I agree with all that say base down, but make sure the carrier you are using will grip the rounds during recoil. (concern for elastic holders)
I would stress thouogh, if a variety are going to be carried, make it two (2) minimum and ALWAYS in the same order/location and train to reload back and forth between selsctions. I also advocated reverse positioning in the carrier. My preferance is the primary load, base down and the secondary selection base up. (if you have time to contemplate the change in projectile you can deal with the different movement. (as I said practice both)
"People won't rise to the occassion...they'll only default to their level of training."
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Be Safe, Be Trained
Life is tough...it's alot tougher if you don't know how to shoot.