To be honest with you, I can do more damage with my Glock 21 in a close quarters situation than I could with my M4. I am trained to react to active threat (shooter) situations both by Military and Civ law enforcement. I would much rather use my side arm than M4 when clearing a building. In my opinion, a rifle is no good up close and personal. I would use my M4 for targets between 50 and 300 yards out, and most buidings are not that big internally. I have an M4 to stay proficient with it. We do not shoot enough with the M4, maybe due to budget concerns, so I have my own. My range quals are in the top 5% of my unit with the rifle and top 1% with my sidearm. I fire my pistol a lot more.