The .280 Ackley Improved has been basking in the limelight since Nosler started chambering its Model 48 rifle for it and offering loaded ammo and unprimed cases. And well it should be as its performance puts it between the .280 Remington on the slower side and the faster 7mm Remington Magnum.
Data shown in the sixth edition of the Nosler reloading manual for the standard .280 and the .280 Improved were developed in 26-inch barrels while those for the 7mm Magnum are from a 24-inch barrel. To level the playing field in a comparison, I'll add 50 fps to the maximum velocities listed for the 7mm Remington, which is a realistic gain for a magnum cartridge when increasing barrel length by two inches.
Maximum velocities for 140-grain bullets in Nosler's test barrels chambered for the .280 Remington, .280 AI and the 7mm Remington Magnum were 3,152, 3,266 and 3,390 fps, respectively. That puts the .280 AI about 100 fps faster than the standard .280 and about 100 fps slower than the 7mm Remington Magnum.
As cartridge efficiency goes, velocity generated by each grain of powder in the three was 55.3, 51.8 and 50.2 fps, respectively. Recoil in nine-pound rifles would be 15.1, 17.2 and 19.5 ft.-lbs.
At 300 yards, the .280 AI with a 140-grainer delivers about 150 ft.-lbs. more energy than the .280 Remington and about 150 ft.-lbs. less than the 7mm Remington Magnum. Other bullet weights stack up about the same. I would not hesitate to use either cartridge for taking any hoofed game in North America.
One of Parker Otto Ackley's first cartridges of the 1940s was the 7mm-06 Improved. It was formed by necking down the .30-06 case and blowing it out to the improved shape with minimum body taper and a 40-degree shoulder angle.
Ackley credited Fred Huntington, founder of the RCBS reloading die company, with being the originator of the .280 Remington Improved. Fred's version had a 30-degree shoulder angle and held a pinch more powder than the 7mm-06 Ackley Improved. Cases were more easily formed by firing .280 Remington factory ammo in the improved chamber, so Ackley eventually abandoned his wildcat on the .30-06 case and started chambering rifles for Huntington's cartridge but with a 40-degree shoulder angle.
The .280 Remington Improved enjoyed modest popularity through the decades, but it did not really take off until gunsmith Kenny Jarrett started chambering his super-accurate rifles for it during the 1980s.
As Ackley described on page 155 of his Handbook For Shooters & Reloaders, he avoided headspace problems with rimless cartridges by intentionally reaming his improved chambers approximately .004 inch shorter than the chamber of the factory cartridge. This created a "crush" fit between the shoulder-neck junctures of the improved chamber and a factory cartridge. If a .280 Remington cartridge is chambered in the .280 Improved chamber having Ackley's dimensions and then removed, the zero headspace fit will have left a faint polished ring at the juncture of its shoulder and neck.
Through the years many rifles have been rebarreled for Ackley's cartridges, and probably just as many have been rechambered. When a rechamber job is done correctly, the gunsmith chucks the barrel in his lathe and shortens its shank by one thread length. Doing so enables him to ream the improved chamber a bit shorter than the factory chamber.
For reasons too lengthy to go into here, the headspace dimension for the .280 Ackley Improved cartridge approved by SAAMI and introduced by Nosler turned out to be shorter than headspace gauge manufacturers had established for the original Ackley version. Some reloading die makers have responded by offering different die sets for the two.
Dies made by Redding for the Ackley version prior to 2011 were marked "280 Rem Imp 40 Degrees." They are still available. Dies for the Nosler version are marked "280 Ackley Imp," which is the SAAMI-approved abbreviation. In the past, RCBS dies for the Ackley cartridge were marked "280 Imp 40 Degrees," and they will continue to be so while dies for the SAAMI/Nosler version will be marked "280 Ackley Imp."
The important difference between the two die sets is in their full-length resizers. Since headspace dimension of the SAAMI cartridge is shorter, the shoulder in a die reamed for the Ackley cartridge won't make contact with the shoulder of the case.
If a die reamed for the SAAMI version is screwed into the press far enough to make contact with the shellholder and used to resize a case fired in a rifle chambered for Ackley's version, the shoulder will be set back for an increase in headspace. But it can be used by screwing the die in just far enough to lightly bump back the shoulder enough for trouble-free chambering of the cartridge. So when purchasing reloading dies for a rifle it is important to know which version of the .280 AI it is chambered for.
If other companies introduce rifles in .280 Ackley Improved, they will likely be chambered for the SAAMI version, and Nosler cases and ammunition will surely work fine in them. But since the original .280 Ackley Improved is a wildcat, gunsmiths who have chambered rifles for it over the past 65 years or so have not always agreed on precisely how deeply the chamber should be reamed.
So to be on the safe side, it is not a bad idea to have the headspace of a rifle chambered for Ackley's version checked by a gunsmith to determine its suitability with Nosler cases and ammunition. Kenny Jarrett, who has chambered more than 1,000 rifles for the original Ackley version, agrees with my opinion. So does Dave Kiff of Pacific Tool & Gauge, which makes chamber reamers and headspace gauges for both versions.