Matt,

Although I haven't responded often, I've read a lot of these threads, and it seems to me that often when someone asks for help they get a whole bunch of replies telling them to buy something else other than what they've already got. If you are satisfied with what you have now, and come to understand the limitations of converting a fowling piece into a deer gun, then go with it. UncleNorby gave the best advice.: Start by buying several brands of rifled slugs and see which ones group the best at 25 and 50 yards. Then buy lots of the ones that work the best and practice, practice, practice, at various ranges out to about 75 yards or so. You can get pretty good results with a smoothboore as long as you understand that you don't have a rifle, and won't get the same performance as a rifle. IF you can group 3 shots within 4" circle EVERYTIME you shoot at 50 yards, then you can feel confident that you can hunt deer at that range. Then you focus on what is really challenging for beginners--getting within 50 yards of a deer.

One more point before anyone else brings it up: any type of sights, or scope will improve your effectiveness over a bead, and eventually you may want to go that way. But if you just want to go hunting, and can't afford to buy everything there is to buy, then pratice with what you have until you are effective with it.
face the facts: you have a shotgun that is designed to point and not aim. don't let that discourage you from getting your venison.