The greatest engineering minds of the GTXFBBQ are needed

[Sad, little man]

I want to design a crude machine made to launch empty 12oz cans (doesn't have to be that far, 10ft maybe). I'm thinking just something that uses spring pressure and a plunger. But, I also want to make it so that it can't be used to launch glass bottles. Obviously an answer would be to make the feed mechanism only big enough for a can, but a bottle could still be fed down the barrel, so that's an issue.

Go!

[G'day Mate]

Trebuchet?

(I know that doesn't meet all of your requirements, but it'd be pretty cool anyway)

[thesameguy]

Launching something that light with so much surface area will be problematic. How many does it need to launch in sequence?

[Cam]

An engineer would probably tell you not to do that. It's not safe. :P

[Godson]

Simple, pneumatic with a valve and PVC piping.
Can face down with the opening facing the air nozzle, paper cone to help displace some of the wind resistance.

[Crazed_Insanity]

You have some weird requirements. Can you launch crushed cans? That way the 'disc' can go further and for sure can prevent bottle launches...

[thesameguy]

An engineer would probably tell you not to do that. It's not safe. :P

An engineer would say "yes we can," a lawyer would say it's not safe.

[Sad, little man]

Nope, no crushing of cans allowed. I know it won't be able to launch them far when empty, but that's ok.

What do you mean how many does it need to launch in sequence? It needs to just launch cans. It doesn't have to be fully automatic or anything.

My main problem is figuring out how to make it so that someone doesn't shove a glass bottle down the barrel and launch that. Maybe it's just as simple as making the end of the barrel be inaccessible. Like make the gun long enough and build a little shooting range around it that eliminates the possibility of putting anything down the barrel.

[Cam]

Who says you need a barrel? Just make a giant sling-shot with some bungee cords or something.

[overpowered]

http://www.xproducts.com/ar-15-soda-can-launcher-accessories-launcher

[Godson]

OK, that's pretty fucking cool

[thesameguy]

Nope, no crushing of cans allowed. I know it won't be able to launch them far when empty, but that's ok.

What do you mean how many does it need to launch in sequence? It needs to just launch cans. It doesn't have to be fully automatic or anything.

My main problem is figuring out how to make it so that someone doesn't shove a glass bottle down the barrel and launch that. Maybe it's just as simple as making the end of the barrel be inaccessible. Like make the gun long enough and build a little shooting range around it that eliminates the possibility of putting anything down the barrel.

I assumed it had to reload itself or something. If it doesn't, just put a "chute" between the outside world and the barrel that is only the size of a 12oz can. Somewhere away from the end of the barrel, put a spring-loaded doodad that will be deflected when something passes over it in one direction and not in the other... like a severe tire damage thingy.

[Sad, little man]

tsg, I thought of that, but a spring loaded thingymabob would probably screw up the trajectory of a can being launched too much.

Ultimately though, I think I might be saved by the weight difference between cans and bottles. I just weighed them, and an empty bottle is about 193 grams. An empty 12oz can is about 12 grams. I did not think the difference was that huge just by holding them. So, based on the difference in mass, I'm assuming any device that is designed to launch cans a reasonable distance would only launch bottles about a sixteenth of the distance, meaning that it's not really necessary to prevent people from trying to launch bottles with it since they wouldn't really go anywhere anyway.

[thesameguy]

If it's far enough back in the barrel, it won't be a problem. The barrel post stop will straighten whatever is wrong out.

That said, what if you make REALLY simple and the entire mechanism is a spring loaded foot surrounding a spike. Just spike the aluminum can onto the foot, no barrel at all. There would be literally no way to keep a glass bottle in place to be launched.