For my Marine Security Guard assignments to the U.S. Consulate (Embassy now) in Panama City and the U.S. Embassy in Baghdad, we had our M1014's (M4's) equipped with the stock Benelli split handguards. They were set up that way for a reason and they worked 100% without fail, as Benelli had intended. The stock handguards may not be "high speed" or have lots of accessory mounting options, but they are actually the best thing for the M4 especially if it can protect the Agro dual piston gas system in both Central America and the Middle East. My detachments were not always indoors with the rest of the State Dept personnel and Contracted Security crews, as we routinely trained with national & local forces (and even other countries' forces) in their varied environments with our issued gear. Our M1014's handled everything like a champ, even when we had a couple of actual incursions by militants and insurgents. For OIF and OEF (& to some extent, OIR), the M1014/M4's stock handguards took on and survived some of the harshest operating conditions and environments and still performed outstandingly well in protecting the dual pistons. Although I was equipped with my M240B, we saw "teams" with their Benelli's looking like they've been through literal #ell but they were still "taking out the trash" with them. As MSG, the only thing we couldn't do with our '1014's was modify the stock set up and functionality of them, lest we'd potentially cause an international incident. That should tell us all something about the stock handguards, as they just simply work and do their designed job. And that's to provide us Marines a perfect ergonomic place to grip to shotgun while we buttstroke the enemy in the teeth... (per DoS, DoD, and RoE regs, of course)
(sorry about the wall of text and rant)
I'm currently using the Agency Arms handguard on my M4, as it is a backup HD shotgun that needed a WML and a foregrip for my Dad. I can see myself with another M4 very soon as a "slick" SBS w/ stock split handguard & stippling.