Question for machinists manufacturing ARs

shaneH5381 · Dec 31, 2024

Hello all,

I was an active member on this site about four years ago, but have been caught up with life and fell out of the sport for a while. Getting back into it and started a new account (can't remember my old account details).

I remember having some conversation on here with a machinist or two who was a manufacturer of AR receivers, but don't know if any are still around. I've got two questions. The first question is how do you all manufacturer matched receiver sets? I'm not talking about one of those nylon tipped screw things and calling a set a matched set, but an actual matched set without a gap or rattling. I was thinking the order of operations would be something like (prior to anodizing and drilling the take down pin holes), mate lowers and uppers to see if they fit together without a gap, and the ones that fit go to the next op where the pin holes are drilled (with the upper and lower clamped together), chamfered and de-burred. Then sent to anodizing. Is this the typical process?

The other question is regarding critical call outs. What are the critical dimensions of both the upper and lower receiver? I have a receiver that I've had issues with for years. I ended up completely stripping the thing and throwing all the parts onto another lower and the problem resolved itself. I mean I bought a new lower and put every part on the other lower and the issue of failing to lock the bolt back on the last round (with subsonic 300 blackout) was no longer an issue. I thought maybe something was just installed wrong initially or maybe a foreign object/debree was causing the problem , so I put the parts all back on the lower with the problem again, and same issue again. I read something on the White Oak website saying one culprit could be the bolt catch. Something about how if it's located wrong it could cause rubbing or something and slow the bolt, so maybe that's it, but really just looking for a list of what tolerances are highly important overall. At this point I'd consider the lower scrap if it wasn't a form 1...

Thanks in advance

Merle · Jan 3, 2025

Without breaking the bank, I'd look at the BKings Firearms M4 Mod-0 receiver set.

BKF M4 MOD-0 Stripped Receiver Set

bkingsfirearms.com

Bigjake83 · Jan 3, 2025

Merle said:
Without breaking the bank, I'd look at the BKings Firearms M4 Mod-0 receiver set.

BKF M4 MOD-0 Stripped Receiver Set

BKF M4 MOD-0 Stripped Receiver Set

bkingsfirearms.com

I paid that same price for a Geissele receiver set during the Hollidays, there's good deals to be had for under $200.

Merle · Jan 3, 2025

Bigjake83 said:
I paid that same price for a Geissele receiver set during the Hollidays, there's good deals to be had for under $200.

On the Geissele set, how tight does the upper and lower fit together? Is the upper receiver dry lube coated on the inside and have a thermal fit for the barrel extension?

strikeeagle1 · Jan 4, 2025

February 7, 2003 Rev 1
TECHNICAL NOTE 55: RECEIVER TIGHTNESS
BACKGROUND:
ArmaLite® occasionally receives questions from customers about the tightness of the fit
between the upper and lower receivers of ArmaLite’s® M-15, AR-10, and AR-180B
rifles.

There is an intuitive belief that a tight fit between the upper and lower receivers is
essential to accuracy, and that loose fit is bad. Oddly enough, this is untrue.

FACTS:

The upper and lower receivers of ArmaLite® self-loading rifles, and all other similar
rifles based on the AR-10 or AR-15 design, are held together by two pins. Unlike rifles
that are cammed or screwed together and can be tightened, the pins allow for slight
movement between the receivers.

It is possible to selectively fit pins and receivers, or to custom ream the pin holes while
the receivers are held together, so that movement is minimized. Doing so reduces the
interchangeability of the receivers, and normal wear results in looseness over time
anyway. ArmaLite® uses the reaming process on its National Match M-15 rifles.

It is also possible to reduce movement by inserting materials such as paper, rubber, or
epoxy somewhere between the receivers and creating a tight fit or bind. Many
commercial customers use a common rubbery device claimed to improve accuracy, and
the Army Marksmanship Unit often uses a bit of epoxy. The AMU Armorers advise that
this procedure doesn’t improve accuracy, but it makes the shooter feel more confident
about their rifle.

The AMU Armorers are correct. Formal testing conducted at Rodman Laboratories in
the 1970s verified that accuracy wasn’t affected by normal receiver tolerances. The fact
is that the bullet exits the bore before movement of the receivers produces a measurable
effect.

