Also, note that this is ONLY MEANT AS A TEMPORARY FIX. It more than likely does not work as well as the bucking and nub combos that are designed specifically for this purpose.
That being said, Here was my problem:
I bought a used Classic Army M15A4, and it was a steal at a rediculous price because the guy didn't know what was wrong with his hopup. So i picked it up and gave it a look. The problem was an easy fix if you know what to look for. The BB was always falling way way short of its mark, indicating that it had no backspin, in turn indicating that there was a problem with the rubber bucking assembly of the hop up.
I broke it down and it turns out the little nub (or nubbin) that pushes down on the bucking to create backspin was not damaged, but entirely missing. Having no new bucking and nub on hand, i started getting impatient because i wanted to USE this thing, and all its fancy upgrades. (tightbore, upgraded spring, etc...) So i started looking around for things to use as a replacement nub. And lo and behold, right on my desk sat the perfect implement.
Step 1: Tools
Standard Airsoft Gun Disassembly Tools
Allen Key Set
Leatherman OR pliers and a good knife.
A Glue stick. (you could even use an empty carcass of a glue stick, you really don't need much. Your saliva may even work in a pinch)
Fine Tip Felt Pen
Step 2: Materials
-Your airsoft rifle
-the knowhow to disassemble (and reassemble) it
-a ballpoint BIC pen. It does not have to be empty, only slightly used.
Step 3: Disassemble Your Airsoft Gun. CAREFULLY.
Again, i'll say that if you do not know your gun inside and out do not attempt this. Do not follow my lead and instead be patient and take it into your local shop and have them replace the nub and bucking FOR you.
Back to the disassembly. Pull apart your gun (keeping your parts in order) and pull out the barrel and hopup assembly. Turn your hop up all the way DOWN (i.e. NO backspin on the BB position.) There will be a place on the uppermost gear where its teeth are not cut, to prevent the hopup from being adjusted to too extreme of a measure. The gears will not turn beyond this point. When your gears will not turn anymore, take your felt pen and mark where the teeth meet on these gears, so that you can reassemble the gears correctly. Take off the little clip (careful, this part can fly) where the barrel meets the hopup assembly, and pull the barrel out carefully, while applying mild pressure so that it will pull out the bucking, brass spacer, and o-ring with it.
Next you will want to pull off the uppermost gear (MAKE SURE TO MARK IT FIRST) and the axle pin where the hopup arm rotates, making sure to save all the little e-clips and things that you pull off. This will leave you able to pull out the part in the next step, which is where we make our modification.
Take your pen and pull out the ink tube. Grab your trusty, trusty knive and clip a short, 2-3mm piece off of the end. then reassemble your pen and put it back in your desk, keeping the little temporary nubbin handy.
Step 6: Reassemble Your Hop Up.
Step 7: Ensure Working Order of Gears and Nubbin, Reassemble Your Gun.
Turn the gears all the way back so that the nubbin recedes into the hop up body, and reassemble your barrel and bucking assembly. Put everything back into your gun correctly, and go adjust your newly-working hop up!
I'll say again, this is only intended as a temporary fix. I intend to go to my semi-local shop at the end of the week and purchase a new bucking and nubbin that is actually designed for this purpose.
Thanks for reading my instructable, please comment and rate as you see fit!