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Aftermarket handguard not flush with M4 receiver


Dankbiz
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Hey guys, was wondering if anyone could let me know if this is a potential safety hazard or not.

I ordered a new quad rail polymer handguard from FAB Defence for my M4 which, for a "custom" design specifically for the benelli M4, did not fit right out of the box. After several hours of sanding down a few areas I managed to get the guard to fit nice and snug and have it snap on with ease. The magazine tube cap finishes the job at fitting the guard as well as it can fit, and the guard itself comes with 5 screws which I screw in at the end.

The potential problem I see now is that the bottom lip is meeting with the M4's handguard cup/slot before the rest of the handguard itself can be flush with the receiver. This results in a gap of around 1mm, preventing the barrel from going "all the way" into the action. It sits over the bolt carrier well and does not prevent the bolt from doing it's job. Switching back to the original handguard I see virtually no difference in functionality, but that 1mm gap is properly closed.

Considering this area is where the shot shell goes *boom*, am I or the M4 in any danger with a compromised fitting?

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Edited by Dankbiz
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My concern is the seat ring on the barrel being seated fully against the receiver. From your photo, it appears to be a significant gap between the seat ring and the receiver. If this gap isn’t closed, you’ll likely experience failure to fire situations since the bolt will not be fully engaged. So any strikes from the firing pin will be minimal. 
 

The overall length of the handguards appear to be preventing the barrel to seat fully. You would have to locate where this is and continue to shave the handguard down to permit a proper seating. You’ll want to compare the seat depth to the oem handguards so you know what to aim for. This is one of those areas where if you remove too much material, it will be just as bad since the handguards will no longer seat firmly. This seating stabilizes the barrel as you ratchet the barrel nut down. 

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21 minutes ago, StrangerDanger said:

My concern is the seat ring on the barrel being seated fully against the receiver. From your photo, it appears to be a significant gap between the seat ring and the receiver. If this gap isn’t closed, you’ll likely experience failure to fire situations since the bolt will not be fully engaged. So any strikes from the firing pin will be minimal. 
 

The overall length of the handguards appear to be preventing the barrel to seat fully. You would have to locate where this is and continue to shave the handguard down to permit a proper seating. You’ll want to compare the seat depth to the oem handguards so you know what to aim for. This is one of those areas where if you remove too much material, it will be just as bad since the handguards will no longer seat firmly. This seating stabilizes the barrel as you ratchet the barrel nut down. 

Understood. Really appreciate this insight, definitely gonna try shaving it down further. Would suck if it ends up being unusable, but it wasn't too expensive of an investment

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On a side note, I noticed you have a Vortex StrikeFire with the cantilever mount. That mount is likely approx 1.5" from center of optic above the picatinny rail. That is really high for a shotgun and that height is for AR-15s. You would be much better off using the lowest mount can can possible get for a decent cheek weld.

I use a Vortex SPARC with the lowest Bobro mount and I had to adjust the comb of my stock up to get a good cheek weld. How are you currently going about getting a good cheek weld with that current setup?

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1 minute ago, NineInchNails said:

On a side note, I noticed you have a Vortex StrikeFire with the cantilever mount. That mount is likely approx 1.5" from center of optic above the picatinny rail. That is really high for a shotgun and that height is for AR-15s. You would be much better off using the lowest mount can can possible get for a decent cheek weld.

I use a Vortex SPARC with the lowest Bobro mount and I had to adjust the comb of my stock up to get a good cheek weld. How are you currently going about getting a good cheek weld with that current setup?

True! I haven't had a chance to take it out to a range yet with this setup and admittedly it's a poor configuration for resting my cheek against the stock. I had a micro red dot before this strikefire but thought it'd be fun to have a configuration that allowed for the use of a magnifier. Again I'm aware it's not exactly conventional nor necessary for a shotgun, but I thought it'd be fun to try for long-ish range target shooting with slugs

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13 hours ago, StrangerDanger said:

My concern is the seat ring on the barrel being seated fully against the receiver. From your photo, it appears to be a significant gap between the seat ring and the receiver. If this gap isn’t closed, you’ll likely experience failure to fire situations since the bolt will not be fully engaged. So any strikes from the firing pin will be minimal. 
 

The overall length of the handguards appear to be preventing the barrel to seat fully. You would have to locate where this is and continue to shave the handguard down to permit a proper seating. You’ll want to compare the seat depth to the oem handguards so you know what to aim for. This is one of those areas where if you remove too much material, it will be just as bad since the handguards will no longer seat firmly. This seating stabilizes the barrel as you ratchet the barrel nut down. 

Took some fine detailing but we got it flush. Thanks for the insight dude!

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5 hours ago, M2_shootr said:

Awesome.....how much material did you have to remove?

Comparing it to the oem plates, the difference in lengths at the bottom slips was just about that 1-2mm visible gap that i ended up shaving down. I also had to deepen the canal of the FAB handguard's custom lower lip where it sits on the handguard slot on the receiver.

Fitting one guard plate at a time onto the frame, they would eventually both allow the barrel to meet with the receiver successfully. However putting both plates together and trying to close that gap was still a problem. So finally, i discovered the inner walls of the FAB guard also needed some sanding down as they were holding on to the barrel's seat ring, not allowing for it to push onto the wall of the receiver.

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