Firstly, the problem:
"When I fire my stampede, it keeps firing even after I let go of the trigger. I have to turn off the safety switch to get it to stop firing."
Before i can explain to you why this happens, I need to explain the mechanics of the stampede's firing mechanism.
See that white part? (ignore the elastic band, that's my solution to the problem) What happens when you pull the trigger on the stampede, is it rotates that white piece of plastic which pushes a switch to start the firing cycle. What happens as the cycle starts is that the plunger tube moves forward. As this happens, it keeps this white part rotated even once we've let go of the trigger. This makes sure that the gun goes through one complete cycle before coming to rest. If it didn't, it could load a dart into the breech, but not fire it. This would then cause all sorts of jamming issues.
What happens when you overvolt the stampede is that you increase its RoF (duh). Eventually, the pieces inside the gun are moving too fast for that white piece to return to the off position before the next cycle starts. As a result, the gun doesn't stop firing until we interrupt the current (by switching it off). The spring pushing the switch to the off position isn't strong enough to turn the gun off between cycles.
The Solution:
We need to increase the force pushing the switch to the off position. I have done this by adding an elastic band to that white piece and attaching it to another part of the gun. Another way I did this in another stampede I modded was to add a small tension spring rather than an elastic band.
Once you have done this, you should find that you no longer have issues with the stampede bump firing.
Also, you may wonder about the other methods I have talked about. Most of the other solutions I have heard for this problem actually slow the RoF of the gun to a point where this issue no longer occurs.