Go to Eldridge (New Reno Arms) and buy as many Magnums, Desert Eagles, Hunting and Assault Rifles, and FN FALs as you can afford. Next, go around the building, enter the back door and use the stairs to descend to Algernon. Let him upgrade all your freshly bought weapons, then go back on the same route and sell them back. Upgraded weapons worth much more than their regular counterparts, but Algernon upgrades for free (not to mention the extra clip)! The best is probably the Assault Rifle: aside from the weapon itself being more expensive, you'll get a fresh clip - which has enough ammo to fully load two regular ARs! I just chose these weapons as Eldridge always has some for sale. These are the available upgrades:

.44 Magnum (Speed Load) [reloading costs only 1 AP]
Desert Eagle (ext. cap) [clip capacity is 20]
Assault Rifle (ext. cap) [clip capacity is 100]
FN FAL NV [gives the same accuracy at night]
Mega Power Fist [more damage]
Super Cattle Prod [more damage]
Scoped Hunting Rifle [higher range, but accuracy penalty at close range]
Improver Flamer [unknown]
Plasma Pistol (ext. cap) [battery capacity doubled]
Magneto-Laser Pistol [better armor peneteration]
Laser Rifle (ext. cap) [battery capacity doubled]
Turbo Plasma Rifle [higher damage, NOT faster like in Fallout]

These are NOT upgrades, the two weapons are independent from each other:

HK G11 -> HK G11E
FN FAL -> FN FAL HPFA
Minigun -> Avenger
Avenger -> Vindicator