You are correct that the 280 case is longer (actually, 0.046" longer) than the 30-06. However, I disagree with your statement that one could put a 280 cartridge in a 270 and that overall cartridge length has any bearing on this...
The poster did not say you could chamber a .280 case in a .270 chamber, so there is nothing to disagree with him about. Remington moved the .280’s shoulder
datum line forward so there would be no chance that a .280 case with a thin neck would chamber in a loosely-cut .270 chamber - there is only 0.007” difference in bullet diameter after all - and not all ammunition or chambers are perfect matches for SAAMI specs. The history of why Remington did this is well documented.
…The 280 and 270 have the same overall case length of 2.540", and 270 brass might be easier to find than 280 brass. Thus, it should be very straight forward to fire-form 270 brass into 280 brass using the "Cream-of-Wheat" process….
Except that there would be considerable
excess headspace if this was done, and a non-controlled feed action might not even fire. If it did then there is a risk of a head separation, if not on the first shot then on subsequent reloadings. None of this is good, see below for the remedy.
…Relative to forming 30-06 brass down to 280, when forming brass for a smaller caliber from a larger caliber case, the brass 'grows' in length since one is constricting the diameter of the case (the brass has to go somewhere)...
You are confusing the headspace datum line with overall case length. You can reduce the latter easily with a case trimmer. If you are necking down a .30-06 case to .280 then you are forming a temporary shoulder from what was the neck of the ‘06 case, not a big problem but a larger shoulder might be wiser. If you are forming a .270 case to .280 then there
is the problem of excess headspace. The usual correction is to neck the .270 case up to at least .338” then sizing it down again in the .280 die. This forms the temporary shoulder at the correct location and all is good to fireform with the correct headspace. An alternative method is to seat the bullets long so they jam into the rifling. As long as this is done with a reduced load it is safe and will result in a case with the corrected datum line and thus correct headspace.
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