AR un-matched receiver build....

S

Skans

Active member
Well, I thought I could save money buying a cheap upper and cheap barreled upper and slapping them together. Probably not a big deal, but I'm running into two issues: 1) the upper and lower don't fit - going to have to do a bit of filing; and 2) one of my takedown pins doesn't fit one (not both) of the lower receiver holes (rear takedown).

OK, so I think I can file down a little of the upper receiver tab to get the two receivers to mate up. What should I do about the takedown pin hole that seems a bit small on the left side of the lower receiver? I've thought about putting the pin in my drill press and use 1000 grit paper to polish it down a bit.
 
G

G.barnes

New member
Sounds like one or both receivers are out of spec. Had a bushmaster like that once. Had to use a hammer and a plastic punch to install and remove the rear pin.
 
G

Gunslick

New member
My advice; buy Anderson Lower receiver and assemble with quality trigger such as ALG ACT, buy BCM upper, go hammer forged barreld upper if you run your weapon hard but the standard mil-11595 E barreled uppers are just fine-its good enough for the Military but for a little extra money you get a more accurate barrel, a little thicker chrome and a more rigid barrel. Look into hammer forging. This is the route I go and I run my ar very hard. Over 10,000 rounds and the upper is still like new with the exception of some throat erosion. Remember, always keep the BCG wet. Use Slip 2000 EWL-it is the best lube out there, research it. Only parts I grease on the ar are the cam pin and slot and a tiny dab on the rear of the bolt lugs.
 
M

Mobuck

Moderator
Over the years, I've seen combinations that simply were not compatible. Tolerances, machining slop, out of spec--all add up to a "no go". I've seen this with "name brand" as often as with "econo grade" parts.
Before you go into sanding, filing, or reaming, I'd suggest just getting a different upper(same brand as the lower). Once you "modify" the upper, it's value is zip for anyone else.
I have (or had, it may have been sold on a lower that it fit)an upper that the previous owner had "hand fitted to his lower" by running a drill bit through the TD pin hole making it about 1/16" bigger diameter. Upon mounting to a known to be "in-spec" lower, I immediately realized that the fit would allow the BCG to hit the buffer tube during cycling which is bad ju-ju. At some point during the previous AWB mania, I got hold of some questionable spec'd lowers and one of those was off enough to fit the "reamed" upper.
 
S

Skans

Active member
Once you "modify" the upper, it's value is zip for anyone else.
Click to expand...

The upper only cost me $199 - included forged upper, barrel, keymod freefloat handguard, forward assist and dust cover. I'm sure the upper receiver isn't worth more than $60 anyway, so I have no problem filing the takedown pin tab to make it fit - if that's all it is, which I haven't completely confirmed yet.

I'm pretty good at fitting parts - I take my time and hand file. The takedown pin hole on the left side concerns me a bit more. I can modify the pin, but it really should fit. I'm not sure how you drill a hole too small. It could be the coating that is throwing it out of spec a little. I'm going to take some measurements this weekend, if my calipers are precise enough.
 
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