Since this is the only ammo that has acted up, I would take these steps.
1. Recheck headspace on rifle. If you do not have a headspace gauge, skip to step 2.
2. Measure to the shoulder of the XM118LR case, and compare to another factory load or headspace gauge. Then compare to a fired case. Basically make sure that the XM118LR does not have a sloppy fit in the chamber. You would need a bushing / sleeve / pipe to slide over the bullet to measure off the shoulder, whatever you use, just use it on all of them so you can accurately compare numbers.
3. Use a hand priming tool and make sure the primers are fully seated / bottomed out in the primer pocket. Pull the bullet first if you are concerned about re-seating the primer on a loaded round.
4. If you have not identified the problem at this point, I would suspect exceptionally hard primers. You could do a diy bolt lift kit / mod with a 38/357 case(to add ~1/8" extra compression on the firing pin spring). Or start looking for a new / stronger firing pin spring.
I do not have any experience with that ammunition; so if anyone else knows better, I would like to hear your ideas.
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