What is the best way to fireform for the 280 Imp?
Should I do like I do for the 7TCU and a couple of others I fireform for and use a one of the heavier bullets for that caliber and seat into the rifling's and use a medium loading? It seems to work fine for all the others. thanks in advance for all the advice.
Steve..........

I have a 257AI and have been involved with minor problems with several others. 280AI should be very similar

Firstly the degree of difficulty of fireforming can be related to the type of action used. With a mauser or derivitive, the claw extractor tends to hold the case back against the boltface. In Rem or Win actions the plunger type ejector tends to push the case away from the boltface and therefore requires the projectile to be hard against the lands so that it pushes back against said ejector,and resulting head space is perfect.

Fireforming loads should be only 1-1.5gn under max recommended for the standard chamber to ensure sufficent pressure that the case fills the improved chamber when fired.

After fireforming then, and only then, check the finished case for uniform length as all cases have a tendency to shorten when fired.

It will be used in a 1909 Argentine action I have barreled and chambered. I didn't know if the Ackley's required any thing different from the others I have fireformed. Looks like the only difference will be to up the powder charge. Thanks.

Steve E: I don't have a .280 Imp. but the Nosler #5 manual says to select a light load of a relatively fast powder such as IMR-4895 from STANDARD .280 Rem. data. Then seat the bullet well into the rifling so it makes good contact with the lands when chambered. This is to hold the case against the bolt face. Of course after forming, load with .280 Rem. AI data.
Bear in Fairbanks

Jamming a long-seated bullet into the riflings will work, but not nearly as well as creating a properly fitted "false" shoulder to headspace on. With a .280 or .280 imp, it's easy to size down '06 brass in your .280 (or imp.) F.L. die. Size just enough that it chambers with a slight resistance, headspacing on the shoulder left where the .30 caliber neck was sized down to .284.
Now seat your bullet normally (clear of the lands) and fire with a near-full power load. Loads fireformed this way usually provide good accuracy and can be use for informal practice and varminting or small game hunting.

By the way, '06 brass will be just a tad short for a .280, but after necking down and firing the forming load, it will stretch a bit. The length usually ends up just about right at the minimum trim-to length, so it will be right where you want it.

If you prefer to use .280 brass, just run a .30 cal expander through it and then size as described above.