Design Generalities

Home Design Pictures Philosophy Other

Main Firing Mechanism

This is the really important part of the whole thing (obviously), but unfortunately it's the part I have the least assurance of getting to work. The basic idea is to run two motors' wheels inside the barrel, in such a way that any BB fed down the barrel is forced between the two wheels and is spit out the other side. I'll have a drawing up as soon as practical to help the visualization, but until then you'll have to deal with my awful ASCII art:
                                  ---
                                --   --
                              --       --
________________________________--   --____________
                                  ---
                               <<---BB goes through here
                                  ---
________________________________--   --____________
                              --       --
                                --   --
                                  ---
The top motor would spin clockwise, and the bottom counterclockwise, in this example. I'm doing my initial testing with some el cheapo 12V motors from Radio shack (about $5.00 each) but I think I'll definitely have to upgrade for the final product.

This mechanism does require a proper balance of torque (how hard the engine spins) and speed (how fast the engine spins). As it is now, the motors are all speed and almost no torque - I can stop them with my hand just by holding the driveshaft. This wouldn't work too well for accelerating even a .2g BB up to 300 fps.

Receiver

This is the thing that holds the guts of the gun together. This holds the interchangable barrel, the two motors, the clip, the trigger switch, the battery input port, and the potentiometer that will pretend to be a selector switch. The pot will actually control the amount of current flowing through the motor, effectively letting me control the output velocity and rate of fire. It will also give me a safety when I put the resistance too high for the motors to turn.

The exterior parts (handle, trigger, selector switch, etc.) will be hand-cut and sanded oak, made from the template shown on my pictures page. The trigger will work as a simple single-throw switch to close the circuit leading from the battery terminal, through the "selector switch" and motor, and the other battery terminal. This is just about as easy as electricity gets, folks.

Modular components

This is where the more visual side comes in. The Heckler & Koch XM8 is configurable in a number of different ways, and I wanted to be able to reflect that in my gun. To do this, I'm manufacturing each of the different pieces I would need (receiver, stock, carry handle, sight, barrel) seperately, so that all I need is about two minutes to change my full-sized rifle into a pistol-sized carbine.

That's eventually, though. Initially, I'm only going to create the components necessary to build the baseline carbine.

Electric Components

I already hinted at this in the receiver section. There's really very little in the way of complex electronics: all that's really needed is the battery and motors(duh), and a potentiometer to act as both a power switch and velocity regulator. A potentiometer is a neat little thing that lets you adjust the resistance going through it between some values (like 0ohm to 1Mohm or something) just by turning a knob. This affects the current that goes through it from a voltage source (your battery) and changes how much juice the engine gets, and then the speed the motor will spin and throw the BBs. After sustained use it may get hot, but it probably isn't something you have to worry about too much unless you fire so much you'd put the recent California governor to shame.
Updated:

Gearing System

This is what is really going to make this a powerful weapon in a compact package. The trick is to sacrifice some torque (which would only be useful for really sustained fire anyway) to get some more speed using the same motor. This does mean, as a side effect, that it may take a little longer to do an initial spin-up of the wheels and it will be a little more sensitive to friction, but the speed increase should really be worth it if it's done in moderation.
The trick here is to use a ratio of two wheels, which will effectively act as a coefficient to be multiplied by the velocity and it's reciprocal multiplied by the torque. To state it in a little clearer way, (assuming E is the radius of the wheel attached to the engine, P is the radius of the wheel that will throw the BB like initially stated, S is the 1:1 ratio speed, and T is the 1:1 ratio torque):

E * S
P

is the speed the BBs will go at, and

P * T
E

So what you get in speed, you lose in torque, but for short bursts this is no big deal. How do we do this? Well, you'll now need three wheels instead of two: one wheel on the motor, one wheel to be turned by the motor, and a corresponding motor more to kill friction than anything else. This is basically taking the initial idea and putting the motor somewhere else, and having it transmit it's power to a smaller wheel from a larger one. I think I'm going in circles here, but I should have a picture up (soon?) to make it a little more clear, but I think you get the idea. It'll make a triangle now instead of a line.

So now, with a motor still only going 11000rpm (instead of the higher-performance and much-more-expensive motors that go around 50000rpm - think $75-80 for a crappy one) we're able to get speeds of 350-375 fps with a 1.5inch dia. wheel on the motor and a .5 inch dia. wheel to throw the BB's. This is only an approximate figure - I don't have the numbers in front of me - but it's a major upgrade.

By the way, that's the speed with just one motor now. If you do happen to use two motors with some mod of your own, I would recommend engineering for extra torque, rather than speed - it would help you with this thing I thought of.

There is one potential problem I came up with a little later: the gyroscopic forces have always been a problem, and may become more pronounced with a combined increase in speed and loss in torque. The worry is that since the wheels are spinning, they act as a gyroscope and work to resist and change in orientation. When you sling the thing around, this force will push the wheel into the axle, and the axle into the wood, and create friction at both points. This corresponds to a loss in speed, which would only be counteracted by torque. There isn't much I can do about this one - maybe put some Teflon or something on the axle - but besides that,I can only hope my crappy motors from Radio Shack can hold out. If not, I may need to splurge and buy some more powerful ones, and change the site's brag from "Build one as cheap as your gas gun" to "Build one as cheap as your gas gun plus shipping from Taiwan and customs 'service'." :P

All conceptual and practical design ideas are my intellectual property, except where indicated. Anyone may use my designs for personal uses, but commercial use is prohibited without my express written consent. Violators will be prosecuted to the maximum extent under applicable law.
The XM8 concept and design is owned exclusively by Heckler and Koch and I make no representations to any ownership of that property.
No one beside myself has any stake in this project, so don't get mad at anyone else.
Click here Brian!