Modern manufacturing processes are unable to produce to dimensions that both allow full
interchangeability AND prevent all movement in all cases.
ArmaLite’s® rifles, except for some of the National Match rifles referred to above, are
produced to military standards of interchangeability to allow full interchange of
components. Tolerances have been deliberately tightened to reduce the movement
between the receivers, but remain open enough to assure interchangeability. ArmaLite®
gets a close fit between the receivers, but normal forging and machining variations may
result in a small amount of play between the receivers or slight side to side receiver
mismatch. This is not a manufacturing defect and has no effect on accuracy.

CONCLUSION:

ArmaLite® rifles are produced to tighter-than-normal tolerances. Receivers will
normally exhibit a certain amount of looseness. This is normal and will not affect
accuracy. Shooters may reduce the movement by insertion of a small amount of material
between the receivers to improve the feel of the rifle, but it won’t improve accuracy.

© 2003 ArmaLite, Inc.

shaneH5381 · Tuesday at 10:52 PM

strikeeagle1 said:
February 7, 2003 Rev 1
TECHNICAL NOTE 55: RECEIVER TIGHTNESS
BACKGROUND:
ArmaLite® occasionally receives questions from customers about the tightness of the fit
between the upper and lower receivers of ArmaLite’s® M-15, AR-10, and AR-180B
rifles.

There is an intuitive belief that a tight fit between the upper and lower receivers is
essential to accuracy, and that loose fit is bad. Oddly enough, this is untrue.

FACTS:

The upper and lower receivers of ArmaLite® self-loading rifles, and all other similar
rifles based on the AR-10 or AR-15 design, are held together by two pins. Unlike rifles
that are cammed or screwed together and can be tightened, the pins allow for slight
movement between the receivers.

It is possible to selectively fit pins and receivers, or to custom ream the pin holes while
the receivers are held together, so that movement is minimized. Doing so reduces the
interchangeability of the receivers, and normal wear results in looseness over time
anyway. ArmaLite® uses the reaming process on its National Match M-15 rifles.

It is also possible to reduce movement by inserting materials such as paper, rubber, or
epoxy somewhere between the receivers and creating a tight fit or bind. Many
commercial customers use a common rubbery device claimed to improve accuracy, and
the Army Marksmanship Unit often uses a bit of epoxy. The AMU Armorers advise that
this procedure doesn’t improve accuracy, but it makes the shooter feel more confident
about their rifle.

The AMU Armorers are correct. Formal testing conducted at Rodman Laboratories in
the 1970s verified that accuracy wasn’t affected by normal receiver tolerances. The fact
is that the bullet exits the bore before movement of the receivers produces a measurable
effect.

Modern manufacturing processes are unable to produce to dimensions that both allow full
interchangeability AND prevent all movement in all cases.
ArmaLite’s® rifles, except for some of the National Match rifles referred to above, are
produced to military standards of interchangeability to allow full interchange of
components. Tolerances have been deliberately tightened to reduce the movement
between the receivers, but remain open enough to assure interchangeability. ArmaLite®
gets a close fit between the receivers, but normal forging and machining variations may
result in a small amount of play between the receivers or slight side to side receiver
mismatch. This is not a manufacturing defect and has no effect on accuracy.

CONCLUSION:

ArmaLite® rifles are produced to tighter-than-normal tolerances. Receivers will
normally exhibit a certain amount of looseness. This is normal and will not affect
accuracy. Shooters may reduce the movement by insertion of a small amount of material
between the receivers to improve the feel of the rifle, but it won’t improve accuracy.

© 2003 ArmaLite, Inc.

Thanks for the reply, and sorry for the delayed response. I didn't get an email saying that you responded. Must have went to spam. I've read this before, and I've also read some things that contradict this. I've read a few debates on this on this website too. I can see the argument from both sides, and don't want to start another debate on the topic that has already been hashed out many times.

