Interesting hobby you've got there - and that's not "interesting" said in anything other than a genuine manner.
Lobbing a paintball 500 ft isn't too hard. The difficult part is making it accurate at that sort of range.
You're going to need to start to define accurate here. If accurate means able to hit a largish tank replica most of the time, that shouldn't be too hard, but don't expect pinpoint accuracy. However, a hop-up (back spin) system on the launcher will help flatten the projectile trajectory, and improve range and accuracy.
Autoloading launchers are a bit more complicated, but as JSR suggested, there is nothing to stop you building something like a blow forward loader. They're not too complex, and are perfectly fine for spheres like paintballs, although the tolerances will need to be reasonable, or you can expect a lot of chopped balls. There are other options if you want them though.
Now, an important word (or really, a short novel) of warning. Spudguns can be very powerful - and there may be problems that arise if you're firing at another "tank" which presumably has someone sticking his head out to see. If you have a launcher that can fire that 500ft, you'll have to know that you won't be able to use it at close range without turning the velocity down a lot - that's basic safety.
The other thing is I can see these "tanks" are made from wooden panels, but again, if the velocity stays up, I have seen launchers put holes through that sort of thing at short range. And I'm sure people don't want holes or dents in their hard work.
I have a launcher that can easily put paintballs over the 500 ft you want, but at short range, it has torn holes in plate steel - so I hope you can see my point that power is going to need to be controlled. Never one to shy away from self promotion, you can see the very launcher doing that sort of thing with paintballs alongside other ammunition in
this Youtube video.
Don't get me wrong, I'm not saying it shouldn't be done, but it needs to be done carefully.
However, this control should be easily if you use a reliving regulator to feed the main chamber. Have a high pressure setting for long range, and turn it down for short range, and because of the reliving design, the regulator will bleed off excess pressure.
(Another reason for the hop-up is that you can still maintain a good range with a lower velocity, making it safer)
Now, for this, you'll ideally need a valve type where you can fill from the chamber, otherwise, relieving pressure from the pilot will only fire the cannon when you don't want to.
The best known variety of valve like this is known as a Sprinkler valve. Now, I'm not sure how easily these are found in Australia, the main reason being I'm British, but here, they're not too common.
It's going to take a little work, but as I can see, you're hardly short on construction skills.
Well, I hope that's given you a couple of ideas and facts.
But, I have one important question - will your replica come with the built in water boiling facilities to allow your tank to make tea and coffee?
)
