I believe accuracy will trump platform, get a 10/22, grab a Mosin, sks, m1, ar, ak, .410, 12g, 10g, go handgun plastic, or steel, .380, 9mm, .40, .45. Each will have pros and cons given a specific scenario. When all is said and done, its going to be using the platform you have ready access to at the moment the situation presents itself. The cover in the area that can be used. The accuracy of the shooter and platform working together. The ammo power, load and projectile. What it strikes armor, heavy winter clothing, summer clothing, bare skin. Step 1 get a gun. Step 2 train and try different loads of ammo to find what works best. Not necessarily powerful, but what recoil is best for the shooter, with the fastest target requisition, while hitting the aimed at target.
Rifle would be my first choice something in 5.56 easy to carry plenty of extra mags. Weight is not fatiguing, good sling setup. Next id prefer a handgun 9 with hollow points for real up close combat. Shot gun would be a bunkered type defensive choice. Think zombie apocalypse in a 2nd fl or higher position covering an entrance while team members searched for loot in the building. Spraying down buck shot, in effect holding back a group while the team arrived with rifles for precision work. Make no mistake gun fights would have a huge psychological effect. A crowd seeing the devastating damage may or may not retreat. Some may even be further encouraged to fight.
No matter what you get train as you would fight, so you fight like you trained. Practice early and often. Practice off hand and one hand weak and strong in case your wounded.