LtK's advice sounds perfect too me. But one thing you might consider since it is a double is that maybe you could bed the barrels to the gripping blocks. Wrap them in a little plastic food wrap and then bed with some good epoxy. I bet that plug will remove pretty easily. I would certainly try it before I got too far into the weeds with trying to machine something to grab whatever is in that chamber. Fiber optic exploration would be really high on my list too.

_________
BrentD, (Professor - just for Stan)
=>/

[Linked Image from i.imgur.com]