This is a set of gun mounts that can be used with a barbette made from PVC pipe. They consist of a mount that goes into the pipe and an insert for the mount to hold the gun. There is also a ring you can glue to the bottom of the turret that has holes for 1/4" x 1/16" hold down magnets. The mount will rotate in the PVC, allowing you to aim the gun anywhere. A screw can be used to hold the mount in the position you want.
These mounts are designed to be used with guns that have a removable up tube. Once the insert is glued into the mount, the only way to remove the gun is by removing the up tube.
The mount system has files for PVC pipes from 1.5" to 3", as well as for the couplers used for those pipe sizes.
I want to give credit to pba for designing this mounting system. He also printed all the prototypes to check fit and functionality. I just drew up the parts.