The 260 SAAMI drawing tolerances for the chamber neck are:
a) .299 - .301" at the rear of the neck
b) .298 - .300" at the front of the neck
As best I can measure with pin gauges, my 260 [long chambered at Douglas] is:
a) .299" at the rear of the neck
b) .298" at the front of the neck
I have used 308 brass, 243 brass, and 260 brass in my 260.
My experiences are consistent, but not a complete verification of the synopsis I offer here of internet folklore mythology I have read about the 260:
1) 7mm-08 brass is the best, good fit between chamber neck and ammo neck
2) 260 brass is second best
3) 243 brass is third best. It gets a donut formed at the base of the neck
4) 308 brass is 4th best. Commercial brass will work. 10% of the time there will be a nasty pressure spike from a pinched bullet. These misfits can be sorted out, turned, or reamed.