Quote Originally Posted by Patrick Chadwick View Post
The snag is that you do not have an "as built" Gras rifle, but a Chassepot (Mle 66) converted to take Gras center-fire cartridges (Mle 66/74). This was done by inserting a sleeved element in the Chassepot system, with a resulting "squeeze" section just in front of the Gras-sized chamber. The /80 modification was the pressure-relief groove milled out behind the chamber to handle burst cases in those early days of brass-drawing technology. The link above is worth studying to understand this, even if it it requires a bit of patience and a good dictionary!
Score another one for Patrick Chadwick!

I don't know about the reduction in bore size, but there's a well executed sleeve in mine. The rifling is good with a very defined origin. But that's because of the sleeve. Seam's invisible about 2/3's of the way around, but the bore scope picked it up after about giving up finding anything. Never did find the back end. Very nice workmanship. Thinking that any reduction in bore is likely due to vagaries in press fit or later corrosion in the joint. I'd about have to section mine to discern anything further.