wondering what holds them in. I was thinking green locktite.

 

I could be wrong, but I thought they were pressed in, nothing more.

press fit

This. It is only intended as an alignment pin although it does provide minimal shear strength.

If it's the barrel extension pin it's pressed into the threads, there is not a hole drilled into the chamber wall to make it fit any deeper. Nada.

Goes to torqueing off a flash hider without holding the barrel - some makers use glues to prevent disassembly and owners have reported twisting the barrel right out of the extension which destroys the headspacing completely. Not a Good Thing.

It also goes to some thinking that the barrel extension is torqued to spec to set headspacing. That's a commonly referred practice one gunsmith posted up years ago, but the whole point of having a barrel extension that screws on is to make setting the headspace easier without resorting to a gunsmith level practice. Colt assemblers on the line can do it as a bench procedure. Goes to owners screwing them right back off when they don't clamp the barrel down tight. You can strip the pin forcing it against the threads with less force than some think. Again, they don't drill a hole into the chamber wall - Bad Mojo doing that.

It's one of the set of very special things Stoner and crew did to make assembling the M16 a lot faster and cheaper than any other rifle - the barrel extension should be given a lot more credit for it's design in having the locking lugs in it, not the upper, because it makes assembly much less a skilled job and with a lot less headaches and takes all the stresses out of the upper doing the job.

I'm not sure I'd want a AR with the locking lugs machined into an aluminum upper if it could be done at all.

An interference fit.

The pin is .0005" bigger than the hole.

The index pin barely touches the threads, if at all.  If you examine the drawings, the pin normally sits just touching to .004" off the threads.  If the pin is on the long side of the tolerance and it is set in maximum depth, and the threads are at the maximum diameter, the pin goes into the threads all of .004".

So the pin is not 'supposed' to go into the threads, and .004" is nothing.



The method of manufacture, assembly and headspacing of the barrel/barrel extension used on the M16/AR was patented back in the 1930s by Melvin Johnson, back when Gene was around ten years old.

Actually, the time required to assemble an AR/M16 barrel to barrel extension is considerably longer than to assemble a M1, M14 or FAL barrel to the receiver.  With an M1, etc you screw in the barrel and when the top of the barrel is indexed correctly you stop turning, and your done.

To assemble an AR/M16 Barrel to the barrel extension, you torque the barrel to extension, put the assembly in a fixture and finish cut the chamber so .300" datum diameter is the correct distance from the front face of the locking lugs, then drill the gas port, knock in the index pin.  Then send it to the plater to be chrome plated.

The advantage of the AR/M16 barrel/barrel extension is that making it is cheaper, much, much cheaper than and M1/M14/FAL barrel and receiver.  The threads on an AR/M16 are not indexed, the threads of the M1/M14/FAL are indexed, that is they the start of the threads have to within a few degrees of a fixed datum point on the barrel the chamber has to be cut to very close tolerance so thread stretch and shoulder crush are accounted for.  The chamber of an AR/M16 barrel can be held to better tolerances for less effort than the other designs and is fixed, a barrel swap on an M16/AR, since in brings its own locking lugs, is a much easier and quicker job and can be done by a unit armorer rather than at fixed repair center or depot.

Also, the method of making the extension is a hell of a lot easier than an M1/M14/FAL receiver,  Most of the precision work is done at one time as a turning.


Aluminum is far to soft, you can put enough there to handle the strength aspect, but the pounding from the bolt would peen aluminum out of tolerance rather quickly.

If the original design was to have the locking lugs in the receiver, the receiver would not be aluminum.

The pin fell out of the extension. Pin lost, but hole is not drilled into the barrel threads. I'll have to look around for another pin and press it in myself if the hole isn't too big. If it is, I think green locktite might hold it. I don't remember where I got the barrel from, so I can't send it back. Wish I had a pin.

Brownell's

Midway

the usual places have them.

(and you'll probably find some other stuff you'll want to buy while searching for a pin....)

I had my pin come loose when I was swapping upper receivers a few years back.  I was able to get another pin (can't remember where - Brownells, maybe?) and tap it in to the barrel extension with a brass hammer.

Any rotation of the barrel extension on the threads of the barrel changes the headspace. This is why it's mandatory to use a barrel vice when removing a flash hider or brake.

The comment on the aluminum upper locking lugs should have been annotated with   Obviously that can't be done.

Finding a description of how Colt installs the barrel extension seems to be proprietary. I keep reading from gunsmiths about machining to a set depth and torqueing the extension. I also keep reading about home builders screwing them right off and in some cases it took less effort than installing the muzzle device.

Since screwing on the barrel extension is how head space is set, I'm thinking what Colt does and what gunsmiths in the field do are two different things. If the chamber was too long I don't see having to shorten the barrel repeatedly and retorquing it to check and set headspace as being a fast and efficient method. I'm thinking it's loose, pinned to the barrel, indexed for the gas port and then machined for ramps. The additional steps do take some time and effort.

At one time there were Colt assembly line photos posted on line which showed the barrel assembly bench, and I noted at the time the bench had multiple barrels, extensions, but the only working tools were the headspace gauge and a hammer for the pins.

If Stoner's team borrowed Johnson's barrel extension idea all to the good, it was being ignored by tradition bound designers who were all basically employed by shops using existing machinery. Hard to move forward chained to the past.

The method I described to install and chamber an AR barrel is how the drawing state to do it.

Once the barrel is torqued to the extension and chambered, the two are now considered a permanent assembly.  When the barrel is shot-out the extension is tossed along with the barrel.  This also theoretically doubles the wear life of the bolt.

It is easier to chamber a barrel from a physically fixed point or plane, like the plane of the locking lugs, than to cut a chamber to an imaginary point like where the lugs will be when the extension is screwed to a receiver, and ensure the gas port aligns at 12:00 +/- 1/4 of a degree.

USGI M14 barrels were only held to .005" installed headspace (not counting bolt variation), AR15/M16 barrels are held to .003" tolerance, (again, not counting bolt variation).

Even with the tighter tolerances, AR15/M16 barrels can be made cheaper, that's why AR barrel start at $100 and go up from there, M14 barrel start at $250 and go up from there.