I have a Magpul fitted shotgun as well, but find the Magpul feels better with a short a LOP. (So tempted to get a 18.5" 590A1 and equip it with FDE Magpul furn)
I like and prefer the factory configuration on my 20" 590A1 for use with a longer LOP.
I just can't gel with the MOE forend when the LOP is over 12" because the rear handstop sits at an odd angle against the side of my palm/wrist area. With a shorter LOP, the handstop functions as it should and rests against the full edge of my palm, under my pinky finger.
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I've kept my stock's LOP at the shortest length (12.5"?) Feels perfect, and like you said the forend handstop works wonderfully. You can hold the pump much further back than with the corncob style stock Mossberg forend.





















































