Yep...Stop shooting when the bullet isn't firmly down on the powder...You will either blow the gun up or put a bulge in the barrel...
I don't like 777 because of the crud ring, prefer Goex or Pyrodex...You may have to go with 2 damp patches or even a brush between shots if you wan to stay with 777...You need to clean the barrel enough that the bullet will smoothly slide down all the way...You might also try a Crush Rib sabot as well...