Nerf Stampede PolarStar Fusion Engine Conversion

Those familiar with the PolarStar Fusion Engine conversions that I do were probably expecting this to happen at some point…

The Nerf Stampede PolarStar Fusion Engine airsoft gun.


  • Powered by PolarStar V2 Fusion Engine (upper cylinder)
  • Integrated box mag, 2000 round capacity
  • Box mag automatically winds when trigger is pulled
  • Easy access BB fill port
  • Prowin M4 hopup
  • Maple Leaf Bucking
  • Madbull 6.03 TBB, 300 mm or longer, depending on configuration
  • Rate of fire: currently 40 RPS (adjustable)
  • Semi auto/Full auto fire select
  • Weight: 3 lbs, 6 oz (unloaded)
  • Painted Nerf StampedeFun factor: off the charts!

After converting almost every type of airsoft gun to use the PolarStar Fusion Engine system, I turned my attention to some non-airsoft gun platforms. The Nerf line of guns was a natural choice. After examining the different Nerf guns available (by going to my basement closet and pulling out the large collection my kids have acquired through the years), the Nerf Stampede appeared to be the best candidate. It had a very aggressive look and had plenty of space inside for the Fusion Engine.

Final wiring

To install the Fusion Engine, the guts of the Nerf Stampede are essentially removed and only the outer shell is used. The Stampede has a safe/full auto switch on the left side of the gun. This switch was reused and it now functions as the semi/full auto switch for the Fusion Engine. For now, to safety the gun the air line is simply removed. In the future I may add a slide switch to the gun so it can be more easily put into safe mode.

To mount the inner barrel and hopup, polypropylene rod stock was lathed down to fit within the Nerf barrel and bored out so it works as a spacer to hold the inner barrel. Slots were cut into the piece that holds the Prowin M4 hopup. The plastic spacers where then fixed into place using screws so they won’t move and it holds the hopup securely against the face of the Fusion Engine.

The Fusion Engine was mounted using a couple of aluminum brackets. The Fusion Engine was aligned by using a long barrel that went through the spacers and hopup block all the way to the back of the gun. The rear mounting bracket was them marked so using the inner barrel as a guide. Since the inner barrel, hopup and Fusion Engine are all fixed into place and matched together, the system has perfect air seal and so far no jamming problems, even with the high ROF.
Box mag test fit. The walls of the box mag were cut down so it fit in the battery compartment.

When converting a non-airsoft gun, one of the largest challenges is determining how to handle the BBs storage and feeding. It can be very difficult to adapt a magazine system and have it work consistently and reliably. With the Stampede, the solution became evident when looking at the large battery compartment in the stock of the gun. It turns out to be a perfect location for a box magazine feed mechanism. By integrating a box mag into the gun, it added a new level of capabilities to the platform. No longer do you have to worry about carrying magazines, reloads, magazine winding or limited capacity. Now you can have extreme capacity and the ability to quickly refill when needed through a fill port on the top of the gun. The MAG brand box magazine works perfectly in that the feed mechanism lays flat in the box mag and has very low profile. The sides of the box mag were cut down to fit the battery compartment, and a little bit of cutting on the Stampede body shell made room for the servo that powers the box mag. A water bottle top was glued into place at the top of the gun, and a foam bulkhead was created to keep the BBs in the back of the gun. When all the extra plastic is removed, the back of the Stampede can hold about 2000 rounds. The back of the box mag was also cut away, and then covered with another foam bulkhead. This allows for easy emptying of the box mag. Simply remove the stock back plate and foam, and the BBs can be poured out of the gun. I have also added self adhesive foam to the box mag and the box mag area to minimize the noise of the BBs rattling around in the body.

Feed line and front mounting bracket

A long feed spring is routed from the box mag to the Prowin hopup. The hopup was drilled out to accept the feed spring and it is held in place by a small set screw. The set screw can be loosened from the outside of the gun so the feed spring can be removed and BBs so BBs can be cleared from the hopup if necessary. The feed spring is supported by a tube that minimizes flexing in the spring and it provides for a relatively straight path for the BBs. This allows the box mag to feed at very high rates, easily reaching 40 RPS without any skipping, using a 7.4v lipo battery.

FCU and Battery stored in magazine

The Stampede dart magazine was cut down so it made room for the hopup, but it now conveniently holds the battery and Fusion Engine FCU. It makes it very easy to access the FCU to adjust settings or change batteries.

The trigger mechanism for the Stampede was removed with the other mechanical guts of the gun. Instead a simple lever micro switch was epoxied in place behind the trigger. The mini circuit board was mounted inside the gun which connects the solenoids, trigger and full auto switch to the FCU wiring harness. The air line was then routed down through the grip and out the bottom of the gun. An MCU (motor control unit) was added to link the box mag to the FCU so it will auto wind when the trigger is pulled.

While the Nerf Stampede Fusion Engine gun has many significant advantages, there are a couple of disadvantages that must be accepted. First, maintaining the gun will take a little extra effort. To fully access the hopup (to change buckings) or to lubricate the Fusion Engine, the gun has to be opened up. The Stampede shell is held together by 36 screws. Yikes! So it takes a little patience, but once it is open, it is very easy to work on any part of the gun. Thankfully you don’t have to get inside the gun very often. The other disadvantage, which hasn’t proven to be an issue yet, is the durability of the gun. It is, of course, a toy Nerf gun, so it is made from relatively brittle plastic.If you fall on the gun or use it as a battering ram, you will likely crack or break the body. It might even get cracked if the body is shot by BBs from a short distance (e.g. 15 feet). But it can be easily repaired with superglue, or necessary, a whole new Nerf Stampede gun can be purchased for $50 or less, so the replacement costs are relatively low (not counting the modification work, though). For me, the pros far outweigh the cons. This gun is a serious monster on the field, despite it’s toy origins.

Painted Nerf Stampede

I’ve built Stampede #2 and painted the body parts to allow the gun to be a bit more stealthy on the field. I’ve also added an outer barrel extension and foam filled extension to accept a longer inner barrel. It has a 425 mm Madbull 6.03 barrel and it is very quiet shooting 420 fps w/.2g BBs @ 70psi using a silver nozzle.

Both guns have proven to be very accurate and reliable. The Maple Leaf bucking has worked very well. The Prowin hopup, which is prone to having bucking lip jams, was modified before installation. I made the BB feed hole oval shaped at the top, using a dremel bit. This allows the BB to “go around” the bucking lip when it feeds, preventing jams.

The Nerf Stampede Fusion Engine gun has turned out to be the perfect airsoft gun for my playing style. It’s extremely lightweight, high-capacity, very accurate and versatile. I can switch between assault, sniper, and support roles with ease using a single gun. It is now my new favorite airsoft gun and my weapon of choice at airsoft OPs.

Now available for purchase on the Products page.

Happy Airsofting!

– Rudy

Watch the Nerf Stampede in action at OpFor Airsoft, Paintball Adventure Park in Taneytown, MD

(also coming soon, Spock’s favorite airsoft gun…)