If there were a difference in barrel lengths, I might buy it, but that's usually not the case or it's often the wrong way around. In any case, unless there is a very significant increase in barrel length, there would be only a marginal increase in velocity and even less in damage (depending on the caliber). The ability of the "named" weapons in Fallout 3 to do as much as twice the damage despite being outwardly identical to their nameless counterparts, and even having interchangeable parts, is what prompted my post. I don't know where poolofpoo got that hollow point bit, but I'd say that a gun magically converting the bullets into hollow points is even more reality-bending than magically making them go faster. I'll buy accuracy, I'll by reliability, I'll buy rate of fire, but firearms can't magically make a bullet go faster or do more damage.
The .223 pistol and rifle in Fallout are special cases in that the differences in barrel lengths could potentially change the damage as much as they do, but they did it completely backward. Most of the damage potential of the .223 round comes from the fact that its very high velocity causes the tiny bullet to fragment upon striking its target. There is a specific velocity at which the bullet can do this, but I can't remember it off the top of my head. I know however that the M4 loses about 200fps from the M16 for its six inches of barrel, and the reduction would be increasingly greater for every inch lost from that. They say the minimum length for an AR-15 to have the fragmentation aspect is 10.5 inches, which is a lot longer than the .223 pistol in Fallout appears to boast. The .223 pistol should do much less damage but does much more.