I use a DE M56A. As far as I can tell, metal inner barrels. Only a few pieces are metal, but they're important bits like the shell door and latch. The paint does come off VERY easily though, so don't scratch it even with a fingernail unless you want the paint coming off. Mine is a full size, solid stock version with a pistol grip. It's pretty damn large and has a similar loaded weight to a real unloaded Benelli M3 (I think slightly lighter, not even a pound of difference), which can be one major disadvantage in CQB. That said, I use it well.
Someone at the CQB field last time I played had the exact same shotgun, but turned it into a pseudo-breaching gun by sawing down the barrel as much as possible and giving it a pistol grip with no stock. It was certainly smaller, but it barely made 200 FPS. Partway through the day, he just ditched it and exclusively sprinted around the field with a Beretta with an extended magazine.