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Old July 12th, 2021, 09:30 AM #1
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Speaking of AR-15's

I pulled out of the back of the safe the Bushmaster Varminter that I've had forever. I recently purchased, from another MDShooter, a 7.62 x 39 upper and thought I would match them up.

For some reason I can't get both pins back in place with that upper. Which then raises questions for me about the safety of shooting the rifle if the fit doesn't seem just right.

Any ideas? Or suggestions for where I might find someone trustworthy to look at the set up and see what I might be doing wrong?
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Old July 12th, 2021, 09:34 AM #2
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Originally Posted by boisepaw View Post
I pulled out of the back of the safe the Bushmaster Varminter that I've had forever. I recently purchased, from another MDShooter, a 7.62 x 39 upper and thought I would match them up.

For some reason I can't get both pins back in place with that upper. Which then raises questions for me about the safety of shooting the rifle if the fit doesn't seem just right.

Any ideas? Or suggestions for where I might find someone trustworthy to look at the set up and see what I might be doing wrong?

What is the brand of the upper? Can you see any visual misalignment?
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Old July 12th, 2021, 10:04 AM #3
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Are either cerakoted? If so, the cerakote can mess with tolerances. Might need a light tap with a plastic hammer.

I have an upper thats off and the front pin hole had a lip on it. I took a light file to the lip and the pin goes in fine.

Small variances like that shouldn't cause issues with shooting. The main issue you'll have with a 7.62x39 are the magazines. Sometimes they don't feed right and the cartridge will hang up. Mine was reaI bad and I had to grind back the feedramps on my barrel extension so the bullet tip wouldn't catch.
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Old July 15th, 2021, 03:31 PM #4
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Quote:
Originally Posted by omegared24 View Post
Are either cerakoted? If so, the cerakote can mess with tolerances. Might need a light tap with a plastic hammer.

I have an upper thats off and the front pin hole had a lip on it. I took a light file to the lip and the pin goes in fine.

Small variances like that shouldn't cause issues with shooting. The main issue you'll have with a 7.62x39 are the magazines. Sometimes they don't feed right and the cartridge will hang up. Mine was reaI bad and I had to grind back the feedramps on my barrel extension so the bullet tip wouldn't catch.
Sorry I didn't respond earlier.

Neither are cerakoted. I had tried the light tap...becoming a less light tap after a couple of taps. No luck.

I can check the pin hole but putting it back together with the original upper doesn't seem to be a problem...
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Old July 15th, 2021, 03:28 PM #5
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What is the brand of the upper? Can you see any visual misalignment?
Sorry I didn't respond sooner.

The upper is a PSA...Palmetto State Armory. I am not seeing any misalignment visually. The upper seems to snug into place just fine and I can get one pin into place but not both.
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Old July 15th, 2021, 03:55 PM #6
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Sorry I didn't respond sooner.

The upper is a PSA...Palmetto State Armory. I am not seeing any misalignment visually. The upper seems to snug into place just fine and I can get one pin into place but not both.

That sucks but it’s a telltale sign that one or both parts are very slightly out of spec. It really doesn’t take much to cause interference.

Take a look at the lower technical spec drawing. Both pin holes are supposed to be drilled 0.250±.001” and they are supposed to be round within .001” and perpendicular to the axis within .002” then their position away from each other measured from center to center can be 6.375±.002” max. Normally there is a bit of statistical average that happens during machining. One hole may be +.001 the next one may be -.001 and they usually cancel each other out in a sort of way. However occasionally you may have an unusual combination of tolerances all at the extreme. Say both holes are drilled .249 and they are at the max limit .002 out of perpendicular to the axis and the overall distance between the holes is right at the minimum 6.373”. If the upper is also at the limit of those tolerances but in the opposite direction you may have .003” misalignment between the upper and lower pin holes. That’s less than the thickness of a sheet of copy paper but enough to cause some interference.




What can you do. Without gauges or QA fixtures it’s not going to be easy to find out which part may be out of spec. If you had a bunch of different uppers and lowers you could swap them around to see if there was any pattern that emerged. It may indicate which specific part was wrong by the mere fact that it didn’t fit onto several parts.

An easy fix would be to lightly hone down the diameter of the rear takedown pin. Maybe just chucking it into a drill using some 220 grit sandpaper you can remove .002” from the diameter and get them to work, without trying to ream out the hole on the upper or lower.
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Old July 12th, 2021, 10:06 AM #7
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Old style lower perhaps? Colt had those lowers early early on which had different pin spacing. I don't think Bushmaster did that but I'm not sure
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Old July 12th, 2021, 10:23 AM #8
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Quote:
Originally Posted by Bertfish View Post
Old style lower perhaps? Colt had those lowers early early on which had different pin spacing. I don't think Bushmaster did that but I'm not sure
Colt had different large pin diameters, not different pin spacing. Bushmaster only made standard pin lowers and uppers.

Loose, sloppy AR’s that fall together are less desirable from an accuracy standpoint. There is nothing wrong with the upper/lower needing a little pressure to go together. There are still tolerances for a “milspec” upper and lower. One being on the large end of tolerances, and the other being on the small end of tolerances will make for difficult assemble. Everything can be perfectly safe and within tolerances, and still be hard to go together.


To assemble:
Push the front takedown pin in. Swing the upper down onto the lower. Squeeze the upper and lower together, and push in the rear takedown pin. A light tap on the pin with a soft faced mallet may help, and is ok. If you have to beat on it, there’s a problem. Several of my upper/lower combos need a light tap to assemble, and a light tap to disassemble.
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Old July 12th, 2021, 10:38 AM #9
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Quote:
Originally Posted by GunBum View Post
Colt had different large pin diameters, not different pin spacing. Bushmaster only made standard pin lowers and uppers.

Loose, sloppy AR’s that fall together are less desirable from an accuracy standpoint. There is nothing wrong with the upper/lower needing a little pressure to go together. There are still tolerances for a “milspec” upper and lower. One being on the large end of tolerances, and the other being on the small end of tolerances will make for difficult assemble. Everything can be perfectly safe and within tolerances, and still be hard to go together.


To assemble:
Push the front takedown pin in. Swing the upper down onto the lower. Squeeze the upper and lower together, and push in the rear takedown pin. A light tap on the pin with a soft faced mallet may help, and is ok. If you have to beat on it, there’s a problem. Several of my upper/lower combos need a light tap to assemble, and a light tap to disassemble.
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Originally Posted by Bertfish View Post
Old style lower perhaps? Colt had those lowers early early on which had different pin spacing. I don't think Bushmaster did that but I'm not sure
The early AR15/M16s had a larger front pin hole on the upper(and lower) where a screw originally attached the front halves together. This should only result in a loose upper-to-lower fit so this should not be the problem.

Some upper rear pin holes aren't properly elongated and require a bit of fitting.
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Old July 12th, 2021, 10:40 AM #10
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The only thing that really matters safety wise is that the bolt locks on a correctly head spaced chamber.
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