Modern Semi Auto Crossbow


March 30, 2011, 09:47 PM
I have had an idea for a while now on how to build a magazine fed, self cocking, semi automatic crossbow. Recently in one of my classes I have been told that I need to complete a business plan, so I figured I should kill two birds with one stone and get school credit for looking into my idea a bit further.

The design will ideally have equal or better accuracy, bolt velocity, and range as a conventional crossbow, but with the additions stated above.

So for anyone who feels like helping me out, let me know if you would have intrest in buying such an item, and what kind of price would be reasonable in your opinion. Also if you feel like choosing between a pistol or rifle style that would be helpful too.

I would love to hear any other opinions on this as well.

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March 30, 2011, 10:13 PM
Where does the energy come from to cock and load your design. In a semiautomatic firearm the energy comes from the energy released in the combustion of the gun powder. This drives the entire system.

In a bow there is no such source of energy from the bolt and all the energy is supplied by the mechanical energy in the bow which is stored from the energy of the person that cocks it.

An electric motor driven crossbow could be a semiautomatic crossbow, but it is after all a power supply and electric motor "cycling" the action of a modified chinese magazine fed bow in place of the person.

Lou McGopher
March 30, 2011, 11:44 PM
You've been watching too many vampire action movies...
How about a magazine-fed, manually-drawn crossbow?
A mechanized semi-auto crossbow is going to need a strong motor, and it would be pretty darn heavy.

March 31, 2011, 12:09 AM
Heck of a mainspring in that puppy.

Take a long time to wind. Probably not light, too.

And then there's the mechanics of having a spring-driven mechanism that draws a bow string having, at minimum, a forty pound draw.

Whoa. I really wanna see that spring.


Thinkin' it would be less work to craft a large-caliber thirty-round semi-auto airgun. Which is probably why things of that sort already exist.

Still trying to envision the spring you'd need for the crossbow.


Sam Cade
March 31, 2011, 12:34 AM
There is an electric self cocking magazine fed crossbow on youtube.

March 31, 2011, 12:57 AM

Uses batteries?

March 31, 2011, 01:28 AM
:what:So that's what the little thingamajiggy is for on all those hunting knives, I always thought is was for skinnin' a deer?:uhoh:

Smokey Joe
March 31, 2011, 12:39 PM
Auto Bow--Is there any call for an autoloading, auto-cocking crossbow? It's gonna cost a fortune to develop a marketable model, and you'll have to price it accordingly.

High-end "regular" crossbows have already passed the $500 mark--just how much is your average customer going to be willing to spend for this gadget?

I help some disabled hunters, who cock their crossbows with the crank thingies available nowadays. Very slowly. Never heard a one of 'em complain that what they needed was a quick second shot.

If, repeat IF, you can turn this idea into a workable model, #1 to do is to hire a really good patent lawyer. I'd say #2 would be to sell the whole thing to one of the large manufacturers, who'd have the necessary capital to produce salable units. Unless you're already a millionaire?

Well anyhow, good luck with it. Please keep us posted. :)

March 31, 2011, 12:59 PM
Your first step is to perfect Perpetual Motion, or Cold Fusion.
Then harness it to cock the cross-bow.
Should be easy!

Even easier would be a 100 pound back-pack with a 12v deep-cycle marine battery and extension cord for the built-in charger.

Seriously, I question how much of a market there would be for a semi-auto cross-bow in the first place.

Target shooting?
Probably not gonna fly under competition rules.

Probably be illegal in most states that even allow manually cocked cross-bow use for hunting.

Vampire slaying?
How many real vampire hunters / potential customers are still out there using cross-bows?
I bet those that are will keep using the same one made 200 years ago and passed down from a long line of vampire hunters in the family.


March 31, 2011, 01:01 PM
It isn't April Fools Day yet! You're a day early.

March 31, 2011, 01:59 PM
Thank you all for you imput so far, I was not even sure anyone would take interest in the thread.

I have looked into other designs out there and I don't like the current electric crossbow design that is on youtube, but I have been toying with the concept. I too feel electric power is limited in this application but I am still mulling it over. That being said my idea is a bit out-there as well.

I have a really unique idea (not copying anything else on the internet) that has a shot at making such a cross bow at a weight slightly higher that an ordinary crossbow.

Parker already has the Concorde that cocks itself with Co2 but I still think it is too slow. You can look at it on youtube.

As for cold fusion, that is going to be in a later model :) But in all seriousness the physics for my design are there on paper (I have talked to my physics professor to check the work as well as the mechanical engineer in my family to confirm it).

I am the typical broke college kid, but I have been doing some cheap R&D in order to get a proof of concept and so far so good.

