This is a 10 Bore correct?
5 Drams is a very light load for a 10 Bore, so your velocities are not unexpected.

Are your balls (the rifle’s, not yours) still cast of wheelweights? If so, they are probably too hard to slug up with only 5 Drams so you will be getting no resistance to build pressure, and not engaging the minor bit of rifling at all. Try 20-1 mix, or even pure lead.

Greener wrote that standard for 10 Bore is 6-8 Drams.
5 Drams is a normal load for average weight 12 Bore rifles.
- Mike