Hey guys - it looks like I'm subject to a character limit since I have a new account - I hope I'm not breaking any rules by doing this but I'm going to post my full topic over a couple thread responses...

I’m having some very discouraging problems with my 80% builds…

I bought a couple 80% lowers and jig from 80-lower.com and the blank lowers came with the rear shelf completed and the takedown pin-holes already drilled. The directions said if the lower came with the rear shelf already milled out there is no more work to be done there.

With the completed lowers I took my newly delivered Aero Precision M4E1 upper and try to fit it up – no good. The front post is tight but fits in between the ears and the pivot pin functions fine. Then when I swing the rear of the upper into the lower it gets jammed. It appears to me that the pre-milled rear shelf isn’t wide enough to accommodate the rear post of the upper. I had the same problem when I previously tried to attach my 6920 upper to both completed lowers but figured I’d wait for my AP upper to get in before modifying anything. I tried test fitting my AP upper on the 6920 lower and it fit great. I tried test fitting my friends Delton (might have been a DPMS) upper on my lowers and that actually fit fine.

So now I decide to shave away the inside of the rear shelf pocket, so I go at it with a dremel (I know I should have had more patience). Anyway, I throw some chalk in the rear shelf pocket and get a feel for where the post is getting hung up and manage to grind away enough material that the post goes in without any problems.

But now the problem I’m having is the rear pin won’t go through the hole of the upper post. In looking closer it appears that the hole in the upper rear post is just a hair too high but I can’t figure out where it’s getting hung up. When I push the upper down as far as it’ll go there isn’t any chalk on the bottom of the post – so it doesn’t appear to be bottoming out in the rear shelf pocket. I can also shine a flashlight from the opposite side along the seam between the upper and lower receiver and can see a small amount of light along the whole length.









So I’m at a bit of a loss for what to do now. I really don’t want to modify my new upper to fit on crummy homemade lower, especially since it fit so great on my 6920 lower. Have you guys had any problems like this before or does anyone have any suggestions? Do you think I should go back to the 80% lower company and write a complaint?

Looks like the rear take down pin hole is off a bit, pick up a round needed file and start lightly working on it until the pin will push through.

As far as seeing light between the lower and upper, that is not uncommon, I have seen it happen on brand new upper level guns in the past.

What jig were you using when you drilled that rear take down pin hole?

Why would you complain to the 80% lower company, you are the one that milled it and drilled the holes, Right?

The rear takedown pin hole functions fine on it's own - it's just when the upper is introduced it seems like one of the following is the problem: the rear takedown pin hole is too low, the upper lug hole is too high, or there is something hung up and stopping the upper from going all the way down. If you look at the close up picture of the takedown pin hole you can see at the bottom of the hole that there is a lip at the bottom which is the upper post sitting a little high.

The rear takedown pin hole comes already drilled from the supplier and is used to secure the template/jig to the lower reciever.

Noting that there was light between the two receivers was just meant to indicate it doesn't appear that the upper is "bottoming out" on the lower frame (not sure if that makes sense).

Based on the above do you still think that modifying the pin hole is a possible fix? I hadn't thought of that yet and would be unsure of where to begin filing...

Thanks

Getting ready to change into someone more comfortable. Name changes coming.

Topic Moved

I don't think I've ever completed an 80% with the rear lug precut thst didn't need additional machining in the rear lug area.  Most were undersized by about. 020-.030. Some needed milled farther to the rear, by the buffer retainer too.

I tried leaving one with the precut rear shelf and only milling the fire control pocket, had the same issue. I always mill out the rest of the rear area now.

Fixed it! It turned out being the upper on the bottom just in front of the rear lug as you said. Thanks for pointing this out - I hate to admit I would have probably never thought to look there. I don't have a small file to work on the upper so I just shaved a little off the lower with my dremel and it works great (in combination with widening/further milling the rear pocket as others pointed out).

Thanks to everyone for their input.

I got my lowers from 80% and the rear shelf was only partially complete.  I still had to mill it.  I'm using the Modulus HD jig. I just posted pictures under "My first mill job".

Just out of curiousity did you get it from 80-lower or 80percentarms? I thought they were the same company but it turns out they aren't.

On page 12 of the instruction manual (https://www.80-lower.com/content/80_Lower-80_Percent_Arms-Easy_Jig-Lower_Machining_Instructions.pdf) for mine it said I don't need to do anything with if the rear pocket was already milled out. I guess I learned the hard way that isn't the case.

80% Lowers.  My jig/instructions are differant than yours.  

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The last one I did, I didn't mill the wall between the FCG and the Rear lug pocket and the gun works fine, you can get away with it on some of the guns,  Colt did the same in their factory guns for a while in an attempt keep them from being converted to full auto function.

The rear shelf of the 80% lowers is not cut to finished size. You do not need to remove the wall between the shelf and pocket if you do not want to, but there is still a few thousandths that need to be removed from the rear shelf area, primarily from the drivers side.

I always enlarge the rear pocket by a little bit and still work on the rear lug on the upper.  I have done a couple that fit perfectly and I didn't have to do any work on the rear, but normally you will have to do something to get them to mate.

I prefer them tight so they require fitting. Use high-spot dye, and take your time with hand fitting (I use wet-dry abrasive paper backed with a fiberglass tongue-depressor for final fitting). If we are going to take the time to custom build a weapon we should take the time to fit/marry the parts we machine.

Just wanted to add a couple pictures of the high spots that were causing problems. It's those little corners where the anodizing is worn off. Instead of shaving that down I just shaved into the lower a little bit. I think if I milled the lower to connect the rear shelf and trigger pocket this probably wouldn't have been a problem. Thanks again for everyone's help.

Mine needed the rear shelf fully milled with ap upper.I have a few more where i didnt mill the metal between the upper shelf and fcg. I figured ill leave it and see how it matches with future builds. As for a dremal me personally would avoid that..

There's nothing wrong with a Dremel - IF you use it properly. The biggest problem with any of the rotary tools is when people try to use the wrong bits, or go at them too aggressively.

You should always double check your width (.500" Minimum) and your depth (.525" Minimum) for your rear takedown pocket on your AR-15 lower...

I have noticed that Aero Precision uppers are EXTRA TIGHT (fat dims) versus other brands and can give you some fitment Issues...

Good luck.

what is your remedy for that?

Just eye balling the original photo, it appears the walls are too thick. In other words not milled enough off each side. Measure, measure, and measure.