While I do not think that this is necessary for a properly machined lower, I have had folks ask me about them so I figured I'd post this.

I set the mill up like this, using the following datum points. The normal "Y" axis zero, the flat area just behind the PG mounting boss for the "Z" zero and .156 (half of 5/16") to the right of it as the "X" zero.



Using a 5/16" end mill I machined along the X axis .265 to the left into PG mounting boss. You do not need to mess with either the Y or Z just leave them locked.





Drilled it out with a #3 bit.



Tapped it to 1/4"-28



Added 1/4"-28 X 3/4" set screw. It sits well below flush when fully installed

This is awesome.

Edit: in essence, could one just use a 5/16 bit, angled straight down on that angled surface, but in line with the rear takedown pin hole?

I never understood the reasoning for the tension screw. Yes it eliminates play between the upper and lower, but that has zero to do with accuracy.

For some, it's not about accuracy. Its about having a gun that doesn't rattle or move around. While that is perfectly acceptable to some, maybe even yourself, I want mine snug. My rifle will be used in hunting applications. And I really don't want it alerting my prey prematurely. I know it doesn't make a ton of noise. But none is better than some. This is why I opted for an Aero Precision Lower. Has the set screw already.

When 3 out of 4 of the shooter's usual contact points are with the lower (shoulder, cheekweld, pistol grip), I'm not sure you can say that play between them has zero to do with accuracy. Sure, sights or optics are attached to the upper and aligned with the barrel, but marksmanship training teaches us proper methods of contact in these places for a reason

The M16 has more issues with the barrel sling swivel attached to the gas block. It's why they invented free float tubes a long time ago and use them in Service Rifle matches. Unfortunately they are steel not aluminum or I would have used one in a dissipator build.

As for the upper to lower play, o rings can muffle that more easily and cheaper, along with ear plugs. The OP was simply responding to a "how to do it" with a knowledgeable answer, but it goes to just because you can doesn't mean you should.

Noise in the field is a much larger problem - and starts with the feet. Knowing how to step making the least amount of noise possible when stalking is one thing, but if your scent is working with the wind, it's still a fail. Rattling and weapons noise isn't automatically a bad sign - I hunt an area where horsebackers and quail gunners frequent, the deer ignore doors slamming or actions getting cocked because they hear it frequently and there is no corresponding danger. They simply stay alert which they do 24/7/365 anyway.

I worry a lot less about my upper and lower fit than my big feet trashing thru the woods after 6 hours of still hunting in dense leaf fall, my scent from lunch on my breath, and the sound of my clothing against itself or brush moving thru it. It also goes to flipping off the safety - I've heard more justification over silencing it as deer tend to react to that sound more. Much less a sling swivel hitting the stock or a stick under foot giving way at the wrong moment.

We think gun centric in gun forums but the reality is there are a lot more things to consider silencing the noise of a hunter struggling thru dense underbrush or shifting his weight in a deer stand or blind - much less the scent issues, etc.

I will say this piece and then shut up. This thread is already derailed enough.

I agree that the hunter plays a much larger part in being quiet. And, where you have a lot of activity going on all the time that your quarry has become used to, I don't have that where I hunt. People are in the fields on a regular basis. That's it. If you go into the woods here, the animals all know something is up. So, you better be bring your A game. Taking care of scent or shifting your weight goes without saying. But those little metallic noises will turn heads really quick. This is the same reason we take our change out of our pockets. Carry extra ammo in a way that it can't rattle. Etc, etc, etc. Making sure our equipment doesn't make undue noise is just one more step in the process. That is why > I < have the screw to tighten mine up. And as long as Aero keeps making them, any build I do in the future will have it too. Or I'll look up this thread and install one myself.


Nice job maxxx93! And thank you for the information you provide!

My CMT lower has a set screw. It puts tension on the rear takedown pin. I find that if I tighten it enough to take out the little bit of wobble there it makes the rear takedown pin very difficult to move. I also feel that putting that kind of tension on the pin over time will create a bigger problem than it fixes. Don't use it.

O rings work well and there is always the acuwedge which I have several in my parts kit that I have never used.

Well, whether it adds anything or not, it's a neat project, I like it, and I appreciate the post and photos. So thanks, OP. I don't think I'll do it to one of mine, but I do confess that I like a tight fit between upper and lower, though Derrick Martin says a little slop between the upper and lower is not an accuracy issue, and I am not going to argue with him.

McMaster Carr sells set screws that have a green colored nylon tip. Search 'nylon tipped set screw' on their site.

The set screw doesn't touch the takedown pin, it only contacts the rear lug of the upper receiver. If it is adjusted properly, it will not put more load on the takedown pin. It is only there to remove any slack.

Well done OP...much more skills than many.  I have multiple lowers with set screw adjustment and have never needed them.  We all do things do our weapons that they don't need,  but make them uniquely our own.