280 rem brass from 270 win

Due to the slightly increase of base to shoulder length of the .280 over the .270 (1.999 vs 1.948), to prevent excessive case stretching upon first firing it is often suggested to first seat the bullet against the lands, with perhaps a reduced load, to hold the case against the bolt face, thereby pushing the shoulder forward to fill up the additional space.
 
Anneal, open up with a 30-06 die, then open up again with an 8x57 die or .338 die, and if you have a 35 cal die, finish with that. Then size down to 280, that way you'll have a "false shoulder" to support the brass against the chamber during the fireforming process.

Jimro
 
Jimro:
Why wouldn't neck expanding with a .30 or at most an 8mm die be sufficient for applying a false shoulder? Annealing may not be necessary.

Tlewis81:
Then gradually resize the neck through the .280 die, checking chambering as you go, until the case will just chamber against the false shoulder. That should hold the case head back against the bolt face upon firing with a normal load and push the shoulder forward.
 
Why wouldn't neck expanding with a .30 or at most an 8mm die be sufficient for applying a false shoulder? Annealing may not be necessary.
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You can probably do it that way just fine, but the smaller the false shoulder the more risk you run with case stretch thinning out the body of the brass instead of blowing the shoulder forward.

To explain with a very simple explanation (discounting the thickness of brass for example) 280 is 7mm. If you expand a fall shoulder to 8mm, you have 0.5mm of purchase area for the false shoulder (a 7mm circle inside an 8mm circle only gives you 0.5 difference between the circumferences). Expanding to 35 caliber or 9mm now puts the 7mm circle inside a 9mm circle, so you get a full 1mm difference which is over twice the purchase area of an 8mm false shoulder (increase the major and minor diameters of an oculus and you'll see what I mean).

Area = pi * r * r. So 3.1415*(4mm*4mm) - 3.1415(3.5mm*3.5mm) = 50.264-38.483= 11.781 mm square for an 8mm false shoulder. Doing the same math for a 9mm give 63.615. 63.615-38.483= 25.132 square mm.

So it isn't that you can't do it, it's just that there are some mechanical benefits from having the largest false shoulder you can before you fireform to already be in contact or close contact with the chamber to resist forward brass flow.

Jimro
 
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