Think the timing may be OK...the little thin lin at the middle leading to the notch isn't uncommon. Bolt just falling a bit early, it still rotates (leaving that drag line) and falls into the correct notch. Can test it by cocking the gun slowly, looking and feeling to see if the bolt has locked into the notch.
Definately better acccuracy with ball that chaves a bit of lead in being seating (and proabably a bit safer from a chain fire). Tripple Seven packs in a bit tighter...smaller garins, so it kind of self compresses leaving more empty cylinder ahaead of the ball. The corn meal works...felt wads would work as well, and are worth a try (but cost a good deal more than corn meal if that's an issue).
Yep.. do tend to catch busted caps...harder to keep that from happening with Tripple Seven as it seems to promote that kind of cap-managling. Are some mechanical modifications that might help....but one simple solution (seeing as you don't much care how fast the ball is going) is to try some Pyrodex RS. Might be a bit dirty, but would be low pressure...and that might keep the cap from fragmenting and jamming up the works. Could also try a differnt brand of cap...seems like they would be the same, but it turns out there is a pretty wide variation in cup harness.
Mehcanically, if you look at the recoil shield, will see a little "ditch" cut into the shield leading away from the hammer slot. IDea is that that ditch is supose to help carry away blow caps as the cylinder roates and let them dump out the right side. BUT most reproductions (1) have that ditch too shallow (2) don't have that ditch lined up with the nipples (3) have sharp burred edges on the hammer's slot in the frame. Take off the burrs aroudn the slit and make it into a slight radius...deepwn and polish that ditch, deeping it off center so that it lines up with the nipples better.
Found that some lighet mainsprings contribute to this...makes it easier to cock, but if too light, the caps tend to blwo back the hammer enough to fall off freely..and they tend to fall into the lockwork.