Interesting idea with the Charge Flak. I haven't tested it yet (although I will; you've sparked my interest). But here's what I know.
If you set a BURST type weapon to 2 or more shots, it first fires each projectile individually, then fires a cluster of projectiles. So if you declare that there are three large projectiles and two small ones, a 2-shot flak gun would fire:
[a small projectile, another small projectile, a large projectile, another large projectile, one more large projectile], for shot 1
and finally a cluster of 2 small and 3 large projectiles, for shot 2.
I imagine charging would work the same way. Charge to one shot, and it fires a cluster. Charge to two or more shots, and it fires in bursts and then a cluster.
EDIT: Tested, and it does work that way. Charge to 2 shots, and it fires 1 burst and 1 cluster. Charge to 3 shots, and it fires 2 bursts then a cluster; etc. What's notable is that with the cluster, it doesn't fire it as a clump, but fires it as one solid line. It's kind of weird.

You should almost definitely not make a Charge Flak weapon.