It *doesn't* use a rifle round. 5mm has "low power PDW cartridge" written all over it. If you consider it to be the same as the real-world 5mm Remington Rimfire Magnum, it delivers comparable energy to ye olde 9x19mm, being a much smaller bullet at much higher speed which translates to penetration at the cost of cutting a smaller wound channel. It is a low powered round optimized for low recoil and good armor penetration (this also explains why the "Minigun" doesn't knock you over when you fire it). 5mm is in the same general area as the tiny rounds used by the H&K MP7 and FN P90. Part of the problem is that, unlike in the real world, video game characters don't fear getting shot or feel pain like living things do. In the real world if you fire your PDW popgun at somebody and hit them half a dozen times with its pathetic little bullets, they are probably going to leave you alone because they are now seriously injured and will die without medial attention. Video game characters have no self preservation or pain which somewhat devalues these things.
It's the Fallout universes 5mm and it is a rifle round as supported by both the fluff and the game mechanics. Look at the breakdown of bullets in the crafting.
http://fallout.wikia.com/wiki/Fallout%3A_New_Vegas_crafting
It has one unit more powder and one unit more lead than 5.56. In fact if you follow the in game crafting logic (looking at the 9mm to 10mm to 12.7mm progression) 5mm weapons are suppose to be more powerful than 5.56 because they consume more resources.
Assault Carbine is not out class by any but a unique SMG; but even that doesn't have the DT bypass as good as Assault Carbine.
With a better spread, you can easily get Gecko's head within you character's vision; while with SMGs (except for Vance's) you better off aim for center of mass.
Because of the way the game engine handles spread as someone else said in the first topic even a 1.5 spread is huge. A slightly less grievous miss is still a miss.
Vance's submachine is pretty much lead to you by the main plot. It has to be as since even though it is unique it's still a 9mm submachine gun. Even the stock 9mm machine gun only does one point less damage, shows up earlier, is more available, and the assault carbine is over 4 times the value of it...for what?
DT bypass is only available through AP ammo and is irrelevant for critters like Geckos as they have no DT.