I've no idea what they slug at, but all seem to shoot well enough with surplus 7.62 x 54. Mine prefer the light ball. I'd assume .310/.311 (that's ~7.7mm for our Euro friends
![Very Happy :D](./images/smilies/icon_biggrin.gif)
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I've got an aftermarket mount for one of my SVT'sand it works well
As to parts, try Surplus Firearms. Google Mark Kubes.
GPC/Numrich has some parts.