Ok, I would like to clear up some confusion on the forming 257 Weatherby brass from 7 mag brass, and I would only use NEW Winchester, PMC, or AGD....NO federal or Remington as they are softer in the case head.
Use Lee Sizing wax in the tooth paste type of container, and let it sit on the cases for a few minutes before running the brass through the die. For this application, Lee was far superior to the Imperial.
If you will put a heavy chamfer, not a deburr on the outside of the case mouth, then gently run the brass into the 257 Weatherby full-length sizer(I used Forster), you will have a 100% perfectly formed case with ZERO bulges, etc. you want the chamfer to just about go through the entire thickness of the brass neck wall, and I consider this critical.
My method is to size the brass down in small increments. Size a little, back off, size a little back off, perhaps 4 small steps instead of just one large step putting a lot of stress on the tensile strength of the brass.
If you have dents in the case, the die ring is covering up the vent hole, move that die ring location.
I never lost a case with the above method, never. I credit this to the heavy chamfer and the use of Lee Sizing lube in the toothpaste container.
The neck will be shorter on the formed brass, with the result of carbon in front of the case mouth in the neck of the rifle chamber. The carbon in front of the case mouth is an important issue if you are mixing factory and formed 7 Mag or 264 Win brass. The carbon ring in the chamber will act as a Crimp at some point on the case neck as that dimension will be smaller. I use a Bronze bristle brush/good bore solvent to get the carbon out of the neck and inspect with a Teslong bore scope. If you do not use regular maintenance on the carbon, you may have to use a brush on a rod, spun by a slow speed 1/2" drill.
I found 250 once-fired 264 win cases from a guy over on 24hr campfire. The web dia was larger in the test rifle those cases were fired in, compared to my reamer. So, I ran the 264 cases through a 7 Mag small base sizer without the expander ball where the neck was not touched.
Then I formed those 250 cases with the 257 Weatherby Forster full-length sizer, with the lee sizing wax on the outside and wax applied to the inside of the neck to ease the trip for the expander ball. Brother and nephew are killing deer and hogs with that brass now.
Their rifles are nothing real fancy, just hunting rifles on non-trued 700's, Boyd's laminate stock, #5 taper, X caliber 9 Twist barrel, a chamber with .030 freebore. They shoot 110g Accubonds and 115g Berger vld hunting at 3600 on 25.5" barrels with a Gentry muzzle break. Recoil is so little, they see the bullet impact on the animal with water flying off the hide, and the bullet impact after it exits. They are ready to get rid of a bunch of their heavier kicking rifles.
Note: the formed brass as it comes out of the Forster Full Lenth sizer with the Lee Sizing wax being used looks like brand new factory-formed brass. They worked up a load with the brass as it came out of the die with groups 3/8" and smaller, then hunted.
None in the family has had a deer do anything but fold up on the spot with the 257 Weatherby, and all of our rifles have either zero to .030 freebore, using R#22 with fed 215's in all guns. I shoot 100g Hornady and 100g Partitions and have killed quite a few with the newer 100g Nosler ballistic tips(3850 fps on a 26" bbl).
I hope to try the Speer 100g Hot Cores next...hogs.