If chambered correctly, and no reason it wouldn't be, he should be able to fire factory .280's - instantly and properly formed brass. That is the normal way most people form their brass for simple ACK-IMP factory rounds.

Poke out the old primers, then he's got the unprimed brass he needs. Even better since they now fit his chamber.

Since I don't like having the normal factory ammo headspace of "up to .007" slop", which is the max. allowable, along with the stretch at the web that occurs with 'short' headspaced rounds, I'd rather make my own brass from '06 brass (or necked straight .270's) and position the shoulder perfectly before it's even fired the first time.

The 2-die set of .280 IMP dies are all that's needed for necking to hold a .284" bullet. Merely neck them until they chamber a crush fit. Once fired, they will perfectly fit the chamber and not stretch for many reloads due to not stretching at the web on that important 'first' firing, that and the well shaped ctg case, of course - stops or slows stretching.

You cannot simply neck up .270's or .25/06's as the shoulder is in the wrong location for a .280 - quite excessive headspace would be the result. .25's and .270s' can be necked up straight or to .35 or .375 (either are fine), then necked down to fit the chamber using your FL die. A pistol expander die can be used as-is or altered, for necking up ctgs. prior to loading for fireforming.