For the life of me, I can't find the rules describing the bonuses given to lower caliber autocannons against air units such as VTOLs and aerospace fighters. If anyone can point me in the right direction with a book and page number, I would be very appreciative.
LB-X's are better, Cluster ammo adds another -1 and longer range
As already mentioned in the Era your looking for FLAK ammo is the AA option for you.
LBX's are "sometimes" better.
Cluster has a -1 to hit, and longer range as stated.
BUT, it also hits in 1 point blocks. Which are extremely likely to not cause any threshold.
Also, sometimes certain aircraft are so thin skinned that you really WANT that 5 or 10 point hit to just core them.
I recently smiled as an LB10X plastered me w/ pellets thinking at worst I'll take motive damage and be far away the next turn. Only to have the concentrated LPL damage core me through the rear and left my vtol a flaming wreck.
Point being, there are advantages to both methods.