Thank for the answers.
Birdhunter50,
I know that it would be a heavy but I know only one gunsmith that can do it and it is far away.
The sleeving, if the barrel is thin, should not increase the weight too much.(I hope)
fuhrmann,
I know it thats why I call it "cape gun" most British combinations were made this way.

I have not find the barrel yet.
If I can not get one on those I should have to use a .45-70 but I much preffer a smaller cartridge.
I will let you know how it ends.
Thanks
MArtin