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The KPA-3, or K'nex Projectile Accelerator 3 (Hey, I had to give it a cool name, right?) is my most recent k'nex gun, and my first to post on this site. Unlike the name suggests, this isn't my 3rd k'nex gun (i've been making k'nex things that shoot stuff since I was like 5), but it is my third in my KPA like of weapons. I built this gun focusing on raw power and accuracy, not aesthetics or rapid-fire. This gun IS powerful. Take a look a some of the damage shots on the last page. The thing that makes this more powerful than most rail-based k'nex guns is that the rail gap is slightly wider than the projectile, eliminating almost all friction. However, that would causes the projectile to slide around when aiming, but i fixed it so that doesn't happen. Since it's a rail-based k'nex gun, it's also very accurate (and it's probably even more accurate when you're not a horrible shot like me).

Gun Pro's:

-Very high Power
-Very high Accuracy
-Wide range of possible projectiles
-Ergonomic feel
-True-Trigger
-Sturdy
-Detachable Barrel
-Bipod (detachable, of course)

Gun Con's:

-Heavy
-Takes a good amount of pieces (especially green rods)
-Bipod connection can break if your not careful
-No Mag

This gun uses a simple mechanism, and i have found no flaws. I find I can cock and load the gun in about 8 seconds. The gun will easily penetrate several layers of cardboard, several cans, or drywall, and is best for close-range shots. I don't sharpen my rods, so i have no idea what this is capable of with a sharp point (please tell me if you try it :) ). I use latex rubber tubing to power my guns, which i got here, but you can probably find it at your local store. You can use rubber bands or elastic, but it won't be as powerful. This gun is not meant for a k'nex war. It should never be fired at any living thing.

If you like this gun, you'll probably love the KPA-4... after i get it designed and build. I'm already in the planning stages for it, so you can expect to see it probably a few months after i publish this.

Step 1: Trigger Assembly

This is by far the easiest step (I wanted to start you all off easy). This makes the trigger mechanism, which we will install in the gun body when we make it in the next step.

Directions follow. (#1 goes with picture #1, #2 goes with picture #2, etc.)

1. This is what you will be making in the following steps.
2. Make these three things.
3. Assemble like so.
4. Insert a white rod in the two gray connectors on the trigger, and snap the last remaining part in the center.

Your done (Yay).

Step 2: Gun Body

In this step we will assemble the main gun body. This is the longest step.

1. This is what the assembled gun body will look like.
2. Make this. This is Layer 1.
3. Make this. This is Layer 2.
4. Make this. This is Layer 3.
5. Make this. This is Layer 4.
6. Make this. This is Layer 5.
7. Place Layer 2 on top of Layer 1 like shown. Be careful to align the brown connectors on the front of layer 1.
8. Put in the blue rods like shown to secure the two layers together.
9. Put a blue washer on each of the rods.
10. Add on Layer 3 like shown. Don't forget about the small section of layer 3, that goes on the front.
11. Insert the trigger mechanism you made in the last step.
12. Add on another layer of blue washers on the blue connectors.
13. Add on Layer 4.
14. Add on Layer 5..
15. Add on two yellow connectors to the blue rods at the bottom of the handle.
16. Put a rubber band on the trigger so it will spring back. If you look at the photo of the gun on the first page, you see that i wrapped the rubber band trough the gun body itself because i installed it into the gun when i constructed it. However, for the sake of easiness, you can do like i did here and just wrap it around the protruding ends of a blue rod.
17. Make this.
18. Insert that into the top of the gun like shown.
19. Slide on a white connector and two blue washers like shown.
20. Cap off the other end.

You are DONE! Now to build the barrel...

Step 3: Barrel

This is the barrel for the gun. It shoots just about anything that you can fit into it, knex or not. The barrel is attached to the gun body by removable pins, making it possible to build different barrel designs to accommodate different projectile designs, but i probably won't be designing any alternate barrels.

1. Here is a look down the barrel you will be making.
2. Make what you see here.
3. Assemble like shown.
4. Add an alternating pattern of blue spacers and gray spacers to the blue rods like shown.
5. Slide on the chain of orange connectors you made, and add the same alternating pattern of spacers on top of that.
6. (Follow the comments)
7. Add on another layer like shown.
8. Now, add on the last layer. This should make a nearly complete looking barrel.
9. Flip the barrel over.
10. Add on the blue rods like shown.
11. Add two gray connectors on each of the blue rods you just put on.
12. Add a yellow rod in the front, and put on two gray connectors like shown.
13. Put a large wheel on each side of the yellow rod, and cap the ends off with gray connectors.

