2mm Energy Charge I believe, The round and power supply were in the same cartilage.
(Also they are coilguns, not railguns ... there is a difference but that is nitpicking a bit)
Bethesda made it a Energy Weapon in Operation:Anchorage, their rational was there was no Sniper Energy Weapon so they made the Gauss Rifle one (by giving it a scope and mark it as a Energy Weapon) and honestly I do not see what is the big deal about it as the round is accelerated to such speeds that will act far more as a energy weapon that as a gun (since its not going to be much influenced by wind and gravity due to its speed).
Also I am going to point out that Gauss weapons were the high end of Small Arms as Pulse weapons were the high end of Energy Weapons, none were scoped, in fact there was no scoped energy weapon until Fallout 3.
I don't care if it does not have a a scope. It works well as a powerful rifle. I am not saying go back to having just the M72 Gauss rifle but have both or very least make the M72 a unique weapon. It was the only weapn I know that can blast an Enclave soldier in Advanced PA apart with one shot. Its a great weapon. A unique Idea weapon. A coilgun/railgun.
M72 Gauss. (And it should be in Energy Weapons.

)
Have it in Energy weapons if the EC is that important to people but that EC sends a metal ball flying at almost the speed of light. Hell Devs remain it to the "coilgun" I'll know its really a M72 Gauss.