You have it in one JP! I have discovered this tonight...

and come to the conclusion that this may be the problem. The error can occur as you say, between the bolt handle and the O-ring. For what it is worth for anybody else experiencing similar problems, GMAC probes are a perfect fit, at least in Crosman and GMAC breeches. The fitting instruction from Gordon is fit the barrel hard to the breech, align transfer port and tighten grub screw. Next check bolt operation, o-ring should NOT be visible.

Thanks to all who responded to this thread.