Crosman 1377/22 5.5mm Breech Riser

This is a breech riser that should fit the common steel breech, you will need a longer hammer pin(use old dremel bits they are the exact diameter, TF port(Use Vinyl tubing for the TF port.), and breech screw (I would recommend tapping the main tube to m3 thread, machining and chamfering a m3 bolt to fit in the breech without obstructing the bolt). With this you can add your own barrel shroud without making it off centre.

It's probably best to print in ABS.
Update: I made it so that it prints