Sorry chap, your just plain wrong. I have come across A2 Fragment Cannons and B2 Rail Guns...so my question still stands.
Nah, he's partly right. There's only one of each type/class weapons. Eg, if you ask for class 2 beam, there are only 3 types: fixed, gimballed and turreted.
I can't really say anything about the letters corresponding to weapon strength - never in the last year occurred to me to observe that as I know the damage relation is: fixed > gimballed > turreted
And bigger class weapons do more damage. If I want to compare them, I use the compare screen - the colours tell you the difference even if they have the same number of "pips" in the DPS row.
DPS is all that matters there (except for frag cannons as they don't have enough ammo to shoot for a whole second) - energy consumption on the other hand is something you have to test yourself with whatever ship/loadout you have, and however many weapons you want to shoot at the same time.
Energy wise on one end you have beams - if you want to do as much damage as you can in the shortest amount of time (or rail guns if constantly fetching ammo is your thing)
The other end is pulse, where you prefer to have sustained fire (and eventually more damage) over a longer period of time. (or multicannons at class 2, cannons at class 3 and 4)