OK fellas...got my mits on a XP-100 rebarreled to the above noted cartridge. The chamber had some headspace issues...cases were splitting on the first firing. So I ended up necking the virgin cases to .277" then back to .257" a little at a time until the bolt would cam closed. This created a secondary shoulder...now the cases do not split and they form well w/ 10gr Unique, TP, COW, then TP again to close off the mouth. I even loaded/fired a single case up to three times (using 38gr Varget and 87gr Hornady SP's) and it's still good. Now I'm looking for some loads to get started. I'll likely be shooting 87gr to 100gr bullets. Also have an AMPLE supply of Varget and RL-15. Thanks. Dan B.