Just gonna state that a 70/30% direct/AoE flak makes no sense when it comes to arming time...
When a huge bulk of damage is there for just hitting the ship regardless of arming time...it negates the reward for being within arming time.
70% (115 damage) direct and 30% (50 damage) means you just need charge rounds at point-blank-range to compensate AoE, which would do 149.5 damage DIRECT (the AoE would only add 65...woooo....) That means with the -25% clip, so 3 shots, you do 448.5 damage direct and 195 AoE. Granted together, they would blow up a pyramidion empty
But then we go to burst rounds, so the damage stays the same, but you now have 5 shots, which, at 115 direct, means 575 damage direct by itself...so really...you don't even need charged rounds if you just want to shoot a ship at point blank range.
Pair it with a second Flak on a Galleon and those Burst rounds total 1050 Direct.
The Charges do 897 direct.
...Lochnagar on a galleon firing all four shots (two in each flak) does a total of 1072.5 WHILE ARMED (which it most likely would be)
With those numbers...I'd stay with Burst Rounds...'cause it doesn't break your gun so it retains its fire rate.