Any one have any experience in reworking the 8mm mag. brass to .340 weatherby? Should the reworked brass be fire formed prior to accuracy testing or hunting? If so how has the brass been holding up?

I have used 8mm Rem Mag brass to form cases up to .411 diameter. A tapered expander is handy. I have one that goes from .323 to .411. RCBS can make them and are relative inexpensive ($12.00). Anealing the brass necks helps as well prior to resizing and opening up the case necks.

I have used 8 M/M REMMAG cases to produce .450 Ackley's in one operation. Put 13-16 grs. of Unique in the primed case, fill with cornmeal and press a 1/4 sheet of toilet paper into the neck with the eraser end of a wood pencil. Hold the rifle vertical and pull the trigger, instant whatever-you-wish cartridge. I would recommend annealing afterwards.