[Kevmo]

Apparently there are barrel extensions. Up to 8” as far as I can tell.
https://www.google.com/search?q=screw-in+gun+barrel+extension&ei=OA_SYYN-tKmq2w-RtYDADQ&start=30&sa=N&ved=2ahUKEwiD-uGF-JP1AhW0lGoFHZEaANg4FBDw0wN6BAgBEEo&biw=1366&bih=625&dpr=1

[Kevmo]

Here it is being discussed to add barrel extensions to pistols, with a picture in particular of a Rossi Judge Revolver pistol and a Rossi Circuit Judge Rifle

https://www.quora.com/Is-it-possible-to-make-a-rifle-out-of-a-pistol-by-simply-extending-the-barrel-How-will-this-work