Round ball, smooth bore, pure wheel weights, about 3 lbs in weight.  7000 grains divide by 16 eachs 437.5 grain or one oz.  rule of thumb start with 1 1/2 oz per bore inch. 3 lb ball weighs more than concrete. But retain your wind age around your projectile.  The ball should roll out it you tilt the barrel down. If it sticks it is to tight for the bore.  Always worm first then load the first round, then worm sponge,worm again.  With a thumb over the vent hole the entire time.  This is done between each shot.Never ever hold the rod tight in your hand.  Palm facing up on the rod, and thumb is never ever touching the rod.  Because if you did have a miss fire your arm does not go down range with the ram rod.

If he is shooting the same size.   Duplicate his load and see how it works.  His charge would be the same as yours.  Easier to copy than start from scratch.
Bill

skratch , nice to see that you got to chance to fire the canon. I bet that it was a great feeling to see and hear the results of your labor.

ron

I've just got to post this.  I figured since the cannon is half scale and my wife is too I got her to pose for the pic. I told her she was Sgt. of the 1st Northern Socorro Co. Light Artillery because she got to light the fuse and could run faster than me. She's a good sport.  If she knew I posted it though she'd probably poke me with the bayonet. 
Still waiting on the mold and cooler weather,,

John....