The characteristics of the guns in MW2 have very little to do with real life.
The UMP is a short-barreled pistol-caliber carbine. It's effective range would be around 100 yards, maybe 150, not all the way across Wasteland. The UMP fires the 45ACP cartridge, the same cartridge as the USP pistol.
By comparison, the real-life SCAR and FAL fire the 7.62x51 cartridge, which is lethal at 1000 yards and more.
The M4, TAR and F2000 fire the 5.56x45 round, which is effective out to 600 yards or so.
The shotguns all fire 12 gauge shells, and aside from the short-barreled Ranger, they would all have about the same effective range in real life, which would be considerably farther than the range of the SPAS in the game.
The Barrett fires the 50BMG cartridge, which was designed for use against armored vehicles. In real life, a single shot is devastating to individuals, even at ranges out to 2000 yards. No stopping power required.
The UMP's cartridge has about 500 foot-pounds of energy. The Barrett's has about 13,000 foot-pounds. There's no comparison in real life.
The developers have to tweak the damage of the guns to insure balance and playability, not realism.
Sorry for the long post, but I like the real things as much as MW2.