This completes the barrel.

Step 4: Bipod (Optional)

This is a bipod attachment for the gun. It helps to steady the gun when shooting from a platform. The bipod is based on the_burrito_master's bipod for the SR-4.

1. This is what you will be making in the following steps.
2. Make this thing
3. Add a beige connector and a purple/blue connector to it
4. Add 4 white rods with blue spacers onto the purple/blue connector
5. Put another purple/blue connector on the other end of the white rods, connect a red rod to that purple/blue connector, and put a gray spacer (equivalent to 3 blue spacers, if you don't have one) and a gray connector on the end of the red rod.
6. Set that aside for now, and build what you see here.
7. Put them together like this.
8. (Follow the comments)
9. (Follow the comments)
10. Duplicate what you just made.
11. Set those aside, and make this thing.
12. Connect the white rods to the ends of the joint connectors on the two legs you made like so.
13. Remember the first thing we made? Insert the red rod in the middle of the white connectors like so, and make sure you position the gray spacer correctly.
14. Connect the gray connectors that are hanging off the top of the legs to the white rods sticking out of the top assembly. This should make the legs able to pivot.

Connection instructions are on the next page

Step 5: Finishing Up

Well, your finally done building. Now all you have to do is put the barrel on the gun and put on the rubber tubing, rubber bands, or elastic.

1. These are the pins you will need to make.

2 & 3. It doesn't take a great genius to figure out how the barrel attaches, but i will show you anyway. I don't think you want the barrel flying off into your face when you cock the gun. Slide the gap in the underside of the barrel on the front of the gun body so that it will make a perfect fit. Then insert the blue pins as shown.

4. If you want to connect the bipod, this shows how to make the connection. Use the yellow pin.

5 & 6. Connect the rubber tubing/rubber bands/elastic to the underside of the barrel near where it connects to the gun body. The trick is to get it so that it will go slack just before it reaches the wheels, and thus after the projectile has exited. That way the projectile won't be traveling down the barrel under its own momentum at any time, and won't loose energy. The rubber tubing/rubber bands/elastic will be moving it all the way. To power my gun, i used a 10 foot loop of 1/8" latex tubing, which i doubled over twice to create a loop of tubing 4 strands thick.

7. To cock the gun, you must begin by pulling the rubber tubing/rubber bands/elastic over the wheels as shown.

8. Now, to cock it fully, pull it back all the way down the barrel and hook it on to the snowflake at the end. The snowflake will not rotate forward until the trigger is pulled, and thus the rubber tubing/rubber bands/elastic will be released when the trigger is pulled.


Inserting the projectile should be fairly straightforward if you've ever built any k'nex gun before. Your projectile must have a hook on it that sticks up above the barrel for the rubber tubing/rubber bands/elastic to catch on. I use red connectors to do this. Insert it so the hook is at the back of the barrel, and slide it into the space at the end of the barrel that is narrower than the rest of the barrel. Picture 9 shows an assortment of some projectiles i have designed, but remember that this gun will fire almost anything you can fit in the barrel.

Now, firing it should be quite straightforward. And remember, use proper safety. Don't point a loaded and cocked gun at someone even if you have no intention of firing it, because there's always the chance that it will break or malfunction and go off. So if you shoot someone in the temple by mistake and kill them, i'm not responsible. And set up a proper shooting range. A misfire (or just someone who has a sucky aim) can put a hole in your wall or break a window real easy. That said, have fun with your new gun, um, i mean k'nex project.

