I have a lot of rcbs shellholder. Would save a bit of $$ if they worked in the hornady trimmer.

Only one way - test them. I do not belief there would be a problem. Only with priming tools and sizing there could be a problem since the primer hight would differ giving a different size on resizing process and/or primer seating dept. This is why Hornady Hand Prime comes with a primer tray for RCBS and a tray for Hornady shell holder.

The older ones, like I have will not work, mine are 15-30 yrs old. I am under the impression, the newer ones, have the same size hole as a Hornady, haven't checked that physically. My older ones have a .270 hole, Hornady trimmer needs a .300 hole to lock up properly.