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One of the coaches in my school asked if it was possible to come up with this:

Something that can fire waterpolo balls from a hopper magazine at a pretty high speed (ROF = 30-40 shots per minute). Air compressor for charging mass volume of air.

In short, a seriously oversized BBMG.

Any ideas or comments on whether it's feasible to build this and how to build this?

Water polo balls? No. Doing anything like this with a ~20cm bore would be essentially impossible.

Individual shots, yes, but not at a ball every second or two.
Why does someone need such a thing anyway?

wow sounds great!
I think it would be possible
but why should you use air power ?
if you have two motors turning in opposit direction next to eachother with a little less space then the diameter of a ball in between the balls can be feed easily, fast and high ROF

if you are ready to spend some $ and time on it then go for it...

I've build a semi --> http://www.spudfiles.com/forums/i-has-s ... 16872.html I suppose it would be fairly easy to scale up

Ragnarok: I thought so too, but then and again nobody in Singapore has any much experience building BBMG's.

sniper hero: I know what you're talking about and such machines exist (soccer ball machines) but trouble is its not easy to find one here.

I didn't mean you to find one but ,ake one:D
I guess you have to search for some semi auto designs pick one and make some changes
then get some materials and start

a mechanical shooter would be much more doable... think tennis ball shooting machine....

I did consider talking about the "flywheel" machines - however, I was assuming that one didn't wish to get motors that near to a pool. While a compressor can be a way away on an air line, the flywheel machines need their motors rather more on site.

Still - a more viable option than compressed air. I actually built a marble machine gun that worked like that once. The little bugger was bloody powerful for being made out of Lego - it'd put marbles through corrugated cardboard, no mean feat for something running off a few AA batteries.

If you could budger for an HPA tank feeding a small regulated chamber and a larg-ish sprinkler valve, I'm sure semi-auto would be feasible, but not at 30-40 rps. The amount of air you'd need to scale up a BBMG design to that size is ridiculous.

This really shouldn't be all that difficult assuming you are going for 0.5 to 1 RPS and you aren't looking for lethal type KEs in the ball. Figure the KE is down around what a person can do by throwing the ball.

A quickie with GGDT using 100 PSI, 10"D x 24"L chamber and a 10"Dx32"L barrel, 2" barrel sealing valve, assuming the ball is 300g puts the velocity at ~80FPS. Which is probably adequate. The only limit on the ROF would be how fast you can refill the chamber.

It would probably be easier, and a heck of a lot more efficient to use a compressed air ram to pull back a big-ass bungie cord then use the bungie cord to launch the balls. Or, use a motor to pull back the bungy ...

I play water polo and i know those balls are HEAVY you would need a really big compressor. I would sagest something like this
http://baseballtips.com/pitching-machin ... eball.html

[/quote] did consider talking about the "flywheel" machines [quote]

is that a flwweel sorry if im just repeating you.

a pneumatic motor is a option:
http://fergusonengineering.com/CPT_specs.html