MO; before you start training for HD or tactical training, you need to learn how to proficiently handle a shotgun, any of them from a single shot to a drum feed Siaga. Shooting a shot gun is not anything like shooting a rifle with sights. Simply put it is like instinctively pointing your finger at a target and being able to hit it. That all comes with experience and practice. This comes from shooting trap, skeet and sporting clays. The real test and proof in the pudding is hunting game birds, ducks, pheasant, geese, ruffed grouse, wood cock, hungarian partridge, chuckars,and dove. I know people who can shoot a hundred straight in trap and skeet, but get them in the field and they have a lot of trouble bagging a game bird. The reason this happens is that when shooting at clay pigeons the are pretty much in constant zone, and there are no surprises.
The whole game changes when you are in the field and the bird erupts from m a totally unknown location. In front of you, behind you, even right out from under your feet. There are plenty of surprises, maybe one bird or several.
The experience you get from hunting game birds is knowledge that you get from the unknown surprise event and you learn how to handle it and hit the target. This is a very good thing to know in a HD situation. not all HD situations have only one perp, some times they are multiple perps and in low light conditions where sights are rendered useless. Shotgun do have sights/beads but they are more for what is called "sight picture orientation" , and not any thing like a rifle sight. Theres a lot to learn about shooting a shotgun, to cover it all would make this post a novel. But I hope that you understand a little bit of what the handling of a shotgun involves.
Take that single shot gun of yours, get a box of trap loads for it and go to the trap range and give it a try. I think you will be quite surprised at how much you like it.