I owned a 7mm JRS at one time, and a 338Hawk too. They both use 30-06 brass with the shoulder set forward, just like the Gibbs.
You do fireform brass, but it is not simple like the Ackleys (where you can just fire standard ammunition and it forms to the Ackley shoulder and case taper).
For the Gibbs cases, there are two (or three) options:
Seat the bullets out long (hard into the lands so that closing the bolt takes some effort). The bullet maintains the headspace during fire forming. I cannot comment on this method, because I never used it. Seemed a little scary to me, but it is probably fine.
Otherwise, neck the virgin brass up (two calibers is recommended) and then back down but just far enough so that a false (forward located) shoulder is made on the neck. Again, the false shoulder is located such that closing the bolt on a round requires some effort. The false shoulder maintains the headspace during fireforming. This is the method I used. It really is not difficult (just sounds like it). For a 270 Gibbs you could probably just start with 30-06 brass and skip necking it up.
Otherwise, the cream-of-wheat method mentioned. I never did this either, so I cannot comment on it. Not really sure what maintains the headspace using this method, but I believe that just a few grains of a fast powder (bullseye) is used, and the pressure spike is fast enough that the brass never gets a chance to move (don't quote me on this - it's mostly a guess).
My 7mm JRS was a 700 KS stainless that had the action and bolt body beautifully skelletonized. It is probably the nicest SS/synthetic rifle I have ever owned. But I don't own it anymore. Just a bit too much work for this lazy SOB.
Today, I'd probably just buy a 270WSM or 7WSM and be just as happy.
