but not both. What's the best solution for this? I'm assuming removing a small amount of material from the bottom of the hole on the upper is the way to get the fit. What's the best way to do it? Thanks. Regards, |
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The two should go together with no modification to either receiver. If at all possible do you have access to other AR receivers (yours, friends, dealer) that you can fit in an effort to see which receiver is out of spec? Then see if you can return that part to whoever you bought it from/manufacturor for an in-spec part. That said, I've used a 1/4" drill bit to chase the holes and clean them out. You can also wobble the takedown hole in the upper receiver a bit to get them to fit. But really you should see about getting a replacement for the bad receiver. That would be best. |
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Good idea. I hadn't gotten that far yet. I tried the RAA upper on 2 separate Bushmaster lowers and an older DPMS lower. It fit fine on each of the Bushys and was very tight on the DPMS, but doable with some persuasion from a small plastic mallet. So, the Anvil Arms lower is probably slightly out of spec then? |
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OK, to muddy the water even more I tried two Bushy uppers and a DPMS upper on that Anvil Arms lower and they fit fine. So to sum up: 2 Bushy lowers and 1 DPMS lower fit the RAA upper and 2 Bushy uppers and one DPMS upper fit the Anvil Arms lower. However, the combination of the Anvil Arms lower and the RRA upper that doesn't work. Where does that leave me? Did I just happen to get an upper and a lower at opposite ends of the tolerance spectrum? |
Then I would almost bet the farm the the lug on the UPPER is hitting the top of the shelf on the LOWER!!! Why> Because the Bushie and DPMS lowers have a lower surface under the hole than the Anvil. Don't screw with the monting holes until you samear the bottom of the upper lug w/some lipstick, and make sure the lug isn't hitting the top of the lower. 
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I've been told we only have "good" lipstick around the house, so a trip to Wal-Mart will be necessary for that test. I did however, crudely gauge the depth of the receiver back there by marking a dowel rod in the AA receiver. If anything, the AA looks a hair deeper than either the Bushies or DPMS that I have. Hmm. |
Are both receivers brand new? If so, sometimes you have to put a lot of pressure at the rear of the upper forcing it to seat properly to allow the two halves to mate. Someomes the pins are the culprit being out of spec & the receivers are fine. If you've already considered this then disregard this post. I just had to pitch two new but out of spec Stag pivot pins that caused a similar problems & order replacements from PKF. Trying different pins on the same receivers might narrow down the issue. Also, many times new parts will not mate properly until they have been used quite a bit. |
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OK, I bit the bullet here tonight and spent some time on this that I didn't have. I swapped out the front pivot pin and, lo and behold, all of a sudden things fit. VERY tightly, but it did go together. So I was thinking, ok, maybe these pins are out of spec. and I just need a new set of pins. However, when I put the original front pivot pin back in, it fit again! TIGHTLY, but it does go together. Maybe all it took, was running the pins in and out a few times to loosen things up. All is well that ends well. Thanks for all the tips and advice. Now, if it will just stop raining so I can try it out! Regards, |
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Sometimes that just happens too. I had forgotten I had a couple of Stag lower parts kits that had bad pins in them. They had a raised burr where Stag had cut the detent slots. I had to take a file to knock them down so they would fit.  Sorry I had completely forgotten about it. 
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