Typical gold-plated overkill for the mission. When I was a grunt, I never did care very much for the M-16. I carried the M16A1 model. It was too fragile (damned plastic stock would break all too easily), too easy to jam (manufactured with very tight tolerances), and the bullet was just too light. I remember firing tracers on the night range and seeing the bullets get deflected about 20-30 degrees after hitting a target.

For close range combat, I thought something like a modified Ruger .44 Magnum carbine with a detachable magazine and selective fire would be just fine. A folding stock would be even better, especially in tight spaces. Straight sided cartridges like the .44 tend not to jam as much as bottlenecked cartridges, and a carefully selected hotload and bullet selection should make the weapon effective out to 200 yards, perhaps more. The big, relatively slow bullet (compared to the 5.56mm M-16 round) has great pentration power through brush, walls, and a lot of body armor. If you need something effective to longer ranges (and hey, there is something to be said for the ability ro really "reach out and touch someone"), then a bottlenecked cartridge in the .243 to 6.5 mm range would probably be a better choice.