Two types could be considered adjustable to change drops.
One with various size bushings, one with a screw adjustment.
I have both. Generally, use the screw adjustable to find the charge I want that works, and then use a fixed sized bushing to insure repetitive consistency.
1st try would be Bruce WIlliams RBWillNJ on this site as he owns and runs the Star Machine site.
2nd try would be Rich Daniels BarrelTester on this sit. He manufacturer.