What ammo is easier to purchase and cheaper?
Remanufactured 223/556 can be had for ~30 cents a round. 308 will be about 3 times that. Both are plentiful at the moment.
5.56mm is the cheapest, especially in the clones of military M193 and M855. You can get it anywhere. Even Walmart has some Federal XM855 ammo boxes that I like.
My neighbor has one chambered for 9mm. That way his ammo will fit both the handgun and the rifle, and 9mm ammo is real cheap and available.
Obviously it won’t puncture a tank at 300 yards but I suspect that if its people on the other end, they won’t be checking the caliber as the bullets fly by.
Not a bad idea ...