Hi Tom, back when I got my Greener Hammergun, I enquired about it & had a brief but pleasant flurry of E mails to/from Graham Greener.I had the same idea with the Greener Martini(converting to a rifle cal'], & asked him what he thought..he seemed very much against the idea due to pressure issues,& warned that such a gun should be sent for Proofing,also to remove the Greener name from the Gun.
But I didn't run the smaller bore rifled shotgun slug bbl by him, which would seem to run much more similar pressures to the original gun than a rifle cal' conversion.This idea seems more reasonable/safe.
He was pissed at a certain Australian site that had a pair of Greener "Presentation" Martini rifles,that he said were bsaically just tarted up shotgun converts, unsafe & mis represented as to grade.
Not much help Tom, just thought I'd throw it up here.
You could always E him, perhaps he'd view your idea differently...but perhaps he just doesn't like people messing with his Great Grandaddys Guns
I must say he was a super patient & helpfull Gent' to a total stranger.....me.
Good luck Tom
Franc
