This is a simple remix I made that allows for a 16mm OD barrel to be used in a Spring Thunder shell. In the picture, you can see I used a 17/32" brass barrel wrapped with some windings of electrical tape to make a tight fit in the shell.

This will be compatible with any 16mm OD or smaller barrel, such as 1/2" CPVC or 16mm PETG, as well as 17/32" brass and Captain Slug's 5/8" aluminum barrels.