The more of a weapon you have the higher the fire rate, up to a certain point. After that adding more of that weapon is just redundancy. Your robot has a certain pool of power, Shooting any gun uses some amount of power and your power will regenerate when you aren't firing. All tiers of a weapon now have the same fire rate, energy usage, and projectile speed. Higher tiers of weapons do more damage. If you had 25 mega versions of the weapon as of 11/14/18 you will have gotten a golden version of the weapon. Plasma and SMG also have carbon 6 cosmetic variants.
Plasma:
Plasma's deal area damage to things in a certain radius. They shoot a very fast volley using power quickly and then have a very slow fire rate. There are 6 tiers of plasma.
SMG:
SMGs or laser guns are rapid fire weapons that shoot individual lasers. There are 6 tiers of SMGs.
Rail:
Rails deal penetration damage and you can zoom in very far with them. They deal penetration damage by shooting 3 projectiles in the exact same spot/line so the projectiles can go through the hole that the previous ones created. There are 3 different tiers of rail.
Flak:
Flak is an anti-air weapon that explodes when near targets in the air. As you hit consecutive shots the weapon stacks its damage dealing a lot more damage. There are two tiers of flak.
LOML:
The LOML or lock on missle launcher, to use this weapon you hover over an enemy player for 3 seconds so it locks on, then you can fire homing missles. Your lock will be broken if they go behind cover or use the ghost module. There are two tiers of LOML.
Chain shredder:
A very high rate of fire weapon that shoots lots of projectiles with a decent spread. It has a spinup time before firing at the maximum rate. There are two tiers of chain shredders.
Protoseeker:
A weak weapon that shoots lots of homing projectiles. There is 1 tier of protoseeker.
Nano-Gun:
A healing weapon that locks onto your allies and heals them when you shoot with a lock. There are 3 different tiers of nanos.
Tesla:
Teslas are the only melee weapon. For tesla to hit you have to be extremely close/touching an enemy with them. They can be somewhat glitchy as the damage detection is done by the players computer not the robocraft servers, so lag can actually be beneficial for teslas. There are 3 tiers of tesla.
Mortar:
Mortars are a slow fire rate large splash damage weapon. They have a short arc of fire compared to plasma but a much larger splash area, and deal a lot more damage per shot. There is one tier of mortar.