When it comes down to it, it's not only about accuracy. I could argue that I don't like gaps between the receivers, such as at the rear of the upper and lower because of small differences in the radius of the upper and radius of the lower. Although small, I'm often in a sandy environment that could more easily allow for sand to find it's way into the set. Or, with my nicer rifle that I don't take out as often, if I spend upwards of $1,000 for a custom Kriger or Bartlin, JP components, an optic with a cost of of well over 1k, having a rattling receiver set just doesn't make sense. It would be the equivalent of building a pistol. I could get a PSA Dagger frame and a bunch of knock off Glock parts, which would function reliably and accurate, would be practical for concealed carry weight wise and all, or I could hand craft a 1911, custom fit the barrel and all. The 1911 wouldn't be better for concealed carry, but there's something to be said about the pride in workmanship and craftsmanship. In the case of one of my uppers and lower, the fit is actually so tight, I can't even get the pins in and there's a gap near the radius of about .005

I'm considering filing some material off an upper via the same type of method as custom fitting a pistol barrel. After getting a good fit between the two, cerakoting the set and doing something with the pins to keep the set pulled together reasonably tight. I don't want to bring anything outside of tolerance in the process. I'd like to get two uppers to swap out on a lower, and have both uppers fit the lower like they were all meant to be together (matching cerakote and all). Even with a matched set, finding a second upper that also matches isn't realistic without doing some manual fitting.

strikeeagle1 · Wednesday at 4:40 PM

For what its worth, according to the 23&P a maximum of 0.020" gap is allowed between the upper-lower receiver.

IMG_8181AR-15 Upper Lower Receiver Gap Gauge 08.04.23 copy.jpg

IMG_8183AR-15 Upper Lower Receiver Gap Gauge 08.04.23 copy.jpg

LRRPF52 · Wednesday at 5:05 PM

The reason for the US Mil-Std specs on receiver fit is interchangeability.

Armorers need to be able to assemble complete weapons from salvaged parts after weapon attrition in explosions, accidents, and damage that can happen throughout the normal course of combat and training.

This is why Mil-Std compliant AR-15/M16/M4s don’t have tightly-fitting receivers.

WindstormSCR · Wednesday at 6:08 PM

There are quite a few different ways to make matched sets of anything in a machining environment, but the best way is simply to tighten the acceptable tolerance on the mating surfaces and have the rough-out step leave a slight excess of material before using a final finish step with a slower feed rate to ensure dimensional accuracy. Then any upper and lower you produce will fit like a matched set

LRRPF52 · Wednesday at 6:13 PM

A common technique many companies have used is to oversize the rear lug width so it press-fits when you try to close the receivers together. You can also move the takedown pin and pivot pin locations .001” to get them to fit really tight for the receiver seam line mate-up.

Depending on how anodizing goes, that will have variances in user experience for those who are looking for tight fit.

Being aluminum substrate, it will wear-in to a comfortable and easier fit with time.

With shooting, the pivot pin and rear lug pin connections will move around a tiny bit.

redbullitt · 2025-01-23T03:03:27-0500

Good call on the confidence angle of it, can see that for sure.

DeathBeforeDismount · 2025-01-23T09:17:08-0500

Autism sucks bro. Carrying about shit that doesn't matter is hell on the soul.

DeathBeforeDismount · 2025-01-23T09:19:43-0500

LRRPF52 said:
The reason for the US Mil-Std specs on receiver fit is interchangeability.

Armorers need to be able to assemble complete weapons from salvaged parts after weapon attrition in explosions, accidents, and damage that can happen throughout the normal course of combat and training.

This is why Mil-Std compliant AR-15/M16/M4s don’t have tightly-fitting receivers.

The best score I ever got on the qual range was from a Mixmaster h&r a1 over stamp that would rattle like 20 degrees in each direction. When I asked the armorer you expect me to qual with this piece or shit he told me to shut the fuck up and I went on to shoot a 37 with it. I learned at that young age that the receiver fit between AR upper and lowers have very little to do with accuracy. This was before the current soft asses got to qualify with optics.

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Question for machinists manufacturing ARs

shaneH5381

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Merle

Gunny Sergeant

BKF M4 MOD-0 Stripped Receiver Set

Bigjake83

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BKF M4 MOD-0 Stripped Receiver Set

Merle

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strikeeagle1

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shaneH5381

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WindstormSCR

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DeathBeforeDismount

Hey, give your balls a tug.

DeathBeforeDismount

Hey, give your balls a tug.

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