AHA! Direct quote from game time

"Reconstruction efforts after the great war inspired advancements in metallurgy and efficient conventional explosives. The two are elegantly combined in the gtw-66 maxim. The maxim is by definition an energy weapon,though it behaves like a high-velocity mass-driver cannon, accelerating uranium slugs along its smoothbore barrel. The maxim has a difficult time penetrating energy-based shields, but it has a devastating effect on hull plating and subsystem armor."
My main gripe about the maxim cannon is the only single thing classifying it as an energy weapon is that it says "The maxim is by definition an energy weapon,".
As far as railgun goes, it says it acts like a mass driver, but it doesn't say it out right.
On another note, "advancements in metallurgy and efficient conventional explosives."
It doesn't say what the efficient explosives are for.
My guess would be that it may use explosives to propel the uranium slug like a standard bullet today.
Though a rifled barrel would be much better than a smoothbore.
As far as the cannon being ragarded as an energy weapon by definition.
Maybe it's the fact that it runs off of your gun energy, and that would seem to be really the only reason, because it doesn't use ammunition, unless people get the enable analog ballistic ammo gauge feature working with scp(like ngtm1r mentioned).
Or if the maxim was a mass driver it'd be using the energy to propel the uranium slugs anyway, but

neglected putting in an ammunition meter for the weapon.
If the gun is to be using any energy at all and not have limited ammunition, it'd either have to be a mass accelerator, or a space-age machinegun(what the description points it out more to be than a mass driver), you'd need my least likely idea for why it's an "energy weapon by definition."
If it used a replication matrix(like replicators on star trek) to replicate bullets on demand, which you're gun energy would be powering.
Like i said, that's the least likely idea.
And since fs2 has no beaming transporting devices(like startrek), they're so definitely not able to convert energy into matter with another device

should have just plain old included an ammunition gauge or something, i mean, the maxim in many ways is not by definition an energy weapon at all. When the term energy is used, i start thinking electricity. Of course when the gun uses explosives to accelerate the uranium slug, yeah that's energy, but it gets too damn confusing calling a machine gun an energy weapon based on that. Not to mention weird, it'd be wierd as hell refering to an m-16, or a pistol as an energy weapon, if anyone gets what i mean besides referring to lasers and emp's as energy weapons?