Question for machinists manufacturing ARs

[shaneH5381]

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]

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

 

[Bigjake83]

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]

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]

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.