its possible that it was the the mauser m71 cartridge but you yanks better now this and the dealer have measure a 11,2 x 62 and will see this when it was the fact.
better idea is a chambering for the 11,5 x 60R450 or the 11,2x60R LK. the first is a german version from the 450 BPE and the second is 410 ga. brass case loaded with a bullet.