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Old March 22, 2010, 12:44   #1
johnmac
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Mixed Manufacturer upper/lower fit?

I had an add for a DPMS upper in the marketplace. Below is why I particularly want a DPMS. Well, I have now found the DPMS upper I was looking for.

I originally bought a BCM upper to go on my DPMS lower and there was a lot of wiggle between the U/L. I could shim out the play but don't like that solution.

So, I then bought a BCM lower. When I matched the BCM upper with a BCM lower. The fit was great without any modifications needed.

I'm fairly new to ARs and was surprised to find what I'm guessing is manufacturers tolerance variance in upper/ lower fit.

I'd like to get a good fit on my lower without having to go with a buffer. I'm hoping to find the same fit experience with DPMS/DPMS as I did with BCM/BCM. But who knows? Now that I have found an upper, I'll Let you know how it fits.

What is your experience with differing or the same mans. quality of fit with Uppers/lowers? Any such thing as a chart as to what U/L fits best with the other?

Last edited by johnmac; March 22, 2010 at 14:47.
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Old March 23, 2010, 00:17   #2
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I had a Colt A1 upper mated to an EA lower with no issues, I then bought a no-name A2 upper (for the cartridge deflector) off GB and after assembling it the holes were a good .025 off I GB'd that one then bought an upper from EA, assembled it and it was a nice precise fit to the lower.

So yes, tolerances DO differ as you (and I ) both found out.
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Old March 23, 2010, 00:48   #3
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d

Colt and bushmaster seem to be good matches.
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Old March 23, 2010, 04:17   #4
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I have one with a High Standard lower and Armalite upper and one with a Spike's lower and CMMG upper and both work fine. I never could tell much of a difference between them and my factory Rock river except for the price.
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Old March 23, 2010, 06:34   #5
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I bought a Bushmaster HB carbine for my first AR. A friend of mine had a RR varmint upper on a pre-ban Colt lower which shot very well so I bought one too, to put on my Bushmaster. Then I decided for the price of a lower I could have two complete ARs. At that time RR lowers were scarce so I bought a Double Star lower, installed a RR 2 stage trigger in it and away I went. The gun shot very well, right out of the box, but the fit between the upper and lower was somewhat loose. I asked around, and from my personal experience, decided that a slightly loose fit between an upper and lower was OK but still wanted it a bit tighter. So.....I found a RR lower and sold the Double Star. The RR lower fits much better than the Double Star did but there is still a tiny amount of wiggle. It shoots as well as, but no better than, it did when it was on the Double Star.

I think your decision should be based on economics and performance. I spent more than I needed to to get a fine shooting rifle with a matching upper and lower. If you have a lower that is of acceptable quality and which fits the upper well enough so that function and accuracy are not impaired, then you are good to go, IMHO.
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Old March 23, 2010, 07:41   #6
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I have run into more out of spec lowers than uppers. I had an early LRB lower that wouldn't fit any upper I had. Once I got it to close there was an irregular gap the entire length so it went away at a gunshow.
Hesse lowers had real problems as well.
I have found that by sticking to quality lowers and uppers I have no problems with loose or too tight a fit. I really like DSA lowers, any upper I have from Colt M16A1 and Colt M16A2 to Model 1 A2 uppers have fit well.
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Old March 23, 2010, 20:39   #7
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Got a funky combo that even matches so far as color, PWA "commando" lower, and a canuck C7 upper.

Most are close enough, and they do make "accu-wedges" if they are a loose fit.
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Old March 24, 2010, 15:56   #8
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Re: Mixed Manufacturer upper/lower fit?

Quote:
Originally posted by johnmac
[B
I'm fairly new to ARs and was surprised to find what I'm guessing is manufacturers tolerance variance in upper/ lower fit.

What is your experience with differing or the same mans. quality of fit with Uppers/lowers? Any such thing as a chart as to what U/L fits best with the other? [/B]
What you are probably seeing is normal tolerance variance that is acceptable even WITHIN a particular manufacturer, yes even for the well known manufacturers. When you buy a complete rifle, you don't have to worry about that upper and that lower fitting each other, but BM, RRA, Colt, DPMS, Armalite, you name it-- some of the factory assembled rifles are going to be tight and some will be loose and given the tolerance ranges, that's normal. They all shoot fine and it is really no worse when you mix and match. As Ssarge and Celtic Warrior noted, there ARE out of spec parts out there, but in my experience they are very rare, especially if you stick with a name brand (or at least one that has a good return policy).

I see you scored your upper, good for you! My advice would have been to take your lower to a gun show and try to mate it with the uppers you were interested in. Some would have fit fine, others would have been as loose as the BCM you tried, and my bet is that you would have found that same variation for the DPMS uppers as well. They don't have to be snug to work properly, although I too prefer them that way.
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Old March 25, 2010, 10:40   #9
johnmac
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Just got my new(to me) M4 DPMS upper. It fits on my lower exactly right. No wiggle, no slop and no big seam between the U/L. And the take down pin slides in with just a tiny bit of effort.

From what everyone has said, Maybe I'm just lucky!

Thanks to all for their input. I learned a lot!
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Old March 25, 2010, 12:00   #10
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There are companies that offer matched upper/lower sets for sale if cash wasn't a concern. Mega Machine comes to mind...

http://www.megamachineshop.com/armsc...03/loader.html

Really nice stuff.

The three matches that worked for me so far (nice&tight) are:

Lower/Upper...

Blackwell/LMT
CMMG/BCM
RRA/DPMS

All manufacturers have tolerances of course, the issue is on how tight they keep them in the process. That opens a whole new can of worms.
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