Years ago I made a long taper .375" expander plug for making .375 RUM cases from 7mm and .300 RUM cases. A little Imperial Sizing Wax in the mouth of the case, and I've had no problems expanding 7mm and .300 RUM cases to .375 in one stroke. Over the years I've probably expanded the necks of over 100 cases to .375 without a single cracked case. I also haven't annealed any of those cases before expanding their necks.
