Can it be done? i know you'd need a new barrel, but the rim sizes are only different by .30mm, the .500 is 14.1 mm and the 7.62x54R is 14.40. Just something i thought about as i was buying shell holders and noticed the 7.62x54R and .500 share the same holder.

My google-fu shows that 7.62 tops out at 57,000 psi and .500 is less than 50,000. So from that angle it sounds possible. The biggest problem would probably be the magazine and feeding issues. The Russian's LOA is almost 1" greater than the Smith. Another thing is that the Mosin barrel contour might (I'm almost sure it does) neck down too much to work with the .5 caliber bullet, meaning you may not be able use an original stock.

Screw the original stock - no problem cutting a new barrel channel in that damn thing :D Been thinking about this but only in 45-70 - would be fun. I know guys have done single shot versions - but I'd definitely want the magazine to work.