GURPS Firearm: Saiga 410K-01 and -04
New article on my blog, it took a moment to figure out various .410G loadings since they're hidden in the GURPS 4e Pulp Guns 1.
LC3 magazine-fed shotgun for modern day adventures, behaves very similarly to an assault rifle, so it could be a 'starting' weapon for a survivalist, guerrilla etc.