12 ga is doable, I have one, 24" long gun model encore. I'm not sure you would want to shoot a 12 ga in a handgun...mine will have screw in choke...doner barrel $360, class 3 work $150, registration $5, transfer fee $75 and reaming/threading for choke (haven't decided who yet) around $100. Total (after S/H) over $700, just for barrel.
It's a lot of red tape involved but, it can be legally done...in my state.... Maybe not in yours.... You'll have to check your laws.