Yeah , made one years ago . went to flea market got front lens in its adjustable sleeve from old slide projector ,for magification. Then eye piece from telescope. When purchasing you can simply test by moving to and aft to see if you can focus on something as deep as your barrel, then at end used right angle prism from old bioculars .
Mounted every thing in pvc pipe that slips [ adjustment] and prism glued at end with side notched. When using I tape small unhooded flashlight works well.
Main thing was looking for was vent hole wear and erosion.