I have an email in to Foxtrot Mike but I figured I would tap the hive for info while I wait. Ordered a 7 inch complete upper from aim surplus. When it arrived, I removed the thread protector to install the included blast diffuser. As soon as I put any torque on the diffuser, the barrel rotated in the upper about a 1/4 turn. I thought to myself “shit, there goes the indexing pin”.

So I removed the hand guard and barrel nut only to find that there was in fact no pin on this barrel. I re installed everything according to the instructions on FM’s website and proceeded to tear shit up with my new blaster. After about 250 rounds, I noticed that the barrel was rotating in the receiver again only this time I could grab it by hand and rotate it. Back to the workbench, I disassembled and inspected everything and can’t find any visible defects in the barrel, receiver face or threads, or the barrel nut. Retorqued the nut and installed the hand guard, everything seems tight  again. 20 more rounds to test fire, it maintained zero and so far I can’t rotate it again.

So by design, the handguard should prevent the barrel nut from loosening up unless I’m missing something. Looking for any ideas as to what would cause this phenomenon. I am running the upper on a foxtrot mike lower, 7.5 oz buffer with .308 carbine spring.

Help me arfcom, you are my only hope. At least until FM gets back to me.

Well bummer... my new FM upper just arrived today... My blast diffuser came attached though...

Not sure why you'd shoot it if you thought there was an issue.... now that said, no gas port alignment to worry about, head space would only care about fore/aft movement, and it didn't go kaboom on you as you put lots of ammo through it



I sent a general inquiry to FM early Saturday evening.... Mike Noonan replied around 930pm.  Think you can wait for them to reply to ya...

I didn’t have any fore and aft movement and everything tightened back up. Just not sure what would cause everything to loosen up as the barrel nut absolutely cannot rotate once the hand guard screws are installed. And there is no real orientation of the chamber required.

So there's no feed ramps on the bbl?

Negative just a concentric cone shape or bevel around the circumference if you will.

In my experience, 9mm barrels usually have a 360 degree funnel feed ramp and may or may not have an indexing pin.  Perfectly normal.  

A rotating barrel is definitely not normal.  Sounds to me like the barrel was not torqued down properly at the factory. If it is remaining tight after you've torqued it down, I would say you have fixed the problem.

Kind of the direction I was leaning. I had to shim the nut after the first time I took it apart to get the hand guard holes lined up. Everything seemed tight but after shooting it, is when I was able to grab the barrel and spin it by hand in the upper. I put new shims in it and torqued it down pretty good this morning. Haven’t heard back from FM yet so I will wait and see what they say before I try it again.

this...

Update time. FM got back to me and confirmed that the barrel nut should just need to be re torqued. Offered to send me another barrel nut, or the opportunity to send the upper to them so they could repair it. I opted for the barrel nut as I have everything needed to do it.

I put new shims on the barrel nut as mentioned and ran another 100 trouble free rounds with no issues to speak of thus far. I will replace the barrel nut with the new one if the issue persists. FM customer support is great.

My barrel has a pin, std guard set and front sight tower.. I also used an old blowed out gas tube to pin the barrel to upper like a gas gun.. cut tube flush with inside of upper