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AR UPPER to LOWER fit


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Can a loose fit cause problems?

My brother's Colt seems pretty loose but I havent noticed any issues(same brand upper/lower). I've seen some sets that does not move at all and does not even have an air gap. Im putting together a bushmaster lower / MUR-1 upper and its a little loose. Just wondering maybe it can cause feeding problems. .

Havent shot it yet, still waiting for gas tube.

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This is one of a few cases where I want a loose fit :D

Really though, a loose fit ensures you can slap whatever upper on that lower or vice versa. Accuracy will not be affected enough to matter. Maybe if you are doing benchrest comp for ultra tight groups you might see a MINOR accuracy difference but not enough for it to matter in real world use. At that point you could bed the upper and lower mating surfaces & lugs to each other, i guess......

Even better is a loose fitting upper lower combo with a lower that has a set screw under the rear lug to tighten it up to your desired tightness.

Edited by vdub5818
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Even better is a loose fitting upper lower combo with a lower that has a set screw under the rear lug to tighten it up to your desired tightness.

That sounds like a good idea...

I was able to tighten up the upper/lower by turning the buffer tube inwards one full turn, then file the tube down until the desired fit where the takedown pins require some tapping(I could still takedown with MY excessive finger pressure). About 1/8 of material was removed from half way up.

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A loose upper to lower fit has no impact on reliability or accuracy. It does, however, effect consistency which in turn effects accuracy. Most will never notice. If you're hunting varmints or shooting from a bench you might. I make sure all of my rifles are tight.

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The accuwedge is little orange piece of rubber that fits under the rear take down pin, normally sells for $5. The cam pin is sold by JP (and maybe others) and just as it says, it replaces the rear takedown pin and you turn it to cam the device to tighten the upper lug to the receiver Some receivers such as Sun Devil, include a set-screw at the rear of the receiver to push up against the rear lug to keep it tight.

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I have seen these tensioning pins fall out a couple of times during a stage at SMM3G...

The accuwedge is little orange piece of rubber that fits under the rear take down pin, normally sells for $5. The cam pin is sold by JP (and maybe others) and just as it says, it replaces the rear takedown pin and you turn it to cam the device to tighten the upper lug to the receiver Some receivers such as Sun Devil, include a set-screw at the rear of the receiver to push up against the rear lug to keep it tight.

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I have seen these tensioning pins fall out a couple of times during a stage at SMM3G...

The accuwedge is little orange piece of rubber that fits under the rear take down pin, normally sells for $5. The cam pin is sold by JP (and maybe others) and just as it says, it replaces the rear takedown pin and you turn it to cam the device to tighten the upper lug to the receiver Some receivers such as Sun Devil, include a set-screw at the rear of the receiver to push up against the rear lug to keep it tight.

Yeah.... a little blue loctite goes a long way on those things.

Tim

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  • 3 weeks later...

i had a ruger sr556 that wa horrible, the fit was so sloppy i could drop a nickel in betweeen the upper and lower.....had horrible groupings. they warrantied it without issue and the new on was much tighter and shot way better

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I guess someone needs to explain to me how a little play between the receivers is going to affect accuracy. The upper receiver, barrel, BCG, optic, and bullet are all locked up together on the upper. As long as the BCG strips a round off of the lower, how is the fit between the two really going to matter?

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I like the "feel" of a tightly fitted upper-lower too, but it really doesn't matter much as far as performance is concerned. I've shot guns that rattled like maracas that would shoot MOA all day long, and I don't think an upper could have enough "wobble" to hinder feeding without the lugs being broken.

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