I bought a 12ga./7x57R combo gun a few years ago. I've been shooting Sellier & Bellot ammo in the rifle barrel & now have a goo

[jfreyfam]

I bought a 12ga./7x57R combo gun a few years ago. I've been shooting Sellier & Bellot ammo in the rifle barrel & now have a good supply of empty brass I'd like to start reloading. I already load for the regular 7x57. If I get the proper shellholder for the rimmed case, can I use the regular 7x57 sizing die? If so, what is the right RCBS &/or Hornady shellholder?

[30-CAL]

I would contact the die manufacture of your 7x57 die you own now and check with them.

[RES1956]

I would answer this question, but instead will give you the information on how to find the answer so you will know how in the future.
Consult your loading manual for the measurements of the 7x57R versus 7x57 mauser, if they are different, then no you can't use the same dies. If the slope of the shoulder is different, no, etc.
It's not really difficult.