K'nex Blow-Back Full-Auto Rifle





Introduction: K'nex Blow-Back Full-Auto Rifle

About: I do K'nex...a lot. It's fun to mess around with new mechanisms, and to learn about basic engineering. I build mostly guns, and love to build new concepts. I will start posting some of my 'unposted ideas' in...

It's finally done, and here it is. This is a blow-back, fully-automatic, controllable, (the shots don't just burst out), K'nex Rifle. This is loosely based off of myOmy's controllable machine gun prototype, and credit goes to him, TheDunkis, and TheFoofinator for coming up with the concept, (they just never got to the point of building it). This gun is very fun to shoot, and I think it is the best concept gun I have ever built.
I think that this gun could be taken to a point in time that it could shoot connectors instead of rubber-bands, and that it could shoot more rounds. Right now, because of so much pressure on the lever in the back of the gun, it can only shoot two rounds. BUT, it does work. These two rounds can either be shot at the rate of a true machine gun, (not burst fire), or they can be used as semi-auto blow-back. This is where I want someone else to come in and edit the system to make it stronger, (if you want to just glue the pieces, I guess you could, but I don't like going beyond taping my pieces, (and this gun doesn't actually have any tape on it, except for the front of the gun to keep the rubber-bands for going into the cracks of the gun)). But anyway, I know that this system could be edited to be stronger and hold more rounds.
Now, right now, I only have a couple pictures and a couple videos, but I will have a full explanation of the system in this next week. So, here are some Pro's and Con's



Awesome blow-back action

Good Range for a rubber-band gun, 20-ish feet.

Looks amazing, yet performs well

Fun to mess around with

New concept

All of the system is hidden in the stock, so it doesn't look ridiculous


Only two shots can be loaded in

Rarely jams

If it breaks, (if you put too many shots in the gun), it is very time-consuming to fix.


Well, that's it for the overall view of the gun. Here are a couple videos...more will be coming. Both of these are shooting two shots.

Step 1: Internals

These are the internals of the gun without the rubber-bands on...as you can see, the actual blow-back slide goes over the whole length of the gun, and the trigger just blocks it from being able to move forward. In the back, the slide will hit a lever, which will activate a hidden trigger, and let a rubber-band fire. When a rubber-band moves the slide forward, the top lever pushes through a hole in the bottom of the gun and lets the rubber-band fly past. When the rubber band is gone, another rubber-band pulls the slide back to load it again. The stock of the gun is really where everything happens. The reason why you can't put any more than two rubber-bands on is because they put so much stress on the hidden trigger and the lever...when the slide pushes the lever, it explodes. I don't know how to fix this unless you want to glue your pieces. Thanks for taking a look, and I hope you build it when instructions come out.


Please leave your thoughts in the comment section!


And once again, I'm gonna stress how I am running out of guns to build. Please leave some suggestions here in the comments, or on my orangeboard, (I check both on a daily basis, so I'll see them).

Step 2: Progress Since It Was Originally Made

Videos on all my progress since I first made the gun.



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    16 Discussions

    Lol just noticed that you siad your running out of guns to build....XD. Oh how I'd love for that to be a problem. I have notebooks full of ideas that I haven't even tried yet. My development time is always the issue.

    1 reply

    Lol yeah. If I ever really need a challenge, I'll definitely ask you. It'll be a shame when I start experiencing your problem....I'll not have a life without K'nex...lol just kidding. What other idea do you have? (just wondering)

    So, I looked at your pics of the mech, and my suggestion right now would be to try and build a bit more of the gun so you can see if there is more space for a different mech, (if you need one). As for what you have right now, I think you're trying to create a bolt action pump shotgun. I am not an expert in this, but I can say that I think it looks pretty good, BUT I feel you should build a trigger next. You're doing really good, and a magazine and trigger is all you need.

    However, I fail to see what you need help with? If you could just tell me, I could help you more, and if you want to, I would ask JonnyBGood about this too, because he's made a bolt action, pump action shotgun before.

    Awesome, I'm glad someone is getting closer to figuring the concept out. However, the flaw of it only being able to fire two shots is a bit important. I knew that would be the first problem I'd have so I spent a lot of my time trying to figure out a trigger system that would be sensitive enough that a single band could advance it no matter how many other bands were on it. Don't get me wrong, it's great that you're trying. It's just that the most difficult part of the concept hasn't been figured out yet, so the rest is kinda moot. But keep at it. You may just figure out a better trigger system yet, something the rest of us failed to do.

    1 reply

    Thanks! This trigger system is sensitive, but not enough...and I just felt that this was too close to success to keep it away from the public. And I found the source of the problem already, so I'm almost done!


    3 years ago

    Sounds cool! Maybe some better internal pictures?

    1 reply

    First off, let me say this is the most thrilling gun you have presented yet! The appearance actually will draw people to build this gun if the concept wasn't enough to entertain them in the first place! Great job!

    As far as performance goes for this gun (from what I understand anyway) this is limited to two shots full auto, due to tension in the elastics? So the firing mechanism ( the device that prevents the wheel from free spinning) needs reinforcement or another possible solution to think about conceptually. One of my professors always told me if there's a part that always breaks, make the device without it. Meaning just replace it.... if possible. Just a thought.

    Anyway, great job! I like where you've been going lately on the semi auto, full auto models and I hope you build on these concepts!

    1 reply

    Thanks for all the compliments! Yeah, you pretty much got the problem with tension in the back of the gun. I'll be posting a vid tonight with more info, and I might show what happens when you overload it. Unfortunately, the part that breaks is needed, but I wish it wasn't. Oh well...I'll figure it out eventually. Right now though, I'm working on instructions, and trying to figure a way to make it small enough to maybe have a blow-back Uzi or something that's the size of a pistol. Yeah, I'll be going on this path for a while, so I hope you like whatever comes next!

    That's a cool looking knex gun!

    Just curious, have you ever considered posting videos of your knex guns in action? Or time-lapse videos of them being built? I think that would be fun to see!

    3 replies

    Yeah, I've posted videos, but they're not very good quality....I'm at school right now, so I can't post any vids yet. I might try that time-lapse thing though.