M4 barrel attachment question

[Laser633]

Hello, all.  New here, and new to Benelli in general.  Sorry if this question has already been asked; I searched a bit and didn't find anything.

I just picked up a new M4 shotgun.  Very excited, I've wanted one of these for a few years now.

I was going through the assembly process, and I hit a roadblock at the very end.  The barrel does not seem to seat flush against the receiver.  At first, I thought that the plastic hand guards were causing the problem, so I tried assembling without them.  No luck.  I looks like the barrel stops sliding into the receiver about 2 or 3 mm before the lip meets the receiver.  This happens with the bolt forward, locked back, or even removed entirely.  The action cycles by hand just fine, so I don't think there's any interference between the bolt and barrel assemblies.

Still, every how-to video I've watched shows the barrel sliding down flush with the receiver easily.  Attached pic shows the best I can get, with a clear gap (I think that lip should be flush with the receiver?).  As I said, the action cycles by hand wonderfully, but I'm reluctant to put a round in there because things are, apparently, not fitting properly.  Am I missing something?

 

[NCswamp13]

Check this area. It should sit flush against that seat. 

[Laser633]

Yep, that looks like the spring in the magazine?  That all looks good, here. 

I'm trying to slide the barrel down over the magazine tube and things seem to hang up right before what I think should be the bottom.  It's not the rings that go over the magazine, and I'm trying this without the hand guards.  That said, watching closely as I slowly close the bolt, it does look like it engages with the end of the barrel, rotates slightly, and slides inside. 

Still...I'm wondering about the gas system now, whether something is hanging up inside.

[Unobtanium]

One randomly wild possibility is that one of the washers on the rear part of the rail was forgotten. Without this lock washer in place, the bolt retaining the top rail will protrude and impinge against the barrel extension. 

[SnidelyWhiplash]

On 6/30/2022 at 9:13 PM, Laser633 said:

The barrel does not seem to seat flush against the receiver.  At first, I thought that the plastic hand guards were causing the problem, so I tried assembling without them.  No luck.  I looks like the barrel stops sliding into the receiver about 2 or 3 mm before the lip meets the receiver. 

Have you tried inserting the barrel without the gas pistons installed? Perhaps that would help you locate what part of the barrel assembly is the problem.

My M4 barrels all fit snug the last millimeter or two inserted into the receiver but not a “something metal is in the way” kinda snug. While inserting the barrel tang into the receiver I’m usually slightly twisting, jiggling, and firmly pushing the barrel towards the receiver; it sounds like you are doing the same.

I just recently picked up my M4 Entry model that had been awaiting ATF Form 4 approval for one year and one week. I purchased it new from a high-volume Benelli dealer so I’m guessing it was manufactured about 16 months ago +/- . This newer M4 Entry is significantly tighter in fit and finish than my other M4’s (also purchased new). None of my M4’s are loose but this M4 Entry doesn’t go together nearly as easily. Maybe a couple of range trips with slugs will help this shotgun adjust and get broken in.

Edited July 2, 2022 by SnidelyWhiplash

[StrangerDanger]

Remove the rear most top rail screw and recheck to see if the barrel will seat. If the tip screw protrudes from the receiver on the inside, it will obstruct the barrel from seating. 

[Laser633]

Thanks for the tips, everyone.  The rear rail screw does not seem to be the issue (I was hoping that it would be that easy, though)

I removed the bolt assembly and tried watching as I slid the barrel in.  On the part of the barrel group that slides into the receiver, the part that the bolt slides along, on the very back end is a small tab.  This tab matches with a small slot machined into the receiver.  That seems to be where the hangup is, the tab does not fit into the slot.  In fact, they seem surprisingly mismatched to my eye.  I'll try to get some decent pictures later tonight.

[RxArms]

Might seem like an obvious suggestion but make sure your magazine tube is fully seated in the front ring… sometimes it can get hung up on the front mag barrel ring but would be more than 3mm… 

so I do not think this is the issue… but ya never know.

Also, you can try screwing the mag cap down to tighten it, see if you can get it flush.  My newest M4 takes some mag cap pressure to seat proper.

Edited July 3, 2022 by RxArms

[StrangerDanger]

Please get a picture of the tab kind of like how NCSwamp13 shows it. 

[Laser633]

Thank you for the help everyone.  I figured it out.

It was that tab and slot that were hanging up.  I carefully filed each side of the tab two or three times, probably taking off a couple thou at most, cleaned it up, and put a very small amount of oil on it: just a light glaze, being sure to wipe off the inside surface.  When I tried to put the barrel back on, it was still tight but didn't feel quite like such a hard metal-to-metal stop as before.  A few taps on the end of the barrel with a rubber mallet and everything eventually slid right into place!

Action works great by hand, and everything seems great.  Now to take it to the range and break it in!

Thanks, again, everyone.  Great community, here.

[RxArms]

Amazing that the machining tolerances are this bad on a new M4 shotgun.. glad you got it sorted.

Edited July 4, 2022 by RxArms