This hopup system is designed after a Apex barrel (for those of you familiar) and worked great when I had it placed on a paintball launcher. Unfortunately, I have since disassembled the piece. I am, however, currently building a launcher for golf balls and will be placing the system on this barrel. I will update this topic with photographs when this is complete. Until then, this is an exert from the book about the topic:
Improving the Barrel wrote:...Begin with a coupling the same size as the barrel. Remove the lip of the coupling so that the barrel will slide through it. Slide the coupling over the muzzle end of the barrel so that the coupling is flush with the end of the muzzle. Mark the opposite end of the barrel and remove the coupling (for reference, the measured length of the barrel muzzle to the mark should be the same as the length of the coupling). From here, measure from the mark towards the muzzle about 1/4 the way and place another mark.
On the muzzle end, at the exact top, measure a small width and draw straight marks from these points to the 1/4 inch mark. The strip should be about 1/4 as wide as the diameter of the barrel. Slowly and carefully trim out the barrel around this strip. Be sure to make the cuts as straight as possible. When complete, rough the inside surface of the slit with sandpaper.
Set the barrel aside and get the coupling. Decide which end will be the muzzle end and which is the back end. Now, obtain four short screws and a drill with a bit the same diameter as the screws. Drill one hole, as straight as possible just a little bit into the coupling at the muzzle end. Drill three holes through the back end of the coupling in the same manner. Work the screws into all of the holes to cut threads into them or use a tap. The coupling will now be called the hopup muzzle.
To install the hopup muzzle, just insert it onto the end of the barrel muzzle so that the muzzle end of the hopup and the barrel match. Rotate the hopup muzzle so that the screw hole lines up with the strip in the barrel. Install the back end screws and tighten them down to hold the hopup muzzle in place. Now, install the muzzle end screw. To adjust the hopup, tighten or loosen the muzzle screw.
For the most part, backspin is going to be dependant on the velocity of the shot. The purpose of the adjustable hopup system is to allow the hopup to be adjusted to fit the velocity, providing the maximum amount of backspin. Adjust the hopup so that the Magnus Effect is present (trust me, you'll know) but no further. Adjusting the hopup beyond that which is needed will begin to affect accuracy and cause the projectile to lift upwards rather than travel straight as intended.
