he is looking for one shots, mags are not going to help at all with that. The long barrel is the only attatchment that will increase damage, but only at medium range. So, it will give you a slight better chance for one hit at medium-close range.
Whenever I use shottys, I only use long barrel, and put my other spots into perks or nades, what have you. if you look down the sights on a shotty then you are either going to get killed or you have enough time to aim without any sights or quickdraw. With mags, you have twelve shots, hit one or two of them... MMS scanner could be useful in certain situations, but not really worth a spot/10. Trust me just hip fire with a long barrel, you will do okay. But if you are looking to do the one shots asap, go hardcore. You'll one shot from twenty feet