TROOPER:
Primary: Any AR or other such gun (Recon, LS w/o FG, etc.)
Secondary: Any gun lighter than the Primary (NF, Element, Firefly, etc.)
Other stuff: Tactical Vest, DT Vest, Stonewall Sword (as a combat knife)
The front linemen, the Trooper is the all-rounder class for any NERF war.
MEDIC:
Primary: SMG or MP-like weapon
Secondary: Any handgun
Other stuff: "Revive Syringes" (medicine or baby syringes; PLEASE NO HEROIN NEEDLES!!!), some kind of Medical ID.
The Medic can revive other downed players or revive them, depending on the context of the game and what "death" rules apply.
HEAVY:
Primary: Vulcan chaingun or Stampede LMG
Secondary: Buzzsaw, Titan Launcher, or other heavy weapon.
Other stuff: Tactical Vest, sandwich (explanation below), fairly large build.
The Heavy is the stereotypical massive man who uses heavy guns. For added fun, you can have a rule where if the Heavy eats a sandwich, he can become invincible for 30 seconds. XD
ENGINEER:
Primary: Any gun, save for heavy weapons.
Secondary: Same as Trooper
Other stuff: NERF grenades, homemade Caltrop grenades (ask for recipe

The Engineer is the one who you'd see if you want a room clear, a flanking position cut off, something built, or something blown up. (Note: DON'T USE REAL 'SPLOSIVES!!!)
loser:
Primary: Longshot, Longstrike, Boltsniper FAR, or other such loser rifle.
Secondary: Handgun
Other stuff: Some kind of camoflague, binoculars (for spotting), a long attention span.
The loser is the unseen killer behind a heavy-as-Hell's-hand loser rifle. Just a heads-up: the don't typically miss.