Smallpatch=- there is NO rear lock bolt, however the back of the breech-plug could be contacting lock bolt.
For a minor crack like that, ultra-thin CA is the glue to use - or to check-drill holes and inject the very thin brand of liquid Acra-Glass from Brownells. I repaired a similar crack on my 14 bore rifle some years ago with ultra-thin CA and it is still repaired to this day and I do not shoot less than 82gr. 2f in that rifle, and commonly with longer range targets, the loads are 140gr. to 165gr. 2F. I suspect 140gr. 1 1/2 Swiss should duplicate my old 165gr. charge, and will try that at some point.