Hello Phil
I do have a set of plans on CAD for the barrel back, at the moment they are scaled for a model of 1.5m in length.
If you have access to CAD I could email you a copy so that you can play with it yourself, if you don’t have CAD I then due to recent illness I wouldn’t be able to produces a scaled down set for a while.
However once produced I could email them to you in PDF format for easy printing.
Paul