R2DA's Weapons can technically be placed into tiers. However, due to the game not simply only being dependent on factors such as damage, certain weapons in higher tiers than others may still be worse than certain weapons in lower tiers. Thus, you must take this so-called "hierarchy" with a grain of salt.
Use your own experiences within the game, influences such as maps and the server's playerbase, and personal preference to determine which weapons truly are superior to others.
All of the melees, the Minigun, the Flamethrower, the Toybow, all removed weapons, the M202, the RPG, the X-Mas Launcher, and the FATAL5 are not included within this list. There are many reasons for not including them — a single, simplified one is that it's too hard to rank them one above the other.
A semi-automatic weapon is a firearm that can only fire one bullet/shell per click. All secondary weapons are semi-automatic.
An automatic weapon is a firearm capable of firing projectiles in bursts, that is, one after the other, as long as left mouse is held down.
A burst weapon is a firearm capable of firing three projectiles in bursts, usually three bullets at a time. Fires one burst per mouse click. These tend to lose accuracy quickly.
Fires one shell that splits up into multiple pellets. Each pellet does its own damage. If all the pellets hit, it deals massive damage. Usually, shotguns are also pump-action, with the DB Shotgun being an exception.
A precision rifle that usually deals a high amount of damage. Usually, snipers are also bolt-action, with the exception of the Dragunov.