Most of the score loss is from ninjas, although yeah being able to shoot down the bandits is very handy. I initially thought score loss was unmanageable when first starting the ninja update, but the dips in score are mostly temporary: every time ninjas make a base you get a massive infamy hit, but not much on the off months.
So yeah, being able to shoot things down is very nice but not critical. Unlocking missions is still the best way to increase infamy.
That said, here's actually a lot of way to get cheap ammo weapons, heres some more ways not mentioned:
-Do every underwater mission you can (aside from jellyfish and sharks). If you get 2 sonic oscillator parts then you can make yourself a nice cannon-tier weapon
-Grey codex starts with a few shots of phantom bolts, which you can use to shoot down a wasp or megapol car with an airspeeder. These always have 25mm cannons as loot iirc.
-Red codex starts with lil-ilya. Although it's not cost effective to shoot down thunderbolts and civilians with spike rockets really. It's a little dicey shooting down a wasp/megapol in an aircar with lil ilya, but it... should be doable?
-Jack's 30mm cannons prize gives you cannons
-Light craft weapons allows you to make them from scrap, although you should probably be rolling in 30mm/50mm parts (crafted with the very simple craft weapons assembly) by then so it's mostly redundant these days.
