What patch thickness, and what lube are you using??? These are the things that reduce group size. Also, measure that bore diameter- don't rely on what the marking is on the barrel. Some barrels are cut with new buttons, and may be a bit oversized, and others are cut with a worn button and are undersized. Measure the bore to be sure!
You should work up from your 70 grain load to about 90 grains, in 5 grain increments to find what will group the best in your gun. A lot of T/C guns shoot an 80 grain charge of FFg, and some with FFFg powder into tinier groups.
You don't give the barrel length or the Rate of Twist of the rifling in your gun. That and groove depth are important in choosing ball diameter, patch thickness, and then even powder charges. Check the spent patches for each round you fire. If you don't know how to read your patches, invest $15.00 to buy Dutch Schoultz's Black Powder Rifle Accuracy System. You will learn how to read them, and get a much better understanding of how to get the best accuracy from your gun.
http://www.blackpowderrifleaccuracy.com/
will give you the contact information to reach Dutch.
You also need to learn about cleaning your barrel between shots for best accuracy, and, both the necessity, and how to read your cleaning patches in the field to make adjustments in your cleaning techniques to conform to temperature and humidity conditions. Off the bench, at 50 yards, you should be able to shoot 2 inch groups. Off-hand, if you keep them all in 4 inches, you are going to win a lot of shooting matches! :thumbsup: