Breakout - Break-action Silly Shell conversion for the Nerf Rival Knockout

After seeing the SillyPilot, I knew I wanted the emotional response of its satisfying break action and auto ejection, but I also wanted the fantastic priming feel of the Pilot's predecessor, the Knockout - which happened to be the only option, since I didn't own a Pilot. So I decided that I'd have my cake and eat it too, and created this miraculous conglomerate of open-source magic.

The Breakout effectively does the same thing that the SillyPilot does, but on the Knockout. Shell ejection is buttery smooth and decently powerful, and the stock spring and plunger tube size is more than powerful enough to launch most of the higher-capacity shells available. Additionally, due to the way the blaster is assembled, I've been able to include a top picatinny rail on both ends of the blaster as well as optional side picatinny for all your tacticool needs.

———

Bill of Materials

• 4x M3x10 mm bolt
• 4x M3x25 mm bolt
• 2x M3x30 mm bolt
• 4x #4 or 4-40 screw
• 1x 016 o-ring
• A good amount of elastic cord

For each FrameWithSideRails, if printing:

• 2x #4 or 4-40 screw

Other tools:

• CA glue/superglue
• Scissors
• Wire cutters/flush cutters
• A Dremel or hand saw
• Needle-nose pliers (optional)

———

Printing

Print all parts once, except for the RailTop - print that twice. If printing the Frames with side picatinny, print the RailSide twice as well. Most parts should auto-orient to the recommended orientation - for the Frames, I recommend printing with the interior side laying on the bed.

If your elastic is 4mm in diameter (such as those intended for use on LeedleDynamics blasters), replace all applicable parts with those found in the “Optional - Wider Elastic Holes” folder.

For high strength, print with 5 walls and 20% infill, but I personally found I could get away with as little as 3 walls and 15% infill. If you want a more rigid BarrelRelease, print it with 8 walls 100% infill. Printing the Tube with those same settings may further prolong the life of the blaster, but I haven’t noticed a difference yet in mine.

For smoother shell ejection and break action, print the Muzzle and BarrelRelease at a 0.16 layer height and everything else at your standard 0.2 layer height. The different layer heights prevent the layer lines from rubbing against each other and making excess noise. 

Make sure to prevent support or brim printing over or inside closed holes. I also don’t recommend brims on the edges of the SidePanels or the ridged bottom portion of the BarrelRelease, as the remnants that can be left behind by brims will interfere with the functionality and aesthetics of the design. In my experience, I had no adhesion problems even with no brims on any parts, but your printer may be different.

———

Assembly

Unfortunately, I don’t currently have access to the blaster to take pictures. If you need visuals, you can check the instructions found on the Printables pages for the Shockout and SillyPilot, respectively. That said, assembly is fairly intuitive and quick.

1. Disassemble the Knockout. Your goal is to remove the barrel, barrel release tabs, and the main blue/red body panels, which should just pop off once the main exterior screws are removed. You should be able to do this without unscrewing the grip, which contains a lot of small parts that are hard to keep contained and arranged properly. Set aside or dispose of these parts and fasteners - you won’t need them for the build.
2. Remove the air restrictor on the Knockout’s plunger. The slanted restrictor is held in place by four small tabs, which you can just cut using wire cutters or flush cutters. 
3. Cut the grip of the Knockout until the SidePanels fit properly. Cut where shown in the photo, as well as everything marked in red. Don’t worry too much about overcutting, but try not to cut into the first hole directly above the top red mark.
4. Glue the Tube and Front together using superglue. The circular tab on the Tube and the circular cutout on the Front should fit together snugly - test fit them before applying glue, and if they don’t fit, sand carefully. The close fit is a big part of keeping the seal airtight and leak-free.
5. Press the 016 o-ring into the thinner of the two circular cutouts on the Front. It should go on the side with the slightly raised circle.
6. Press the Tube and Front onto the front of the plunger tube until it bottoms out. The fit is fairly tight, so no glue should be required. Make sure the Front is properly oriented before you proceed - the narrow, notched edge with the screw holes closer together should be pointing up.
7. Press the Frames onto each side of the grip and screw them together with 2 M3x25 bolts in the two top holes and an M3x30 bolt in the bottom hole above the trigger guard. The rear portion of the Frame where it meets the top of the grip may be a tight fit and might not always seat fully without the screws.
8. Screw the Front into the Panels with four M3x10 bolts.
9. Screw the RailTop(s)/RailReceiver into the Muzzle and/or Panels, respectively, using two #4 or 4-40 screws each (four total).
10. If attaching side rails, screw them in using two #4 or 4-40 screws each (four total). two #4 or 4-40 screws each (four total).
11. You now have two options. If you would like to tie the elastic on the ejector mechanisms and the Muzzle without it attached to the blaster, jump directly to step 13 and do step 12 last. If you would like to tie all the elastic Muzzle connected, continue to step 11.
12. Slide the Muzzle into the tabs in the fronts of the Frames and screw it in with a single M3x30 bolt. Don’t tighten it completely - make sure it can rotate freely with gravity. If the Muzzle still does not rotate freely, the grip is probably improperly cut and rubbing against the hinge of the Muzzle. 
13. Slide the BarrelRelease in between the tabs on top of the blaster and screw it in with a single M3x25 bolt. Again, don’t tighten the bolt all the way down so the BarrelRelease can still pivot properly.
14. Tie the elastic for the EjectorCatch through the rearmost set of holes running through the two tabs on the bottom of the Muzzle. I recommend doing this before adding the EjectorCatch itself to more easily tension the elastic.
15. Slide the EjectorCatch in between the two tabs on the bottom of the Muzzle, and screw it in using one M3x25 bolt. Check to make sure the EjectorCatch is properly tensioned and swivels freely.
16. Tie the elastic on the Ejector - one strand of elastic on each side, with the knots on the flat side.
17. Slide the Ejector into the front of the Muzzle until it sits flush with the front of the Muzzle and clicks into the EjectorCatch. If it doesn‘t click, it may be upside down. Make sure the elastic strands you just tied feed through properly to the other side - you should be able to grab them (needle-nose pliers help here) and pull them through.
18. Tie the other half of the elastic for the Ejector - you will likely have to pull very tight to get the tension high enough for proper shell ejection. This is the most difficult step in the entire assembly, and it may take a few tries. Thankfully, the deep holes in the Muzzle will easily hide even the largest and ugliest of knotslop.
19. Tie the elastic for the BarrelRelease. The elastic feeds through the curved hole on each side of the Frame and through the hole in the side of the BarrelRelease.
20. You’re done!

