[KrashRider]

Took a couple of inner barrels I have, the hopup, and all the buckings to a machinist I know. The results of some tests and measurements basically came up with the same thing told to me by the gun doc. The bucking lips are being pushed in by the air nozzle chamber which has a different diameter than the barrel section. Depending on the type of lips on the bucking and thickness this change in diameter pinches the lips differently causing a constriction. This was checked using three different barrels, the Prome, RA-Tech 6.01, Magpul PTS Masada stock. All barrels resulted in the bucking lips being pinched inward. The thinner buckings pinched less because they weren’t as thick at the mating end. The thicker buckings pinched the most due to their thickness. Then some of the buckings had a different length of lip as well. The machinist put a 45 degree bevel at the change in diameter to allow the bucking lips to retain their angle instead of being pinched inward. The Prome barrel size is still a factor but not as much now. After moding the hopup chamber all three barrels have no real bucking problems anymore. Most of them all work but still with some slight varying degrees of performance. Using the same bucking on each barrel and testing it. The fps is basically the same for each bucking on all three barrels. Each bucking has a little different fps across the board, except a little higher on the RA-Tech 6.01 probably due to the tighter bore.

Test result using the Prome barrel and modify S100 spring:
MadBull Shark – still will not work due to the three bumps inside the lips of the bucking pushing inward.
Guarder clear – works 405 – 410 fps
Prome purple – works 410 – 415 fps
ARS – works 390 – 400 fps
Madbull Blue – works 405 – 410 fps

The picture attached it the mod made to the MadBull hopup mating end. The machinist said if you wish to DIY it. You have to be careful take it slow but it can be done with an over sized drill bit to get the taper angle started and then smooth out the taper with a dremel.

Of course all hopup mods you do at your own risk. We did a lot of measurements and rechecking numbers before doing anything. It was done in small stages and checking the fit of the buckings each time. Then using a bb pushing it into the hopup making sure it would enter the barrel with very little force.