JamesAirsofterAgent

Yeah for sure there are quite a few things that could potentially go wrong with reassembly.

- ensure the hop rubber (the sleeve) is fitted correctly, with the protruding line seated in the long groove in the barrel and the protruding nub on the opposite side seated inside the little window in the barrel.

- It is possible to push the barrel with the rubber installed both too far into the hop unit thereby smushing up the feed lips and distorting them or equally not pushing the barrel and feed lips in far enough, either scenario will almost certainly run you into feeding problems. 

(sometimes with a very tight setup a small amount of silicon oil can help - just don't get it any where near the hop window)

- ensure the plastic barrel clip is engaged properly, with the two recesses fitting correctly into the grooves on the barrel - some times these can go on and you'll think its in place, but in fact the two recesses are just resting on the edge of the groove - this can cause the barrel itself to move, thereby moving the cut out window away from the hop up arm rendering the mechanism useless.

- custom hop nubs (especially flat nubs) can be too wide for your existing hop window and will require an equal amount of light sanding on each side until it comfortably fits into the window - the test is to press the nub down and see if it springs back up under the elastic force of the rubber without getting lodged. 

This could cause you an inability to push the nub down or it could cause the nub to become stuck, so that when you raise the hop arm the nub remains in its position in the hop window.

- Also do check the condition of all the hop mechanism dials and arms, It's not that common but some times these little pieces can become a bit minced.

Defintley, take the barrel and hop out, dissemble, then  reassemble checking at every stage.. to be safe, do clean the rubber on the inside of the window. You can see if the mechanism is working in its most basic form by looking down the barrel and adjusting the wheel while observing the nub lowering and rising.

Hope this helps dude!

Sounds like an issue I had when I hadn't reinstalled the nub correctly.

@Cr0-Magnonoh yeah, that's another one - make sure the nub is engaged properly with the hop unit arm, if it doesn't lodge into the arm it may be displaced to the front or rear of the arm most likely resulting in a zero hop scenario. 

Some people like to super glue the nub onto the arm, which is fine for a standard nub, though not necessarily for a flat nub otherwise you may get a sloped hop surface - the tiniest blob of silicon gel can help keep it in place for installation.