I believe you are correct about Forster not making a bump die for 260. I cobbled one together by having the hole in the top of a .243 die honed out to let a fired .260 case pass through, and it worked, but the neck in my chamber was just too sloppy to use bushing dies well - had to size down too much and couldn't control runout with bushings. I finally ended up using a Redding F/L body die for sizing the case body and shoulder to give me the bump I wanted in conjunction with a Lee Collet die for sizing the neck. Had to turn the diameter of the mandrel on the Lee die down a little to control neck tension, but it seems to work OK.

A customized F/L die would be the ticket - haven't gone that route (yet).
Elkbane