Well, my prototype seems to be working out better than I thought. Here's the setup I have:
I've heavily modified a Crossbow Attack (2007 Castle) set to allow a 1x12 brick to pass through it sideways. I've got said 1x12 brick (the kind with holes, this is important; I also lined the entire top of the brick with flat, studless plates, also 1 stud wide) passing through this gap, and every few holes I've stuck one of the ballista bolts the crossbow set uses as ammo.
It's tough to visualize, I'll take some snapshots in the morning. But the best part is that it works; once the loaded "clip" (aka the 1x12) slides into the gap, I can fire rounds one after the other, because the clip drops down, auto-loading the next bolt. Granted, you need a little bit of altitude to properly use it the way I've designed it, so I mounted it onto the stripped chassis of the lower half of my Hybrid Rescue Tank.
To be honest, I wasn't intending to keep track of ammo this way, I just thought it would be a fun way to create a semi-automatic Lego weapon of sorts.