Hey all.

I've searched online and while I've found a few mentions of this I didn't find anything concrete. So I'll ask yall.

While back bought one if those Colt 9mm smg parts kits off gunbroker. Built it on a decent lower and it shot great. Got home from the range today and when cleaning it I noticed the barrel nut was loose. I took the nut off and the barrel out to make sure there was no damage and to my suprise there is a very thick shim/washer between the barrel nut and "shoulder" that's machined into the barrel where the extension would be on a regular 5.56 barrel.

I'm assuming this is how it was assembled at the factory? The barrel is a Colt 9mm barrel with all the proper markings and the overall condition leads me to believe it has never had the taper pins removed or the fsb off.

The "shoulder" on the barrel that mimics the barrel extension shoulder is very thin and oddly has a smaller outer diameter than the shoulder on a standard barrel extension. The washer is actually thicker and has a larger outer diameter than the machined shoulder on the barrel.

Does anyone knows why Cold made the barrel with such a thin shoulder that pretty much requires the thick shim/washer? Also, would the torque be the same 30 to 80 ft lbs as a standard AR barrel?

I'm hoping one of the Colt gurus here can fill me in.

Thanks in advance.

Unfortunately, 9mm barrel nuts coming loose is not uncommon.

The shim is normal.  Why it is there, I don't know.

It is a strange choice. I've been searching online for info or exploded diagrams in the hopes of revealing the reason behind that design but no joy. People mention it with Colt and BCM 9mm ARs but havnt found an explanation. The shoulder on the barrel is so thin. It looks so weak. Especially for a blowback smg that originally was fill auto capable.

I'm just going to put her back together and keep enjoying shooting it but I'd love to know more about that barrel design. With the barrel nut just hand tight its clocked in a position that looks like once torqued (and the barrel nut would align with the gas tube if it had one) it will fall in the 50 to 60 ft lb range. Which is perfect. But that thin and tiny shoulder. Ugh!!!

For the Colt 9mm 'pencil' barrel upper, I (like others have done) have a cut down (to clear the bolt key) gas tube in place to keep the barrel nut from ever backing out, even though it's aligned and torqued to spec.
MHO, YMMV, etc.  Be well.

It has been a number of years back (maybe 10 or more?) and I bought a Colt 9mm pencil barrel from I believe Numrich for a build.  It too had the same issue.

From what I can recall there was a batch of these that were made incorrectly and needed those spacers. The Colt and BCM barrels were the only ones available at that time that were chrome lined.  Maybe the BCM were from the same supplier?

I actually ended up returning mine for a refund.

What I can recall was that the shoulder/lip on the barrel wasn't wide enough so what happened was the barrel nut would bottom out on the threads against the receiver before it hit the shoulder/lip on the barrel.

So the barrel wouldn't be held snugly and could move in the axial direction ever so slightly.  The spacer was simply a large washer that added enough thickness to allow the barrel to be torqued properly.

I've been reading the responses and without the washer the barrel nut would bottom out before the barrel was tight.

The shoulder is very thin and smaller in diameter than the shoulder of a standard barrel extension.

I can see the thickness having been screwed up by the manufacturer and turning them too thin but the diameter too? They must have really been having an off day with specs and their calipers. Lol.

Still, I wonder if there is some actual reason they did it this way. To keep the barrel nut tight without a gas tube? To combat some issue with straight blow back in the AR? One can only wonder as engineers are crack heads.

Either way it shoots fine and there apparently are many others just like it with thin shoulders and washers that shoot fine.