The rifled shotgun slug was developed to help stabilize the slug by imparting spin.
PeteM:
This is simply not true, mate. The 'rifling' on the slug is to allow a crush-fit through the choke without raising pressure. Actually, it's not rifling at all: the fins are simply angled so that lead smeared off the fins as they are crushed in the choke is wiped into a space behind the fin, so the build-up doesn't raise pressure further. Invented by Wilhelm Brenneke I believe. Brenneke's tests showed that a 'rifled' slug rotates about one turn in 50 metres, certainly not enough to spin-stabilize the projectile. Smooth-bore slugs depend on the 'shuttle-cock' principle for stabilization, in one form or another (either attached base-wad or hollow rear skirt, or sim.).
Wikepedia perpetuates the mis-information about rifled slugs spinning to stabilize, but the Brenneke web-site will set you straight.
Hope this helps.