I am realistic in that I know the likelihood of this contraption working in a way that is safe and of any quality is slim at best, but I like to tinker and I have to do the school project anyway, so what do I have to lose? I figure I would spend the same time on another hypothetical project that would never become real for sure, so I might as well have some fun with it.

So to everyone who was nice enough to share your ideas, If you could for the sake of my project if nothing else let me know if you would buy something like this if it were a real working item on a shelf at your local Bass Pro Shop and what kind of price would you be willing to pay. Your answers, good or bad, will be included in the business plan project I am working on.

Oh and to everyone who questions the need for such a thing, I agree that this is likely an "answer to a question that was never asked". My response is so is a Harley Davidson fat boy chopper with the ape hanger handlebars. It does not do anything particularly better than any other motorcycles but people think they are cool and fun to use and Harley has sold tons of them because of it. That is one of the main points of the question; will anyone want this just because they think it would be cool?

March 31, 2011, 02:26 PM
I can imagine a very low powered semi auto cross bow that is cocked via slide a pump action shotgun. I'm thinking similar power level to one of those small crossbow pistols I've seen before. Maybe it could be designed with a hand and pawl and gear reduction so that two or three pumps on the slide are required to fully draw the bow.

March 31, 2011, 02:32 PM
Other possible power sources:



piston and combustion

Ok, the piston and combustion idea has me thinking. What you would need to do is fabricate a miniature pile driver and adapt it to draw your bow string. Use the internet to learn about the construction and operation of a pile driver. They are a very simple compression ignition engine and thus use diesel or oil type fuel.

March 31, 2011, 02:34 PM
The pile driver concept has a lot of similarities with an open bolt machine gun. I think this is the way I would attack the problem.

March 31, 2011, 02:49 PM
Using a powder actuated or gas actuated nail gun as the basis you might be able to drive a piston to cock the thing.

March 31, 2011, 03:21 PM
I actually built a small lever action (if you can call it that) pistol crossbow, a combination of the Cho Ko Nu Chinese repeating crossbow and the 80lb cobra crossbow pistol. I don't have pictures but it is my take on the "xobra" repeating crossbow design which can be seen on youtube at The guy in the video does not have plans out to my knowledge so I just kind of ran with the concept. The credit for that design definitely goes to him though. The problem with this crossbow is that it only has like a 7 1/2 inch draw, so at 80lbs of force (which seems like an overstatement) it does not deliver much energy. The bolts fly at an advertised 165 fps but they also weigh less than a pencil.

There is a similar design that has no magazine made by a Swiss company called the Twinbow 2 It has an 8 inch power stroke but a 350 lb pull to compensate.

March 31, 2011, 05:14 PM
The guy with the pistol crossbow and the gravity fed arrow magazine is on the right track. I would say that is your starting point and work on improving the magazine and the cocking mechanism. Worry about useful power later.

I believe you are going to find out using a self powered device to draw your bowstring for you is a waste of time. The reason I say this is the very first firearms were essentially arrows shot out of barrels using explosive powder. So people already went through the mental exercise of developing an "automatic arrow thrower" and settled on a tube with explosive powder to throw the arrows...and then the arrow was gradually modified and refined until it became a lead bullet.

You could take hso's idea of using a nail gun to draw your bowstring. But look at what a nail gun is. It is a miniature arrow thrower. You could just grab the nail gun and shoot modified nails with it and forget about the crossbow.

The process of progression went from using a spring(a bow) to propel an arrow, to using explosive combustion of chemical powders to propel the arrow. By using combustion of some kind to draw the bowstring(compress the spring) and then using the spring to propel the arrow, you are increasing complexity and getting no benefit.

But if it is just a novelty thing and a puzzle you MUST solve, then have at it.

March 31, 2011, 05:19 PM
Too late. Saw one in sportsman's mag the other day. Uses a little CO2 cylinder (like from a pellet pistol) screwed into the butt. Don't remember name or price though. Sorry.

Smokey Joe
March 31, 2011, 07:21 PM
Auto Bow--You asked, would I buy an autoloading crossbow, and what price would I pay for a working one, stocked @ Bass Pro or similar.

(Pls note: I took my first archery deer with a crossbow this year, so using a Xbow is something I've been working on. Used this one because it was given to me by another hunter, an enthusiastic archer, who himself had upgraded.)

If an autoloading crossbow were stocked @ Bass Pro, I'd walk right on by it, unless it were (1) at least as accurate and as powerful as the "regular" crossbows available, and (2) cost less than the super-expensive "regular" crossbows out there. Say, $500 for a round figure, is more than I'd consider paying. No, on second thought, go below $400. Get it in at around $350 and I'll really look into it.

But to be seriously considered, it has to be effective, trouble-free, quiet, workable in the woods under bad conditions (rain, cold, snow, etc.) and above all, accurate.

And I know that's expecting a lot.

Question asked, question answered.

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