The only way to do it is to offset the bore of a .17 barrel in a shotgun stub. I made one up in 5mm RF Mag, (stimpylu32 has it now) and, while it worked great, it was a huge pain in the butt to make and I don't think I would do one again. You would be better served to just buy the whole gun and be done with it. I just finished failing badly making up an offset bore 14 HMR barrel, that's right a 17 HMR necked down to .14, don't try this at home.

Larry