You are right, the SSG only comes in 308. I'm just curious, because the magazine of SSG3000 is quite long, does it mean it can use 7.62 x 54R too? I don't know. Isn't it the difference between 7.62 x 51 and 7.62 x 54R is only the length of the casing, but same diameter??
The magazine might accept it, but the gun won't
Please refer back to, or retake your firearms course and REALLY pay attention when doing the practical where you verify the caliber of ammo you have with the caliber labeled on the gun.