My first post here so please be gentle.

I've been hovering around this excellent forum for sometime now as I have a great interest in how things used to be done back in the day (since reading several books by Sir Samuel White Baker) In fact this particular forum inspired me to go out and buy an old but in good condition Greener GP for slugs and round ball(target only, big games a little hard to come by in the North of England)
Well every question I needed to ask has already been asked and answered here with the exception of one.

I am experimenting with round ball black powder loads in my humble Greener with great success but BP makes a real mess of plastic cases and with this in mind I've purchased at great expense a box of 12 gauge Magtech brass shells. My question being is there a method of crimping the ball into the brass shell?

As Im sure the members of this forum are already aware magtech brass has an internal diameter nearer 11 gauge than 12 so wads ect. can be a problem with standard 12 gauge offerings being a more than sloppy fit. I've managed to get round the wad problem now all I need is a reliable method of crimping the case closed such as I've seen in many pictures.
The picture in the thread Side by Side Slug Gun, What should I do? shows exactly the crimp I'm trying to acheive.

Any help with this would be greatly appreciated.

Regards
Badger