G&G FN F2000 PolarStar Fusion Engine Conversion

The G&G FN F2000 is my favorite bull-pup design airsoft replica. The gun has some really nice features. Being a bull-pup, it has a very long barrel relative to it’s size. The AEG has an adjustable spring guide for variable FPS, the trigger has a unique full-auto mode that supports both single fire with a half pull, and full auto with a full trigger pull, and the upper receiver and barrel can be removed from the gun with a single push block. Even with all these features, I knew the gun would be even better with a PolarStar Fusion Engine. But the F2000 uses a custom gearbox so it would not be a straight forward conversion to the Fusion Engine (FE).

After a bit of thinking on the problem, it came down to three different issues that needed to be solved. First is how to mount the Fusion Engine within the F2000 so it can replace the existing gearbox and piston, second is how to make a nozzle that works with the F2000 hop-up and third, how to handle the unique trigger mechanism.

I decided to solve the trigger problem first. I wanted to maintain the same functionality as the AEG where the full-auto setting supports both semi- and full-auto. I first tested the FE to see if it supported the same behavior, and it does. If you press and hold the trigger on the FE it will run a single firing cycle and stop. If you then press the full-auto switch while the trigger remains pressed, it immediately cycles in full-auto mode. This is the behavior needed for the F2000.

To start out, I took the F2000 apart and removed the gearbox. It comes out very easily and cleanly and you are just left with the two body shells. There are other resources on the Internet that provide instructions on how to take the F2000 apart. Be careful to not lose the tiny fire selector detention pin and spring.

A couple drawbacks of the trigger mechanism on the F2000 is the long trigger throw, and the sometimes rough/sticky trigger pull feel. To fix the sticky trigger, I filed down the guide rails on the trigger just slightly, which allowed it to move more freely within the guide slots.

Just a note, to remove the push rod from the trigger, rotate it down, then it will pop up and out easily.

I also filed down the plastic push rod guides to prevent binding.

To replace the trigger and auto fire micro switches on the FE, I used two small lever micro switches, picked up at the local Radio Shack.

I attached JST wiring connectors to the switches, then mounted them to the body of the F2000 using 1/2″ 2-56 bolts and nuts (also available at Radio Shack).

For the semi-auto trigger, I mounded one level micro switch parallel with the trigger rod. I bent up the lever so the rod would close the switch when it is pulled back. I have it mounted so that the slightest pull on the trigger would trip the switch, and it would remain closed to the end of the semi-auto trigger travel. This picture below shows the location of the push rod when the trigger is in normal/safe position. It is just barely touching the bent switch lever.

This is when the push rod is fully pulled in the semi-auto mode. Note that the micro switch lever matches the end of the push rod, and the push rod barely touches the full-auto micro switch lever.

For full-auto mode, I have the full-auto micro switch mounted perpendicular to the push rod. When the trigger is fully pulled, it closes the full-auto micro switch, while keeping the trigger switch closed also.

Mount the semi-auto switch first, making sure it closes properly and stays closed through the whole trigger pull. Mark the body through the holes on the switch and drill holes to mount it. Do the same with the full-auto switch, having it fully closed when the trigger is fully depressed.

To mount the Fusion Engine within the F2000, I first had to determine the nozzle line within the gun. The F2000 uses a modified version 6 gearbox, and the stock nozzle is offset from the center line of the piston. The FE would need to be mounted centered on this nozzle line. Is used a line laser to help identify the nozzle line within the gun. I placed the stock gearbox in the right half of the gun body, and setup the laser to make a parallel line with the front of the body and centered down the nozzle and along the piston.

I then removed the gearbox and drew a line on the body following the laser line.

It turns out that the nozzle line follows a line already identified on the body. The line follows the seam between where the body side is flat, and where the the body begins to curve to make the top.

To mount the Fusion Engine, I removed lower half of the Engine. The trigger circuit board can be detached and just a few additional screws removed and the entire lower half can be removed. This part of the Fusion Engine won’t be used, and it further reduces the overall weight of the gun.

Then made two L brackets to mount the Fusion Engine cylinder and solenoid mechanism. One bracket will be used at the front, making use of the front mounting bolt, and one bracket will be used at the back of the cylinder, which uses a M5 bolt.

I started by measuring the center of the back bolt, based on where the cylinder would sit. The center point would be inline the same as the center line of the body half.So I used a straight edge and ruler to determine the center point height to the body half. This turns out to be 1-1/32″.

I then fabricated a bracket out of 1-1/2″ angle aluminum. The foot of the bracket is smaller so it mounts cleanly to the flat part of the body. I slotted the holes to allow for some adjustment in case it’s needed later.

I mounted the bracket using #6 flat head screws and I counter sunk the holes so the screw heads are flush with the body. This picture shows the rear cylinder bolt in place, centered on the body half.

To mount the front of the Fusion Engine cylinder, I made a smaller L bracket and slotted the holes to allow adjustments. The cylinder head need to rest flush up against the plastic brackets behind the hop up BB guide. I used single #6 flat head screw to mount it to the body, and the existing Fusion Engine bolt to attach the bracket to the Engine. The slotted brackets allow the cylinder to be shifted left/right and up/down. Test fit the cylinder with both body halves assembled together and make sure the nozzle is centered right above the hop-up BB guide.


Some plastic tabs on the body needs to be cut away to make clearance for the cylinder.

The F2000 hop-up needs to be modified to properly match up with the FE cylinder. Some extra plastic around the hop-up needs to be cut away.

The modified hop-up should fit cleanly into the face of the Fusion Engine. It’s important to make the cut away plastic even and symmetrical around the nozzle opening so it is centered properly with the Fusion Engine nozzle.

To make the nozzle, I used the same technique as I did with the A&K Masada. I first measured the length of the nozzle when it is in firing position.

I milled down a stock Fusion Engine nozzle to be the same size as the stock gearbox cylinder nozzle. I then cut the plastic nozzle to the right length so it also protruded out from the Fusion Engine the same distance. I used gel Super Glue to attach the nozzle to the modified FE nozzle. (Note the nozzle pictured below represents the nozzle, but it’s not the exact nozzle I used. I used a spare MP5 nozzle, but the “shoulders” on the nozzle are too high, so they had to be milled off. It works just fine, but I’ll have better pictures of a proper F2000 nozzle when I get a chance to make a proper one. I’ll also show the final length of the stand alone modified FE nozzle).

To hook up the trigger and full auto micro switches, I added two JST connectors to the back of the trigger circuit board.

I drilled a hole in the bottom of the body to allow for the air hose. The hole has to go through a few layers of plastic, including the battery compartment.

One other modification to the F2000 is to fix the rubber stock cover to prevent the metal plate from falling off. Simply heat up a piece of metal and melt down part of the plastic tab holding the plate, fixing it in position.

Here’s the F2000 with the Fusion Engine and trigger micro switches installed before assembling the body halves.

Once put together, there is plenty of room in the F2000 for the FCU, trigger circuit board (which can be covered with some heat shrink tubing), and battery. (The extra connectors and wires are there since this is the Fusion Engine borrowed from my Stoner 63)

The F2000 is an ideal gun for the PolarStar Fusion Engine. There is plenty of room inside the gun and the trigger mechanism is easily adaptable to the FE. The gun is outstanding as a regular AEG. With the Fusion Engine it becomes are a real monster. The compact form factor, longer barrel, instant trigger response, and crazy fast rate of fire, makes this customized gun a dominating weapon in both field and CQB games.

Happy airsofting!

– captainbingo