As for caliber choice, your high demand rejects, will be what is most produced and what is easiest to pick up off the ground when push comes to shove. Of course the Zombies never drop their ammo. As for other choices look to the guys that have been doing short barrel, rifle calibers, for years. TC, not a big fan I love my Handi’s, but there is good info to be gleaned. If they not offer it in short barrels there is probably a reason. But we insist on reinventing the wheel. As for the stock I have 2 ultra varmints and I love them in Varmint calibers for shooting prone or off the bench long distance it’s the ticket. Find one of the new synthetics, I like the one H&R is putting on the ultras. A muzzle break that really reduces recoil, not just helps to control muzzle flip should only be used with hearing protection. Not too many of us who have truck guns behind the seat grab our ear muffs first. Limbsavers recoil pads, and stock weight go along way. You spoke of different weight bullets. Find one that will be good all around. Going to the ammo pouch for Pigs then looking for something else for deer? Why? The idea of a truck gun is convenience, and always at the ready. My thoughts on scopes the biggest reticle, at the lowest power. Good balance between light gathering and quick target accusation, especially in low light conditions. Give it all a long lasting protective coating and you can put it behind the seat and forget about it till you need it. Just my two cents worth.