Gunsmithing Misadventures in Amature Gunsmithery: 80% AR lower edition

[molonlave]

Ever have a sloppy fitting AR upper and lower? I'm specifically referring to the rear portion at the take down pin. Well, I found myself facing this unacceptable situation through all fault of my own.

There are shim kits to remedying this situation. It works, but is a real bitch getting the shims to stay on both sides of the upper takedown pin lug while reassembling and pushing the pin through. Then each time you disassemble, having to deal with the shims falling down in the trigger pocket.
The photo below shows the product on the pivot pin lug, but you get the point.

 

[molonlave]

The print calls out .500" width for this area of the receiver to allow for the upper takedown lug to sit. My receiver measured at .525". Analyzing the error revealed that too much was milled away by .007" on the left side and .018" from the right side! So, I came up with making bushings that could be pressed in from the inside of the trigger pocket. Thus, allowing the outside of the receiver to look unaltered.
My biggest problem was how to counterbore the rear take down pin holes from inside of the trigger pocket.

Enter this cool little counterbore:

Insert the cutter into the trigger pocket, slide the drive shaft through the takedown pin hole of the receiver and into the cutter and engage them!

 

[molonlave]

So, cut a .375" dia counterbore .080" deep from the inside of each takedown pin hole. The left bushing had a .007" thick x .400 dia flange while the right had a .018" x .400 dia flange. Pressed in and here we are:

View of outer aspect

 

[molonlave]

Lesson learned: be careful and hit the numbers on the damn print!

Was this necessary? Probably not. It was shooting quite well prior to this modification. But, it just bothered the hell out me. To the point where I was going to scrap the receiver. I just needed to try to solve this. Funny thing is, the engraving was not up to par from a cosmetic aspect. So, I have a newly engraved 80% lower that looks perfect. Now I just have to make sure I don't fuck up when I get around to milling it.

Stay tuned for the C7CT episode of Misadventures in Amateur Gunsmithery

 

[pmclaine]

Huh....could a similar fix bring a receiver with "egged" trigger pin holes into spec?


Addendum - Im guessing trigger pins may be able to "wallow", I mean anti walk pins are an industry, but in the back of my mind I know I will never experience it.

 

[Trigger Monkey]

What about bedding it to take out the slop?

 

[molonlave]

Pmclaine: that would probably work for triggerpin hole

Trigger monkey: bedding would definitely take out the slop.

 

[sjmpcc022]

I applaud the fact that you did something, knew you messed it up, and it sounds like it kept you up at night.

That is an old school trait, it used to be common, not so much these days. It's called pride in ownership. For that you should be commended.

Now, I could make suggestions that would have saved you money/time. That is not my point.......................

I would really like you or someone else to explain to the world how you would bed an Ar 15????????????????????

 

[264win]

I would really like you or someone else to explain to the world how you would bed an Ar 15????????????????????

It requires a few of the little blue pills and some good whisky.

 

[Trigger Monkey]

I would really like you or someone else to explain to the world how you would bed an Ar 15????????????????????

Aluminum tape, did you think I was talking about using Marine Tex or something? Maybe bedding was a bit of a misnomer, I remember back when the Ruger precision rimfires were coming out, guys were using aluminum tape to "bed" the actions.

I've read about guys bedding AR-15 receivers using traditional bedding compound but dang, talk about a giant PITA to keep the compound from going everywhere.

 

[sjmpcc022]

Aluminum tape, did you think I was talking about using Marine Tex or something? Maybe bedding was a bit of a misnomer, I remember back when the Ruger precision rimfires were coming out, guys were using aluminum tape to "bed" the actions.

I've read about guys bedding AR-15 receivers using traditional bedding compound but dang, talk about a giant PITA to keep the compound from going everywhere.

I very much thought just that.

Bedding would generally be thought of as a buffer between the action and the stock of a rifle. This buffer would ensure that said action is seated in said stock in exactly the same rigid way every time you remove it for whatever reason you remove it for. As a secondary, it would also prevent the recoil lug, or the front of the action from recoiling it's way through the magazine area of your stock clean through to the butt area of your stock.

Given that an AR15 possesses almost none of the features I mentioned, I'm still lost on how one would bed an AR15, aside from the blue pill and whiskey mentioned above.

Keep in mind, also, that an AR15 more closely resembles any Canadian individual featured in the show South Park than it does a traditional rifle. IE......the top and bottom of their heads aren't attached by anything except gravity, no take down pins!

 

[molonlave]

I applaud the fact that you did something, knew you messed it up, and it sounds like it kept you up at night.

That is an old school trait, it used to be common, not so much these days. It's called pride in ownership. For that you should be commended.

Now, I could make suggestions that would have saved you money/time. That is not my point.......................

I would really like you or someone else to explain to the world how you would bed an Ar 15????????????????????

By all means, fire away! I'm all about learning, and this is part of the journey. Please share any suggestions/insight.

 

[slambeach]

Drilling and tapping the receiver for a teflon tipped receiver tensioning screw also works (if your 80 isn't tapped) to take up any play. Another tip before machining, is to tape off the sides of the fire control area, before you drill the holes, (I have good luck with that aluminum HVAC duct sealing tape), It keeps the aluminum curl, that sometimes develops when drilling the selector hole, from whipping around and scratching the anodizing. Taylor Tactical sells the tensioning screws, along with other goodies.

 

[molonlave]

I was wondering what that threaded hole was for. I just encountered it on a new 80%. My older receivers didn't have that hole. I figured in was utilized as a fixture point for production.

I'm assuming you don't tighten the tension screw to the point you can't slide the takedown pin. Otherwise you would have to remove the grip each time you disassemble.

I'm not sure this would have taken out the horizontal movement of the upper receiver rear lug. I wasn't experiencing significant vertical movement.

Interesting idea though, thanks for sharing!

 

[buffalowinter]

After the Army, slop in the receiver doesn't bother me. Is there not supposed to be slop?

 

[roostercogburn98]

After the Army, slop in the receiver doesn't bother me. Is there not supposed to be slop?

View attachment 7800167

So……lots of duct tape used for rattle check on that ambush mission I guess