2" groups at 75 yards is not all that bad. That rifle as it stands would be more then able to take whitetail out to 100 yards. On average my side locks with open sights shoot 4" and on some days I get lucky and they shoot much better, but I really do not expect better. When I clover leaf at 100 yards, I am thrilled to say the least. You have to remember, you're shooting a 1/2 inch projectile. If you just touch hole to hole, you're at 1". Big bullets make nice holes.
The way to shoot better groups is with better sights. A good peep sight might help close that group up even more. If legal, you might look in to a scope for the rifle if that is what interests you.
Another option is try different projectiles, change lubes, even change caps, and see if they can shoot a better group for you. I really would not be too disappointed if my rifle was shooting 2" at 70 yards.
Of course the last way to shoot better groups is to shoot more often. The more you learn about how the rifle shoots, the ignition time, the hold of the rifle, and your sight picture. All of these can be worked on to shoot better groups..
Good luck with your rifle.