Real world I would probably go with a Ak-47 or Ak4 (G3) since there would be easier to find ammo and spare parts. I live in Sweden so I guess Ak4 parts would be easier to find and we have a shitload of ammo for them (from 60-80 awaiting a Soviet invasion that never came.) In about 6 months I would go with a Ak5 (FNC) since I'm joining the military and that's the rifle I'll use there.
But much more interesting is the prospect of fictional weapons, and I have no idea how no one have posted this weapon yet, shame on you. PORTAL GUN! In a fight I't would be below average in performance but the thing I would use it for would be to rent a apartment in central London and another one in New York set up a portal between those two and charge 5 $ per transit, since there was 2,871,657 passengers between those two airports in 2008*. That would give me 14,358,285 $ per year and the cost of upkeep would be two apartments per year.
Considering British Airways (the marked leader) would charge 723 £ (approx. 1141 $) for a transit there would be almost no one that would choose to fly instead of using the portal. I would probably soon take over almost all trans Atlantic travel because of price and comfort further increasing my income. I would not show the portal gun to anyone.
*According to
Wikipedia.