Thanks Daryl,
My idea was to use a pistol ML barrel as guide or to make a short with a square steel rod twisted.
Most of the barrels I have are 1-30 to 1-48".
I have measure the barrel:
OD: .829"
ID: .694"
Wall: .0673"

If I rifled as you said .02" grooves the wall will be .0475"
If I rifled to a groove diameter of .715" it will have a .057" wall.
Is it enough to stand the pressure?

The other option is to cut 2.5" off the barrels and get a 25" cilinder ball and shot gun.

I will have some time to think as I will get guns papers in three weeks.

Thanks
Martin