This Instructable will cover how to upgrade your Nerf Mega Magnus by replacing the spring, and removing the Air Restrictor. Note: If you upgrade to a 15lb/7kg spring, then you won't need to remove the Air Restrictor, as a spring that strong overcomes the Air Restrictor resistance, even without removing it.
Step 1: Unscrew the Upper Slide
Remove any darts from the internal magazine before beginning.
Take the four screws out of the upper slide. I usually leave the screws in the screw wells, as some of them are different lengths, and they'll need to go back into the same place.
Step 2: Separate Blaster Body
Before separating the body make note of the position that the tension springs are in, on the aperture blade (the orange flange that holds the darts in). When reassembling, these need to go back in place.
Some blasters have "Do Not Tamper" stickers, you'll have to split those.
Then unscrew the 9 screws on the blaster body. I usually keep the screws in their screw wells, since they are different lengths, and they need to go back into the same spot.
Carefully pull apart the two halves of the body. Be careful the screws don't bounce out of their wells.
Step 3: Lift Barrel Out of Body
You will not need to unscrew any internal parts, you can work around the trigger mechanism.
Make note of how the dart retention aperture blade is seated. It will need to be placed back in the same position. In the mean time, remove it, and set it aside.
Lift the barrel from the body of the blaster, from the end with the front sight post. Once the front comes up, the barrel can be pulled away from the spring.
Step 4: Replace Spring
If you have a more powerful spring (available on Ebay). You can replace the old spring at this point.
Since the springs will look almost identical, if you get them mixed up, just press the two ends together like a "Chinese Finger Trap". The more powerful spring will be much more difficult to compress.
If your only modification is replacing the spring, skip ahead to reassembly.
If you are removing the Air Restrictor (not necessary with the spring upgrade), continue on to the next step.
What kind of performance can you expect with different mods?
I took 4 blasters, and modded 3 to various stages, and took them to a paintball range to have tested. Unfortunately the chronometers they used wouldn't detect the nerf darts. But they recommended going to a large local indoor gun range that has an indoor archery range, and using their archery chronograph.
It worked like a charm, and the staff at H&H Gun Range had a good time playing with the Nerf blasters.
So, the average for the 4 blasters are as follows:
Stock Blaster: 74 fps
Air Restrictor Removed: 79 fps
Upgraded Spring: 90 fps
Air Restrictor Removed & Upgraded Spring: 90 fps
Step 5: Air Restrictor
The Mega Magnus has a closed system for the Air Restrictor, which makes removal difficult.
Step 6: Clamp Plunger
At the back of the barrel assembly, there is a plunger that moves through the assembly, there is a retention ring that keeps it from sliding out the back of the assembly. This is what the spring presses against when assembled.
You'll need to make sure the plunger remains at the back of the assembly. You can tape it in place at the rear. Alternately, I prefer to clamp it with some small vice-grips.
Step 7: Grind Down Air Restrictor Center Post
When you look down the barrel, you'll see the front end of the Air Restrictor assembly.
I like to grind the center post down using a dremel.
I drill down as far as the dremel wand will reach.
Step 8: Boring Out the Remainder of the Air Restrictor
I use a 1/2" Hole Bit, which fits perfectly down the barrel once the center post is ground down.
You'll have to put it on some sort of drill extension to get deep enough.
However, you have to be careful not to drill too deep, and go through the diaphragm for the plunger.
If you stretch out the barrel assembly, and hold it up to a bright light, you should be able to see the shadow of the diaphragm for the plunger (which should be clamped in place). This is the maximum depth you can drill to without ruining the blaster.
Make not of that position, and hold it up to your drill. Note the depth on the drill extension. Once you start drilling, don't let the extension go past that point.
When you start drilling, you'll need to hold the barrel firmly. I recommend letting the front sight post rotate all the way around until it is pressed against the palm of your hand, in the direction the drill will rotate. Otherwise, once the bit catches the Air Restrictor, it will forcibly flip the barrel, and the front sight post will catch your hand (which will sting - lesson learned).
Once the bit begins to spin freely again, the first part of the Air Restrictor has been separated. Do not drill any deeper.
Step 9: Removing the Remaining Air Restrictor Pieces
I use a bent coat hanger to remove any pieces that get stuck in the barrel assembly.
After fishing out the upper part of the Air Restrictor (if it didn't come out with the drill bit), you'll see the lower part of the Air Restrictor.
The lower part of the Air Restrictor will be a ring, with a hole in the middle, flanked by two "C" shaped holes.
Fish out this lower Air Restrictor ring with the coat hanger. There will also be a very weak spring with the Air Restrictor assembly, fish that out as well.
Step 10: Cleanup
You should now be able to see all the way down the barrel to the far end where the plunger diaphragm is located.
I used a baby bottle brush to get the debris out of the barrel assembly.
You can also flush it out with water, but you'll need to wait until it is completely dry before adding lubricant and reassembling.
Step 11: Lubricant
If you want to add lubricant to make sure the plunger moves freely, I recommend lithium grease.
Traditional WD-40, is a penetrating lubricant, which will eventually break down the rubber O-rings. I don't recommend it. Likewise, I have been advised that if you use a silicone spray, to make sure it doesn't contain acetone as that can break down the plastic and rubber parts as well.
Lithium grease, or a silicone lubricant spray will work well. Just point down the barrel, and give a small spritz. Too much will just make a drippy mess.
Step 12: Reassembly of Barrel
Place the spring on the rear spring retention post.
Place the front of the spring into the barrel assembly plunger
You'll have to compress the spring a little to get the barrel assembly in place.
Make sure the three orange holes on the barrel assemble align with the three red posts on the blaster body, for proper seating.
Make sure the white trigger post is seated in the trigger well of the barrel assembly.
Once the rear of the assembly is in place, rotate the front sight post back into position over the screw well.
Step 13: Reassemble Aperture Blade
Place the aperture blade back in position.
If it helps: There are two clips on the left and one on the right.
Step 14: Prepare to Reassemble Body
You'll need to make sure the internal ammunition magazine spring is seated properly (and stays in position) before reassembling the body.
You'll need to make sure the spring stays above the screw well on the right, and stays above the shelf on the left
If you are lucky, you'll have gotten a blaster from a production run that has an extra retention shelf built in on the left.
Step 15: Reassemble Body
Place the body in position and align the body so that the Aperture Blade springs are in place on the Aperture Blade. If you need to, you can nudge the spring a little with a small screwdriver.
Press the body into position, clipping into place starting from the front end of the blaster, working your way to the back. Be careful that the magazine spring doesn't slip down out of place during the process.
Step 16: Screw Body Together
Screw the 9 body screws back in place.
Place the Upper Slide back into position.
Screw the 4 Upper Slide screws back in place.
That completes it!