My oldest son did something like that a few years ago with a Crosman 1088 pistol and the barrel and housing from a Daisy 856 pump rifle. He cut out all but about 6 inches of the Daisy soda straw barrel and fitted it in place of the short Crosman barrel and trimmed the housing to fit so he could use the rifle's front sight.
It did catch the eye of deputy sheriff on patrol while we were testing it in the yard. I just knew I was going to jail when he pulled in with us standing there with the LBP (long barrel pistol). But once he saw it was an airgun he was cool with it. Told us to be safe and have fun and then left.
Anything that is complex is not useful and anything that is useful is simple
- Mikhail Kalashnikov