———

The Fun Part

This blaster will shoot every standard Silly Shell on Silly’s official file set. Max FPS varies, but you can take a peek at the Shockout’s FPS table to get a rough idea of what numbers your Breakout is likely producing.

I found that most everything flew fairly well out of my Breakout. Mega and Rifled Mega shells worked remarkably well and felt oddly more powerful than most other shells. My airtight Elite 3x was also surprisingly powerful, and felt equivalent to the performance I saw from my Battle Axe. Singled Rival performed similarly to when my Knockout was stock. My 4x Rival shell refused to fire all four rounds in one shot - my guess is the required pressure to force all 4 foam rounds out of the shell at once was too high for the stock Knockout. My MXL shell was also rather lackluster in performance, perhaps due to similar issues to the 4x Rival shell.

Extra long shells, such as Bottlenoses and Snipers, will not fully eject due to their length. Holding the blaster at certain angles or flicking the barrel down during ejection may also have adverse effects on ejection reliability. If looking to show off the ejection, MXL shells eject notably better than other shells due to their light weight.

A loading trick is to drop the shell into the barrel without priming the ejector, then flick the blaster upward with some authority. The inertial force will both close the break action and push the shell the rest of the way into the barrel, fully seating the ejector against the catch.

———

Credits to:

SterlingSilver, whose open-source Diamondback Knockout bodykit served as the base for all of this madness

Cake, whose open-source Shockout bodykit helped tremendously with creating this blaster by allowing me to mate the SillyPilot to the Knockout quickly and easily

Dom's 3D Print Shop, whose SillyPilot inspired me to create this in the first place and whose open-source files let me model the break-action pistol of my dreams headache-free

Spyr, whose remarkable Yeethammer inspired the shell ejection seen both on the SillyPilot and the Breakout

Sillybutts, who created the fantastic ecosystem of Silly Shells this blaster uses

———

Changelog:

9/2/25: Added UNSs, formatting

8/28/25: V1.2 - 4mm elastic cord parity

• Added optional wider elastic holes on all applicable parts to allow parity with Breakout and LeedleDynamics 4mm elastic cord
• Added instructions for printing certain parts
• Corrected model of BarrelRelease to remove the nonfunctional rear “ironsight” cutout

8/25/25: V1.1 - Customization release

• Adjusted the height of RailTop to better accommodate a wider range of picatinny attachments
• Adjusted the shape of FrameLeft and FrameRight to create a more symmetrical profile
• Added FrameLeftSideRails, FrameRightSideRails, and RailSide parts
• Added RailReceiver part
• Added additional instructions where necessary
• Formatting

8/20/25: V1 - Initial release

-----

Unfortunately Necessary Statement:

Disclaimer: This is not a toy. This blaster is meant for hobbyists, not children. Blaster must be operated correctly to avoid injury. I am not responsible for any injuries from this blaster. Please use a bright/blaze orange muzzle and barrel cap on this blaster. Consult local laws before building this high powered blaster.

More Unfortunately Necessary Statements:

NONE of the files found anywhere on this Printables page or account are to be used to produce blasters for commercial sale without first contacting myself and, if applicable, the designers of any and all of the original filesets, and negotiating a contract. 

Additionally, NONE of the files found anywhere on this Printables page or user, or any account under the name crispy_CORNDOG are to be downloaded, modified, sold or otherwise utilized in any manner by the business known as Frontline Foam, or any of their affiliates. By downloading this fileset or any others published by crispy_CORNDOG on any platforms, you agree that you are not in ANY WAY under or working with or for Frontline Foam and/or related to Frontline Foam or Derrikk Sun. 

Violators of the above statements, or of any other legal clauses in the Creative Commons License, will be prosecuted to the fullest extent of the law.