An excerpt from a Nitro Express post from pwm of 2005:

"The 11,2x60 can be made from any brass from 300 win Mag to 458 win mag. You have to cut the belt from the case and trim the rebated rim to 8mm Mauser dim.. Best is the 458 Win Mag, annealing, full length resizing and trimming to 60mm – ready. Have found some 300 Win Mag cases, will also anneal this brass and blow it out with a fire form load in the chamber. "