What is wrong with just shooting the Magtech cases and then load them with your conceived load? Once-fired is only slightly different than un-fired.
I prefer re-assign to sacrifice: I could easily use bullet and charge in an S&B case.
(the V0 values for fps and ft/lbs are almost identical)
This is not something I had initially planned on: I was happy using my once/twice fired S&B cases, but when experienced reloaders advise me to use new brass for my intended load development, I listen!
If no option exists, then I go with what I've got.
However, when there are options, I explore them even if they do seem off-the-wall...
Truth is I don't know how crucial new brass is to this, but I take safety seriously and I want to develop sound instincts about how to do stuff, hence I listen to recommendations and don't question them too much!
With respect to your own suggestion: why would you decap the cases? Just work-up the loads with the factory primers. You should resise the case mouths after removing the bullets, which you can do without decapping by removing the decapping stem from the sizing die.
Yes, I've actually resized primed cases when I realised a batch of .38s had weak charges: what a royal PITA pulling all those bullets was !!
As for the why: simply because it seems prudent to develop a load using the same components I'd later have to rely on if I chose to make yet more of these cartridges.
Just out of curiosity, what are you planning to do that would cause you to sacrifice brand new factory ammo?
275gr hard cast .430 bullets at 1050-1200 fps from a 4" Redhawk.
I'd settle for 1100-1150. Not the hottest of loads but still fairly potent.
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Karma. Another word for revolver: because what goes around, comes around!