What you'll need: 2 dispensers (Side by side) 1 Sticky piston 1 redstone block 1 block (any) 2 redstone dust MANY arrows
Place your dispensers like so; The side-by-side setup will allow the triggering setup to work properly. Load the dispensers with as many arrows as you like; this can be done at any time you choose up until firing.
Next, place your sticky piston behind the dispensers, one block to the side (your choice), facing inward. Position is vital, as it's the piston that drives the redstone mechanism.
Next, place any block you wish next to the piston on the far side of the dispenser, like so.
Place your first bit of redstone dust on top of it.
And another just here. What you're going to do from here is place a redstone block where the crosshair is positioned, behind the dispenser closest to the piston. This sends a signal up the redstone dust trail to the piston, pushing the redstone block away, breaking the signal and turning the piston off, causing it to retract and reactivate, switching itself on and off rapidly. It's this that gives the dispensers their rapid fire; each position of the piston fires an arrow. Want to turn it off? Just hit the lower redstone dust to shut off the cycling signal and bring the piston to a stop. You may end up with the redstone block pushed off of the piston, but simply mine and replace the block in the correct position, re-place the redstone dust, and you're firing once again!
If it all goes well, you should be lobbing arrows like nobody's business!
Upgrades, you say? Well, add some hoppers and chests like this and voila, you've got expanded arrow storage! Just place arrows into the chests and they'll auto-load into the dispensers. For those who -really- want to blot out the sun, add one hopper and chest -above- each of the dispensers as well for 4 chests worth of self-loading arrow mayhem.