I hooked up three 9.6V batteries to my nerf vulcan. Now I need to mod the battery compartment to hold them and build some longer chains. It goes through 25 rounds in about 3 seconds.
This was inspired by ManaPotion's 500 RPM Vulcan mod. I've reached their fire rate, but hope to push it a little farther with some heat sinks and better motor cooling. One more 9.6V battery should push it to 666 rounds per minute.
Super-charged Hexbug runs about twice the speed of a stock bug.
I modified the body to accept the slightly longer body of a zip-zaps motor and hacked the battery compartment to fit one more cell. Faster motor plus more voltage equals one fast bug. I don't know for sure what the circuit is rated for, but I don't suspect 4.5V will have an adverse affect on it's life.