Yes, is possible to move the case shoulder forward at a press. Have to take the neck to much larger diameter - perhaps .44 caliber or larger - so sort of straight wall - smooth out the old shoulder completely. Then use final sizing die and work neck back down to desired size, which also brings the new shoulder and new shoulder angle down with it. From what I have read and done, is a technique used to transfer the "headspace" within the chamber from the belt or rim, to the case shoulder. End up with basically "zero" possibility of case head stretch. Works well with new 303 British in a "longer" or bigger than normal chamber. Using the rifle's chamber as the "gauge" for when you just so have it back far enough. Still needs a more or less full power first firing to do the final, full forming to the chamber, though. And then likely only ever doing "partial" full length sizing on subsequent loadings, so shoulder does not get pushed back too far - unless head spacing on belt or rim is okay.
COW case forming, for sure, uses a primer, and small amount (10 to 15 grains?) of Unique or similar shotgun powder - not, at all, using regular rifle powder for COW case forming. The last Fed 210 and Fed 215 primers I bought at Cabelas were 0.10 each to me, so using COW method going to use two to make a "new" case - one for the COW load, and one for the first full power load. Is sometimes a LOT cheaper than buying cases, if you can even find any to buy - I did buy some 7mm Weatherby Magnum cases - Weatherby brand - last fall - $235 for 80, so $2.94 each. A much better deal to spend 0.20 or 0.30 each on primers, powder and COW, if you have or can get something to make your own??
