I agree with Darth on this one. When I was issued one, the HK MP5 became my favorite to bring along on high risk operations. It only slightly edged out my previous favorite which had been the Remington 870. I imagine the fact that it was select fire (semi-auto or three round bursts) and that I could easily carry several spare magazines each holding 30 rounds, that it was a better size for tighter places and that it never once bruised my shoulder all helped have it win me over.
Now if I was stuck within the parameters of your game, it must indeed be a game because I would not be doing it your way in the real world, I would choose a Ruger Model 99/44 Deerfield carbine with 20 round magazines (remember this is a game so I am choosing my fantasy mag size). My ammo would consist of both hollow point and solid point rounds. In the hollow points I would likely opt for something like a 180 grain jacketed hollow point. As for the solid point, I would go with a 300 grain semi jacketed round. I would carry several additional magazines, some containing the lighter weight HPs nd some with the heavier solid point rounds. Yes I would also carry a pistol, probably two of them as backup weapons. They would likely be exactly the same as one another or at least each would be capable of firing from the exact same type of magazine as the other. A pair of Glock 19s come to mind and so does a brace of Glock 21s. Again, I would carry a mix of ammo for the pistols. Some mags would be loaded with 115 grain HPs and others with 230 grain FMJ ammo. Most would hold the HP ammo but there would be at least one with FMJ.