9mm M&P barrel and a couple of 9mm mags. Then you can shoot 9mm in the gun. Perfect for the prepper such as yourself. Or just in case you find a good deal on a bunch of 9mm ammo.
A spare striker assembly would be a good thing to add to your list.
Added to the list, keep the ideas coming people!! Do you think just stocking a extra set of all internals would be the best option? and maybe 2 extra of the striker and other major pieces?
What spare parts should i keep on hand for my full size Smith and Wesson M&P40 (I'm one of those crazy "Preppers" and would like to know what to keep on hand to replace if things break?)