Step 6: Damage

For you viewing pleasure...
(All these were with unsharpened rods shot from about 5 feet away)
do u know if the barrel being shorter will work?
knex man go byby *sniff*sniff* he was my friend i dont like it when my knex friends die <br/>
haha nice makin friends i see no harsh feelings
You should make a scope for this, maybe like a dragunov's where the scope mount is at the side so it doesnt block the bullet.
Wow I'm going to make this but does anyone know how far it goes?
Broke the 4 star barrier!<br />
cool<br />
finally, a good-looking sniper that doesn't use 30 y connectors haha. Nice job, 5 stars.<br />
what is the range <br />
nice
you must have SOOO many pieces in order to build that....... gj looks great too bad i cant build it XD
i like the last pic! btw i might build this when i take apart my heavy cannon. it took up all my useful pieces haha
It's a good gun, but most of it has been done already, I was expecting something like this to come out when Oompa Loompa posted his Goliath MSG. The wheels do balance out the tension in the barrel so much more power can be added without the barrel breaking. The gun is basically a longer version of that with an SRv1-like design. Although it is a good gun, I can't understand why people are giving this a five star rating, it is simple and the mechanisms have been done before. I rate it 3.5.
I wasn't looking to invent something new or complicated, just make the most powerful gun i could design. I made a couple rail-based knex guns off this site, and found the main limitation was the friction created by the barrel. Making the barrel wider than the projectile really eliminates almost all friction, increasing the power greatly. Adding on the wheels were actually something i did after i already built the gun, and at the time i honestly had never seen a knex gun with wheels on it before.
Ima go ahead and use this for myself, is that allright with you?
That's all fine, I wasn't suggesting that you were doing something wrong, I was justifying my rating. I said all of that to protest against a 5 star rating, since everyone who has commented have been blown away by the instructable. Those are the people who I was really talking to.
The one thing that got me rating this 5* was the last step.<br/>
Yeah, but I rated after considering the gun as a whole including the damage it inflicts.
PS- oompa loompa deleted everything but his noise maker, so I wouldnt blame him if he didnt see the Goliath.
Yeah.
actually he should post this because the goliath MSG and all oompa-loompa's other creations are gone.
Yeah, I heard that recently, I don't understand why he would do that.
he says he wants to "clear up some clutter" to one person but to another he says he wants to "recede into the shadows" on instructables. i really do not understand it.
What a Jerk, I wouldnt even care if the ibles were bad, but they are very good, so Im upset and mad at him.
Someone should make a forum for posting any pictures that they have from his instructables to make a slideshow. That way, people will still be able to see his k'nex weapons. PS: When I say someone, I don't mean myself.
yeah, we could make a slideshow. Call it Oompa loompa: Greatest hits.
Yeah, and then we'll sell them to the masses and have concerts and put them on cd's, and, and, and! Lol, just kidding, someone should make a slideshow though.
I think of the knex ibles as music on the hot 100 or something.
Your not crazy or anything... lol.
We should have a "top knex guns" list or something.
Ratings determine the best guns, what more do you need?
no they do not, we get hater attacks on ratings for no reasons.
Yeah, you'll have to figure out a way to list all of the best guns though, it won't be easy unless it's completely your opinion and then the list will lose credibility. Maybe you could compile all of the k'nexperts opinions.
ahh, forget it. My OSSR got a 4* rating 2 days since it was posted.<br/>
i know, he made us all mad.
Whoa?!? where did you get those elastic bands? (picture two)
<a rel="nofollow" href="http://www.amazon.com/Grafco-Pure-Amber-Latex-Tubing/dp/B000TVPK00/ref=sr_1_2?ie=UTF8&s=hpc&qid=1248054414&sr=8-2">http://www.amazon.com/Grafco-Pure-Amber-Latex-Tubing/dp/B000TVPK00/ref=sr_1_2?ie=UTF8&amp;s=hpc&amp;qid=1248054414&amp;sr=8-2</a><br/>
how far do u think this gun can shoot in range
Thanks
Argh! I only have one of the wheels at the end of the barrel what else can I use?
no u cant i tried
lol the bullet describings are kinda funny. well the first 2 are. i like the gun nice job :)
i agree, i did not even half to make the gun to figure out that it wouldent be very new. And to add on that i dont see why you just make the barrel smaller nd increse the tension of the elastic bands, or if you cant find a band strong enough just add an ammount or bands insted, but over all i thing it is a *3.
I think you mean it takes a lot of green rods not green connectors, but anyway cool gun. About how far does it shoot?
Yeah, that's what i meant. Fixed. As far as shooting, it will go about 50-60 feet held horizontal depending on the projectile. I actually designed this more a power gun than a distance gun, meant for heavy damage at close range.
I figured out why it goes 50-60 feet. The whole barrel is a shallow rail, loaded with friction. If you suspended the ammo in the air, like the SR-v2, the range would increase by a lot.
i wouldnt go near tha t dude. nice gun btw
i made the trigger without looking at the last 3 pics. i feel so special
50-60 feet ?? mine shoot's around the 160 feet ! i know sure that you can get that to , i use rubberband from the blokker < dutch shop and they are realy realy good !!! zo maby you gonna try my gun and maby can you on this way see how poerfull it realy could be ;) farther is it a very nice gun ;) 4*
50-60 feet held horizontal. I'm sure if it was aimed at an angle, it would fire around 200 feet. I just don't have the space to be doing those type of tests.

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