Typically, for the first shot you're fire-forming. Thereafter, you're using a die correct for the specific case/chamber.
RCBS in particular, and Lee (and others) have .257AckImp dies. You'll want a set of the specific dies, just as you would for typical cartridges.
You can get neck sizing specific dies too. It really depends on the individual chamber in the rifle. In the early '80's, I traded for a Ruger #3 in .22Hornet, not knowing it had been recut to .22 K-Hornet. It was cut so poorly however, that even with K-Hornet dies, I couldn't get but 1-reloading before the cases split in the body. The chamber had been cut too large and was out-of-round... "Wobbled out" was the term a consulting/inspecting gunsmith used to describe what had happened with the reamer when the chamber was cut by an unknown individual. I traded it to an aquaintance who I informed that the chamber was "modified". He wasn't a reloader and wasn't concerned that the fired cases were unusable......