If it were me, only because I already have the powder, I'd use Hodgdon's starting load of 4.9gr of Universal or 5.6gr of CFE-Pistol.
The Universal load is ~8,000 CUP for 721fps and the CFE-Pistol load is 9,800 CUP for 784fps.
Ed
I'll agree with the others in not lapping unless you have to, having done it a few times myself. Rounding over the rifling edges at the forcing cone is not beneficial, and can promote bullet skidding in the rifling. It's also easy to go too far and end up with an oversized bore. 100 bullets is way too many, I wouldn't want to use more than 1/4 of those at most and even that would be a lot.
However, if you do go ahead, the loads above are
way too fast. You want BB gun speeds, and ideally will have the bullet barely exit the muzzle. Doing that part right means adjusting your load as lapping progresses, and, you will have to determine the load yourself.
Use the fastest burning powder you can get, Bullseye, Clays, etc and start with small charges, probably not more than 2-3gr. Last time I did this in a 45 Colt I ended up using a 22 Short case as a dipper.
