I love my .30 Gibbs. I neck out 30-06 brass to .338 then resize it in a 30-06 die until it chambers in the Gibbs with bolt resistance. Then I just fire form with 30-06 starting loads using a faster powder so I don't have so much. You could also neck down .35 Whelen brass,but I would use an intermediate .338 die first. I have found that I can use H414 with full cases and no pressure signs. BTW, supposedly P.O. Ackley would simply fire form 30-06 ammo in the M98. The extractor claw would hold the case for reliable ignition and firing. I never tried it because my Gibbs is built on A M700 action and I would be really leery of trying it anyway. If I did try it, I would have the bullet seated out so it would be jammed into the lands to help hold the case tight against the bolt face. Personally, I would use the neck up-neck down method which holds the case against the bolt face. Also, case capacity is almost identical to the .30 Short mags new you get more reliable feeding plus you have advantage of having one more round in the magazine.