The information from Dave Manson is interesting--partly because the difference in headspace is so small--which can result in something I personally experienced when Nosler first introduced the .280 AI as a factory round.
To promote the "new" cartridge they decided to take a couple of gun writers on a pronghorn hunt in Wyoming, one of them me. Nosler wasn't building rifles then, so I had a Ruger No. 1 chambered for a smaller 7mm round rechambered by a local smith. But there weren't any headspace gauges available, so I took him some unfired Nosler cases--which at the time were headstamped "Norma." He used a .270 Winchester Ackley Improved reamer and a 7mm neck reamer to do the job.
The rifle shot very well with both the factory ammo Nosler sent me (which was loaded by Norma, in the same headstamped brass), and with various handloads. But I also had some .280 Remington ammo on hand from various projects, and decided to see how it would fireform in the new chamber. It formed fine, without a hint of excess stretching.
