If you have one of the Mossberg shotguns with the shorter action tube, all factory 14" SBS M590A1s and most 3.5" chambered models, you have have been disapointed when you found out the MOE forend dosn't fit.  People with 3.5" chambered guns have options, the easiest being to just order the longer action tube, but if you have a SBS the longer action tube won't even fit on the gun.  Magpul still has the only mounted photo on the Mossberg MOE page showing a M590A1 SBS with the MOE forend despite the fact that it is not a direct fit, not even close.

I modified two MOE forends last week to fit my two SBSs once I got the measurements and thought it through.  First off, the forend needs to be relieved at thee rear to clear the tension spring between the tube itself and the bars.  As shipped, the action tube won't even slide all the way in the MOE.  It stops about 1/8" short of the molded in ribs of the MOE.  Clearancing the MOE on both sides lets the action bar slide all the way up against the ribs like it is supposed to.  This should be an issue on any 500/590 series; 14", 18", 20"... and was mistake on Magpul's part.  To address the length I first used a 1 1/2" hole saw to to the rough work.  It is a very snug fit and was pretty much self centering.  It bottomed out after cutting in 1" so it got me most of the way there.


I then used a Dremel cut-off wheel to start removing the center portion.


Once the rough work is done you need to remove the remainder of the ribs and make the inside profile match the front of the MOE so the anti-rotation insert will slide in.  A combination of Dremel cut-off, router style, and sanding drums will work best.  My 22mm Craftsman socket was a very snug fit in the MOE and worked very well as a guide to make sure all the ribs ended up even.  You actually need to remove 1 1/8", the difference between the 6 1/2" and 7 5/8" action tube, for this to work best.  Once this is done you can isert the correct anti-rotation insert to match your barrel profile and tighten up the nut.  I used the orange MOE for the in-process photos because it is much easier to see the work but i did one black and one orange.


The bottom black MOE shows the gun in battery and you can see how the MOE actually cover the barrel lug just like the 16" barrel in Magpul's promo photo.  It was worth the work to me but I just wish magpul would have considered this before they went into production.  If they had just molded the forend this way to begin with they could have just included a 1 1/8" tube section to use as a spacer on the longer action tube guns.

Thanks for this. I have a 14" 590A1 coming one of these days and would like to install a Magpul forend. I will refer to this when the time comes. Hurry up ATF.

I wasn't aware it wouldn't fit on the SBS. It looks like the photos are all of the front end cutting back the 1 1/8" required to get the anti-rotation ring in the right location. Can you post a photo of the relief required at the rear of the forend to let the forend slide on? I can't really visualize what you are describing. Thanks.

Look at the photo looking down on the tube.  You will see the flat spring that goes over the tube and under the end of each bar.  The forend just needs to be clearanced for the ends of the spring to have room.

After some searching I found this photo on another forum. He said he removed all the material from the rear and the MOE doesn't cover the mag tube cap. Brazos, did you make the mod at the front for a specific reason? Did you consider removing the material from the rear?

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Multiple reasons actually.  The MOE is molded with taper at the contact point to match the action bar tube at the rear and on the anti-rotation spacer to match the nut.  The rear of the MOE also has a multiple step profile the clear and contact the action tube bar.  As mine sits now when the action is back the MOE is only 5/8" away from the receiver.  If I had removed the 1 1/8" from the rear of the MOE it would be going back over the receiver 1/2".  This would require further opening up the inside of the MOE because the receiver is wider than the widest part of the action bar assembly.  If you have a side saddle on the receiver a large chunk would need to be cut out of the MOE to clear it and it would look like crap.  It does not look like he is using the anti-rotation spacer either.

Thanks for the explanation. It makes sense. I have an e-mail in to Magpul to ask why they don't allow for two different length action slide tubes, especially since the more "tactical" Mossys have the short assembly. Hogue and Surefire seem to have got it right with the use of a spacer or a longer nut to secure the forend.

Brazos, how did you confirm that the 7 5/8" action slide tube won't fit on the SBS? It seems like it should fit since the additional length of the longer action slide tube would be inside the MOE forened, and the forend is in the same position relative to the rear end of the action slide tube and mag tube.

Does the question make sense? If the MOE is in the same position relative to the mag tube, why doesn't the longer action slide tube fit? Does it interfere with the foend running fully forward?

I've had multiple factory 14" M590A1 Mossbergs for fifteen years, I am very familiar with them.  The action bar tube nut on a factory 14" Mossberg M590A1 sits only 3/8" away from the barrel lug when the gun is in battery.  The action bar tube that is 1 1/8" longer will physically fit on the gun but the gun would not be able to go in to battery.  The bolt would stop 3/4" away from the chamber and the gun will never function because the action bar tube nut will impact the barrel lug.

Thanks. Modding the MOE looks like the only option.