I love how every time I go to the paintball field there are all these 12-14 year olds with these $500 guns that mom and dad bought them. When I was their age all I had was a stingray II.
No only are they expensive to buy, but you'll use a lot of paint with it. I can make 500 paintballs last me 3-4 hours, these kids are going through a case (2000 balls) every 1-2 hours. I probably get as many kills as they do.
Ok Ok, enough ranting....
I'd suggest just buying one. or getting it for christmas. There is alot to consider in this:
You cant use your average dc motor salvaged from an old drill or dust buster. (I tried
) You need a geared motor or better yet a servo motor. Assuming you do find one of these for pretty cheap, then you have to make up an augger to move the balls around in the hopper. and mount this rig to your hopper.
Then you also need to figure out the sensor at the neck of the hopper. (IR beam ect) and mount that to your hopper. YOu'll need a ciruit to turn the servo on when it doesnt sense a ball. Op amp maybe?
So assuming you get everything made up, and connected to the hopper, then you have to worry about a paintballs blasting into it and damaging it.
I cant even count how many times I took a hopper hit...
Sorry to put down your idea, but I went down this road, and you are better off to just buy one, (Maybe used off ebay?)