Can a Mattress Spring Increase Tennis Ball Launcher Velocity?

[bkamer]

Me and two friends need to build a tennis ball launcher using nothing but stored mechanical energy to propel it. Our concept is similar to a mortar or cannon. We tried some comercial springs from Home Depot and even a spring from inside a sprinkler head, but the tennis ball does not have enough velocity to even get out of the barrel. We are considering using a mattress spring, but need some ideas on how to increase the velocity. Any suggestions are welcome. Thanks in advance.

 

[Sirus]

Think about reducing friction. If that doesn't help too much, the only options using a spring would be to reduce mass, increase spring constant, or compress the spring more. As you most likely know, this is due to:
$$F_{net}=ma$$ and
$$F_{spring}=-kx$$

 

[NoTime]

Is the spring end of tube open?
If not and the ball fits snugly in the tube, you have problems with air pressure.