IRL: Modified AK-47. Modifications are titanium armour and reinforcement, a detachable grenade launcher below the barrel and a retractable bayonnet on the side of the barrel. I choose the AK-47 due to it's respectable stopping and penetrating power, ease of locating ammunition, and it's overall reliability. Armour and reinforcement, along with bayonnet make it at least functionnable as a melee weapon (like a short spear) so if I run out of bullets then I can keep up.
Fantasy: Warhammer 40k Lasgun. Not the most powerful weapon in that universe, but it's still pretty powerful by modern standards, can be recharged my warming the clip up, and is scary dependable. Also, bayonnet. So yeah, I'll go with the trusty companion in either case.