I've spent quite some time in the past to check out ammo alternatives for several guns, including the Light Flak. Besides Greased, Charged and Burst Rounds seemed viable when I looked at it on paper. However in practice, it does not work as well. On many occasions, the Armor was rebuilt too quickly, especially when two players tanked it. Fast rebuilds are common on the Pyra, Squid, Goldfish, and even Spire. Mobula is one of the few cases in which rebuild is much longer. It is much more important to prioritize high DPS for Armor breaks:
DPS while firing
Greased DPS - 479 (598)
Burst DPS - 317 (381)
*Brackets indicate buffs
The most used ammo on the Light Flak is and will be Greased Rounds, for good reason.
Also, D was just joking.