There are actually two ".280 Ackley Improved" chambers, and RCBS supplies dies for both. The version with the pedigree that harks back to Ackley himself has a 40-degree shoulder, while the other is a modified version with a 30-degree shoulder said to feed better. I don’t buy it, and judging by popularity ratio, neither do most others.
As an aside, when Nosler presented the 40-degree .280 AI to SAAMI for standardization, the base-to-shoulder length was shortened by a few thousandths. As a result, some variation from older .280 AIs exists in chambering reamers, dies, etc. Be aware of this condition and safely load for your rifle accordingly. Consult with your gunsmith or rifle manufacturer if you need help sorting it out.