There' s one advantage to using the whelen brass, you can leave a little .35 cal bulge at the bottom of the neck so you get a good tight crush fit in the chamber for the fireforming load. You can do the same by necking 06 up to 338 or 35, and then back down to 8mm. That will assure the case head is tight against the bolt face when you form the shoulder. You will lose about 100 fps velocity on the fireforming load, so if you are very serious about getting the most and very consistent loads, you would be well advised to shoot the fireforming load as part of your brass prepping and then start load development with fireformed brass. In my 33806 I like to neck up to 35 then down to 338 leaving that little bulge, and then seat a bullet that is in firm contact with the riflings and fire form with a load that would be a starting load in any manual. No sense in burning more powder than you have to just for forming a case. I form cases in standard chambers by seating the bullets to touch the rilfes. You want to make sure you use enough to fully form the shoulder though, or you will lose velocity on the initial firing as the case is finally formed. Or you can just neck the 06 brass up and load and shoot. Bear in mind that if you mix fireformed and non fireformed brass, your loads will show very wide extreme spreads in velocities, like 150 to 200 fps. I usually fireform cases in batches of 50 or 100 and use them for offhand practice. Take a couple of guns along so you wont be tempted to cook your barrel by shooting too fast. I mostly use surplus powders and corelokts since thats the cheapest way.