I've seen nothing in any of the descriptions of SR HMGs which would lead me to believe that they fire .50 BMG rounds.
All the descriptions include things like hip-pad braces and under-mounted tripods.
All the pictures include standard stocks and such.
They're not M2s, that's for sure.
As to sniper rifle damage being higher, I agree with JS to an extent. Damage is abstracted, and the level of precision in a sniper-purposed weapon allows for such a higher amount of in-built control as to justify increased damage from better placement. Still, regardless of whether you're using a sniper rifle, or an ill-designated HMG, whatever you're shooting at is in very bad shape.