A friend just bought a rifle chambered for this cartridge and I'm trying to help him with components. In looking for dies I found these. Take note that Lee states these are for the original .280 Ackley Imp.

https://www.midsouthshooterssupply....ey-improved-limited-production-3-die-set

The RCBS dies state they size to SAAMI specs.

https://www.rcbs.com/dies-and-shell...als/rcbs-280-ackley-improved-nosler.html

Redding has a tech bulletin clarifying how their dies are cut and labeled.

https://www.redding-reloading.com/tech-line-a-tips-faqs/133-280-changes