AR receiver lapping... It's been eating me

[rickyrick]

Ok, I have a receiver just laying around... A slow build I guess. I've been considering lapping the receiver face but some info is spotty. It makes sense to have a nice square face but also been thinking about the potential for corrosion, especially since it will probably be Stainless.

7075 aluminum and stainless are pretty far away from each other in terms of reactive metals. Thats a bad thing. I also understand that they must be in together in the presence of electrolytic material in order for the reaction to happen. Seems simple, just seal the joint, yes? Well many suggestions out there advocating the use of RTV in order to keep water out.... Not so fast, RTV contains stuff that may turn into a material with electrolytic properties when exposed to high temps (according to my research)

Looks as though recoating the receiver face would be counter productive because the consensus say that the coating process may have caused any unevenness to occur.

What's the prevention that can be used?

 

[bedlamite]

What's the prevention that can be used?

Use the same moly grease on the face that you use on the threads.

 

[TMD]

Unless you plan on plugging your ar into a light socket or connecting a generator to it electrolysis isn't an issue. After lapping some people use grease, some use antisieze compound. Some even use loctite. Take your pic. Unless you're going to be changing out the barrel its a non issue.

 

[FrankenMauser]

I've lapped.

To finish...
I thoroughly cleaned the upper, smeared moly grease on the raw aluminum, and torqued the barrel on. If the receiver ever goes tits-up from galvanic corrosion, I'll replace it. A3 upper receivers are cheap.

But... one of those uppers went through sweat, rain, sleet, snow, dust, dirt, mud, and even black powder loads, over the course of about 14 months after lapping; and it really wasn't ever cleaned (wipe down the BCG ... or maybe not). And when I pulled it apart last month, the lapped surface still looked pristine. I'm not worried about it.


As for lapping, itself....
I have the tool, so I may try it again in the future. But if someone asked me if it's worth it, I would have a definitive response of "my results were inconclusive..."

I had a notable improvement in performance ONE time - we're talking 300-400 percent improvement in group size. But that was it. The rest of the times I tried it, there was no measurable improvement, or actually a negative change.

If you asked whether or not I thought it would be worthwhile to lap a more expensive receiver, such as my MEGA SBU side-charger, I would probably just say, "No."



Edit:
Just as a point of reference... I was a helicopter crew chief in a previous life, with much of our operations centered around salt water, and our base of operations 1/2 mile from the Gulf of Mexico. I am very familiar with galvanic corrosion, and had to deal with it on a daily basis.
...I'm not worried about it on my lapped receivers.

 

[impalacustom]

You don't need to literally plug your metals into something for galvanic corrosion to happen, you need an electrolyte, 2 dissimilar metals. The exchange of electrons from the least noble metal will create it's own electricity on a very small level. When we built boilers we would use anodes to prevent it to an extent.
Also remember aluminum will create it's own barrier of aluminum oxide as soon as it's exposed to air, but yes it can corrode, maybe in 100 years but not anything to worry about.

 

[tirod]

I've lapped. Basically the operation is meant to square the nose of the upper so the barrel extension lug is holds the bore axis in the same line, eliminating some of the windage and elevation variable.

NOTE: most high end AR builders don't bother for their guaranteed MOA guns. Its usually noted as an extra expense option. That right there is the professional gunsmiths opinion of it. They don't bother unless they run out of windage or elevation - lap to check - fire again and see if it helped. Sometimes, not. It wasn't the problem in the first place.

What some do use the squaring tool for is when the barrel nut won't go to the index to pass a gas tube under 85 foot pounds. I've done that, too - a few turns lapping and try again. The nut lined up and things were fine without worrying I stripped the threads.

As for corrosion of steel and aluminum: If there is a worst case situation out there, take a look at your alloy wheels mounted on a steel hub with steel lug nuts. Even after years of winter salt you still get them apart, install new tires, torque them down, and don't have a care in the world. Very few even bother with grease or antiseize on the lugs or add any protective coatings to prevent corrosion. Nope, just load up the family and whizz off at 70 mph around corners with just five rusting lug nuts holding a 25 pound wheel assembly on to force a 3,500 pound vehicle to change course.

And yet there are worry warts who say that the AR will corrode and even fail if we allow the barrel nut to come into contact with the upper. I say "Chicken Little." There are M16's in service with the AF with low serial numbers that have never been rebarreled - Montagnards in the highlands of Vietnam still shooting their M16's with broken handguards - nobody has any "Warning of Use" messages out for the last 45 years.

But surf the net and you will find a couple of hundred recalls on lugs, stud, and wheel bearings. Yet most people do nothing about preventive maintenance.

You tell me whether a weapon that is likely never to be used in self defense should get an extreme preventative in assembly, or the cars you put millions of miles on that can kill you if a wheel fails.

I see a huge lack of perspective about it. It's just an internet argument to see who's the primary monkey dancer.

Ook Ook

 

[zukiphile]

As for corrosion of steel and aluminum: If there is a worst case situation out there, take a look at your alloy wheels mounted on a steel hub with steel lug nuts. Even after years of winter salt you still get them apart, install new tires, torque them down, and don't have a care in the world. Very few even bother with grease or antiseize on the lugs or add any protective coatings to prevent corrosion. Nope, just load up the family and whizz off at 70 mph around corners with just five rusting lug nuts holding a 25 pound wheel assembly on to force a 3,500 pound vehicle to change course.

I understood the problem to be between bare aluminium and stainless steel.

Aren't aluminium wheels painted?

Aluminium body to steel frame contact is a problem in old range rovers.

 

[MarkCO]

All aluminum wheels are coated and the steel they are contacting is mild steel, not stainless. Not even close to a similar condition and tirod, for being so sure of his opinion, should be ashamed of the analogy he proffered.

Most 7075 uppers are pretty good and have a square facing already. If you have a 6061, well, not worth even messing with it.

 

[rickyrick]

I think you all have confirmed my suspicions, it is not seeming to be a needed practice in most cases. The tool is cheap enough to have around as one's AR collection grows.
I was a heavy truck mechanic for quite some time, when aluminum wheels were used next to steel a non-conductive shim was place between then. I went to school and changed careers some time back. I am now an electrical technician, I have some knowledge on the subject, it is a possibility, but is not easy to predict corrosion.