RCBS Whisper Loading Die Question

[HotLead]

I have owned a T/C Custom shop Encore barrel in .300 Whisper for several years now, and love it (even though it is a 1:10 twist). I contacted SSK and purchased my first couple hundred pieces of formed brass and a set of RCBS .300 Whisper loading dies.

Now that I am in the process of building an AR upper in 300-221, will I be able to load for it with my Whisper dies and shoot safely? Or will I have feeding/ chambering issues? I don't know enough about the differences in chamber dimensions between all the variants. Thanks in advance for any answers.