It's not actually a rifle, it's a carbine. A carbine is a intermediate length weapon designed for intermediate ranges (generally 150 yards or less). For close quarters, rifle caliber bullets overpenetrate and can cause a lot of collateral damage (going through walls and whatnot). Also, the recoil is a lot higher with a rifle caliber bullet. Take the MP5 for instance, most SWAT teams and whatnot use these because of the low recoil and the good stopping power of the 9mm. Also fully loaded magazines weigh less with pistol bullets (doesn't matter in the HP really because of the 10rnd capacity, but you asked about them in general).
IMO the perfect weapon for HD would be a semi-automatic .45ACP carbine. Highly accurate, higher bullet FPS with the longer barrel, lots of stopping power, and not as much penetration through to your neighbor's house.