Sir, I have just heard from a bunch of dedicated muzzleloading shooters from the USA, any and all of whom have more BP shooting experience in their ear-wax than I have in my entire body, and HERE is the advice -
'"The hollow base bullets shot in an Enfield have a habit of blowing out the skirts if they are fired with too great of a powder load. This needs to be recognized especially when using a powder that creates higher pressures than real black powder, like 777 does.
For this reason, if you wish to shoot your 777 in your Enfield I recommend that you limit the powder load to 50 grains maximum. Even with this small powder load the skirts of your bullets may blow out as they leave the muzzle thereby ruining any possibility of shooting tight groups.
If your groups are poor with a 50 grain load of 777 try reducing the load to 45 grains."
Best
tac