You can only legally own that if it’s preban in NY. Our state is gay and law abiding citizens can’t own weapons with magazine capacity in excess of 10 rounds unless they are made before 199x (exact date escapes me). Either way for HD use a 12 gauge shotgun really is your best choice. I would recomend a remington 870 tactical. To answer your other question about ARs being available in basically any caliber let me elaborate a little bit. The lower reciever or bottom half of the weapon is detachable from the upper which constist of the barrel and firing mechanism. It’s more complicated then that but this is the simple explanation. So you can buy a good quality lower reciever and different uppers in various calibers to suit your needs. However if you want a good quality AR I would only buy a S&W, colt or bushmaster. Those are some of the higher quality firearms. You could also look into getting a STAG piston rifle which is beneficial if you will do a lot of shooting and don’t want to worry about fouling the gas tube. Either way to end a long ranting post if you think you will ever have to use the rifle to defend yourself or someone else figure on spending over 1000 dollars for a high quality one or you may regret it later. Also you can buy an 870 tactical at dicks for about 500 bucks new, so that would be your best bet for HD use.