The Nerf Vulcan's rate-of-fire (RoF) is voltage dependent. By default, the stock battery sled holds 6 x 1.5V D Batteries which combine to provide 9 Volt of power to the vulcan's motor, this supports a RoF of approx. 3 rounds per second or 180 rounds per minute.
To increase the RoF, a popular method is to increase the voltage to the motor. Here is how it can be done in a modular manner with affordable easy to find parts.
Step 1: Get the following items from an electrical supply store.
1 x Battery Holder (holds 10 x AA Batteries)
1 x Battery Holder Snap-on Adapter
2 x Insulated Wire Leads (Red + Black w/ Crocodile Clips)
Step 2: Attach everything together and install the AA Batteries (in this example, i used 10 x 1.5V AA Batteries which provide 15 Volt power in total). Please take note of the Positive (Red) and Negative (Black) wire leads, do not let them short-circuit!
Step 3: Remove the Nerf Vulcan's stock battery sled and clip the Red and Black wire leads onto the connection points (clip properly and note the wire lead colours!).
Final Step: Test out your upgraded Nerf Vulcan!
Here are the Test Fire Videos of my stock vs. upgraded Nerf Vulcans:
Nerf Vulcan RoF - Stock 9 Volt Power (180 RPM)
Nerf Vulcan RoF - Upgraded 15 Volt Power (300 RPM)
Do note that as expected with most electrical components, increasing voltage to a motor might invariably shorten its operating lifespan.
I've also tested increasing the voltage to 18 Volt and 24 Volt by adding more batteries (and also tested with other battery types like R/C car Li-Po batteries), the RoF gets even faster! But at those crazy speeds i could smell the motor start to overheat even after firing just 2-3 ammo belts, therefore do be careful when increasing voltage to those limits! Don't let your Vulcan's motor burn out prematurely.
This upgrade is easily reversible back to stock voltage (just detach the clips and revert back to using the stock battery sled) and its very modular too, so you can adapt it to your own preferences. :)