Search:

40mm. The simplest I can think of is a 'L' hook mounted horizontally, the hook entering the muzzle. Black shrink wrap the tip, paint the shaft of the 'L' hook same as your wall.

And a similar...

40mm

Super magnets epoxied into a concave wooden mount screwed onto the wall (drill hole through wooden block and split to make two holders).

Will hold barrel invisibly. Here is a drawing I...