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.