What do you put on barrel extensions?

[Rifleman_04]

The Certificate of Analysis will be reflected to state: “Shelf life of AeroShell Grease is 6 years from the date of manufacture.

Sure but does it go bad? Do I have to disassemble my guns every 6 years and reapply with new?

[lysander]

All while not saying a thing about greasing the forward facing side of the barrel extension flange which is a critical step to miss in the process. Though oddly, it's also not mentioned in any manual.

It is a critical step, depending on what part you have in the vise.

If you have the vice in the barrel (per the manual), greasing the front flange doesn't matter. Turning the barrel nut cannot turn the barrel, it is held by the vise, however it can turn the upper, but only through the friction from the greased threads, which is relatively small. Because the transmitted torque is small, the load on the index pin is minimal.

However, if you are holding the upper in the vise, turning the nut will transmit torque through friction with the barrel extension flange to the barrel, which in turn will put a pretty good load on the index pin. Greasing the flange will reduce this considerably.

[lysander]

Sure but does it go bad? Do I have to disassemble my guns every 6 years and reapply with new?

"Shelf life" = how long it can sit on the shelf and still be good.

The grease on the threads is only there for assembly. After everything is torqued down, the grease no longer serves a purpose*. You could put the whole thing in an oven at 150 degrees for two hours and evaporate all the grease out and you would be fine.

________________________
* Depending on how long it has been assembled, it might make disassembly easier.

[lysander]

Using antiseize will increase removal torque vs grease for a given installation torque, I think. Anecdotally, that effect seems more significant on threads larger than 5/8” in diameter for some reason. Maybe one of our engineers can illuminate this topic better.

Lubrication of threads allows for higher clamping loads for the same applied torque, as more of the torque is used to stretch the bolt rather than overcoming friction.

Over time, the oil in grease or anti-sieze evaporates off, giving you and unlubricated joint. Because the clamping force is higher, it will take more torque to loosen. (You might see what appears to be grease residue on the bolt, but that is just the thicker, the oil has boiled off.)

[Rifleman_04]

"Shelf life" = how long it can sit on the shelf and still be good.

The grease on the threads is only there for assembly. After everything is torqued down, the grease no longer serves a purpose*. You could put the whole thing in an oven at 150 degrees for two hours and evaporate all the grease out and you would be fine.

________________________
* Depending on how long it has been assembled, it might make disassembly easier.

Any change in resistance of galvanic corrosion?

[lysander]

In the Army we used a high temp silver colored anti-seize on the tank exhaust grill door bolts. That stuff works good on threads and holds up good to high heat. Might be good on AR barrel installation. Anyone who has walked behind a running Abrams tank knows the heat it puts out. I've seen them start grass fires. I saw one pivot steer in a circle in the middle of a riot to clear everyone out when they were blasted with exhaust.

Silver anti-sieze works in high temperatures, because after the oil has evaporated, or boiled off by the heat, it leaves both surfaces coated in a thin layer of silver, and silver-on-silver is very slippery, and keeps both surfaces from corroding.

The same reason you see a lot of silver plated screw and/or nuts for high temperature applications.

[lysander]

Any change in resistance of galvanic corrosion?

Most of the galvanic corrosion protection is handled by the anodized layer.

Anodizing converts the outermost layer of the aluminum into aluminum oxide, which is a great insulator. No electrical connection, no galvanic corrosion. Unless you have a very large break in the anodized finish where it is in contact with steel, you are fine.

EDIT:
Anodizing is why you do not worry about steel hammer-trigger pins in aluminum holes, or all those other steel parts attached to the aluminum uppers and lowers . . .

[556Cliff]

It is a critical step, depending on what part you have in the vise.

If you have the vice in the barrel (per the manual), greasing the front flange doesn't matter. Turning the barrel nut cannot turn the barrel, it is held by the vise, however it can turn the upper, but only through the friction from the greased threads, which is relatively small. Because the transmitted torque is small, the load on the index pin is minimal.

However, if you are holding the upper in the vise, turning the nut will transmit torque through friction with the barrel extension flange to the barrel, which in turn will put a pretty good load on the index pin. Greasing the flange will reduce this considerably.

100% correct to the blue highlighted paragraph.

But to the red highlighted part I will still say that greasing the forward side of the barrel extension flange is critical no matter if you are using a Reaction Rod type tool or upper receiver vise blocks.

You are correct in pointing out that when using barrel blocks (as per the manual) or a Reaction Rod that: "Turning the barrel nut cannot turn the barrel, it is held by the vise, however it can turn the upper, but only through the friction from the greased threads, which is relatively small. Because the transmitted torque is small, the load on the index pin is minimal."

However, the barrel nut will still bind up on that un-lubed barrel extension flange. If you have a timed barrel nut, this binding will prevent you from advancing the nut and gaining alignment for the gas tube. Also, any binding there will have your torque wrench indicating a torque value that isn't actually fully reaching the threads of the upper receiver because the barrel nut is seized up on the dry flange. So in my view, greasing the forward facing side of the flange is critical for a few reasons, no matter the tool you're using to hold things in place.


Edit: I also need to again point out that although the transmitted torque when using the Reaction Rod may be minimal to the indexing pin, that it is still enough to cause the indexing pin to embed into the side of the indexing pin notch (with everything properly lubed) even at only 30 Foot Pounds of torque on installation. As shown in the picture below... I can imagine that if I had slathered the rear side of the barrel extension flange and the entire portion of the barrel extension that slips into the upper socket with grease that the damage may have been worse.

[556Cliff]

And here's what 70-ish Foot Pounds of torque looks like when using upper receiver vise blocks if you only lube the threads and forget to lube the forward side of the flange.

[tangolima]

If you have the vice in the barrel (per the manual), greasing the front flange doesn't matter. Turning the barrel nut cannot turn the barrel, it is held by the vise, however it can turn the upper, but only through the friction from the greased threads, which is relatively small. Because the transmitted torque is small, the load on the index pin is minimal.

However, if you are holding the upper in the vise, turning the nut will transmit torque through friction with the barrel extension flange to the barrel, which in turn will put a pretty good load on the index pin. Greasing the flange will reduce this considerably.

I don't think it is correct. Action and reaction. The barrel nut mates with upper receiver. When the nut is torqued, there must be equal and opposite torque (minus thread friction) applied to the upper to keep it from turning. The question where is that counter from.

Clamping the barrel, including action bar, the counter torque comes from the index pin. Not good. Clamping the upper. The origin of the counter torque is obvious. Definitely not through the pin. Good.

I understand the army manual specifies clamping the barrel. For what I don't know. But it doesn't make sense. As an employee of a defense contractor, I worked with government officials on occasions. Despite their technical titles, not all of them know what they are doing.

-TL

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