One factor is the .270's SAAMI maximum average pressure is 65,000 PSI--as high as they allow for ANY cartridge.
Half or more of the reason the .280 AI gets more velocity is SAAMI's MAP for the standard .280 is 60,000 PSI, because it was originally introduced in Remington's semiauto rifle. When SAAMI accepted the .280 AI they standardized the MAP at 65,000.
Just "Ackley Improving" most rounds doesn't gain much extra powder room, and in fact in some basically none--as in the .35 Whelen AI, because there's no enough shoulder there to gain significantly more room.
The reason reason most Ackley Improved rounds gain significantly more velocity over the standard version are handloaders "work up" loads until they see "pressure signs," then back off a little. This generally means pressures a lot higher than data or factory ammo for the standard rounds.
About the only real advantage in AI'ing any cartridge is reduced case-stretch, due to the sharper shoulder maintaining headspace more firmly in the chamber.
Great info here. Thanks
