tightening up AR-15 upper-to-lower connection

[DarthNul]

Not too long ago Brownell's had Bushmaster stripped uppers & lowers on sale for $49.99. It seems I just couldn't pass up that deal so I got one of each. I figured I'd put together another AR "eventually". Well, that plan changed into building a "no excuses" gun for high-power service rifle competition. I want the rifle to be good enough so that when it's done, there will be no doubt that it's MY fault whenever the nine ring (or worse) gets perforated.

I am concerned that these units wobble considerably when they are pinned together. What (beyond an "accuwedge") can I do about this? I remember something in the CMP rules about it being legal to put a screw through somewhere around the pistol-grip area into the upper, but I don't know anybody who's done that, or even talked about doing that. How common is that?

What other effective options are out there?

 

[cryogenic419]

I've not heard of anybody doing that screw mod, I've seen it and how it works but don't know anybody using it.

Accuwedge seems to be a more common practice.

I've heard of using an o ring on the front pin to help.

There is this thing which has been around forever but have not heard of anyone using one or if it actually works.

http://www.jprifles.com/1.5.1.7_small_tp.php

Armalite has this to say about receiver fitment.

http://www.armalite.com/images/Tech Notes\Tech Note 55, Receiver Tightness.pdf

 

[Dixie Gunsmithing]

I take it you are talking about the receiver pivot pin at the front of the magazine well? If so, you could turn one for a tight fit. Generally pin fit is 0.002" under the hole size, for a tight fit that will pivot. The takedown pin should be close to the same.

 

[MarkCO]

There are several lowers that come with the receiver already threaded for a 10-32 screw to tighten the upper to lower fit, I have two.

The JP pins are another option. I think I have an accuwedge somewhere too.

But all an effort in futility. Upper to lower fit does not affect accuracy!

Getting the barrel nut torque level at the upper end, if not beyond "spec" with a slight interference fit of the barrel extension into the receiver, yes important. Working the threads in and using anti-seize on the barrel nut, yes important. A very good barrel, yes important. Everything else (sans shooter technique) is unimportant in terms of accuracy in the AR15 platform.

 

[macsgunworks]

building is a leaning process nothins free

were are the pins the loosest. have you fit the pins with the upper and lower apart to determan were you poor fit is ? upper or lower ,hinge or lock pin ? also if it is as bad as you say you may have colt pin size upper and commercal smaller pin lower. if thats the case drill lower to take larger colt pin's but like you have heard loose will not realy effect how it groups

 

[DarthNul]

Thanks for the replies!

Yes, I am talking about a loose fit in and around the pivot and takedown pins. The little blocks (not sure of the correct term) that have the holes through which the pins pass, have about 0.004 on each side to rattle around. I have not measured how loose the pin/hole fit is.

I would agree that in most cases this fit is not a factor in accuracy, but when I'm doing standing position in high-power competition, I make absolutely no contact with the upper to wedge anything or otherwise prevent rattling. Maybe that won't matter, but in my mind, it's most likely to matter in that situation. This is after all, my "no excuses" rifle.

 

[Dixie Gunsmithing]

If done for a tight fit, the holes are drilled undersize, and then reamed with a chucking reamer, to no more that 0.002" over. One could easily make the hinge pin, by turning a head on one side of a round rod, and threading the other to use a nut with. Leave the shank unthreaded, so it has a solid surface through the holes of the bosses on the frame. Essentially, you would be making a small shoulder bolt.

 

[HiBC]

The principles of Geometric dimensioning and tolerancing have been applied to the parts.
Its easy to precisely control the OD of a pin.(to less than .001)Its slightly less easy to control the diameter of a hole.

You'll get slightly more variation positioning holes.

They do seem to control the diameter of the pins very well,within tenths of a thousandth...and they all seem to run .245 or so.(Its been a long time since I measured).

A minimal diameter pin buys more tolerance for the hole and positioning.

I simply made new pins the one time I had a real loose upper/lower.I duplicated the design,just a larger OD.Gage pins make it easy to determine size.
As has been mentioned,its more about "annoying" than important.

 

[jcj54]

Service Rifle

Service rifle rules do not allow "external modification".Anything you do with special pins must externally duplicate factory pins. Any screws holding upper to lower must not be visible externally. Good luck.

 

[CowTowner]

There is this thing which has been around forever but have not heard of anyone using one or if it actually works.

http://www.jprifles.com/1.5.1.7_small_tp.php

I have one of these on my AR in 7.62x39 Russian. It does keep the upper and lower firmly together - no wobble. Is it legal for CMP? I don't know. I use the rifle for hog hunting.
I'll say this, make sure you have a screwdriver and the allen wrench with you in the field if you install one of these. No, don't ask......