Snooker wrote: ↑June 5th, 2023, 8:52 am i got mine from a RC Airplane shop, in German they are called "Stellring". If i can trust the Translation website the propper english term is "adjusting ring" or "setting ring". They're availlable in different sizes and forms.
Unfortunately that design is incorrect, the ones on the Proton Pack were a single piece, with a small grub screw set into the side in order to hold it onto the mechanism that extended the Proton Gun barrel.
CPU64 did sell them as recently as 2014, so it might be worth getting in contact with him to see if he still sells them:
LINK.