Dies for one camber used on another?

[Muleskinner]

Is this presuming your 300bo die set is not small base? They do sell SB 300bo dies also, which I bought since all my 300bo are in AR format..

RCBS AR Series Small Base 2-Die Set Taper Crimp 300 Blackout

The RCBS AR Series die set is a must for any progressive reloader. Each set comes packaged with a full length small base sizing die and a taper crimp...

www.midwayusa.com

Right. My 300 BO die set is a Lee pacesetter set. It's not small base. It works fine to resize actual 300 BO brass but for some reason I cannot figure out converted 556 brass requires I use the small base die just to properly size the base of the brass when loading supersonic compressed loads. 23.1 grn of CFEBLK fills the case. All I can figure is the compressed powder is bulging the base when I seat the bullet. Sizing the base with the small based die prior to powder appears to fix it. Actual 300 BO brass doesn't require it. it's weird.

I don't have the problem when load subsonics..