Is it feasible to fireform Creedmoor cases using 260 Rem brass?

Yes, you have to do some work though. The 260 has a shorter neck, you'll have to ream the inside and possibly cut some off the outside.

I used some .308 brass for mine and it just required inside reaming. I annealed the cases prior to sizing. I sized them down in 3 steps with Imperial Die Wax.

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Thanks for the info.