One of the reasons it is so hard to balance scrounger, and thus why it took so many passes (and it’s still not balanced) is likely because of the issues I raised. Had scrounger been a flat ammo percentage boost from the very beginning, I think balancing it would have taken very little effort.
And for the weapons that get infinite ammo from scrounger - would your suggestion be to shoot more and pick up less ammo?
This kind of misses the point. I’m not saying the ammo sustain at the low end is not enough, and I’m not saying the ammo sustain at the high end is too much. I am saying (and @alsozara correctly reinforced) that the ammo sustain on the low end and high end are too far apart, and depend too much on crit rate which is already a very very powerful and highly utilized stat.
I’m glad you brought this up, actually, because with guaranteed crits you get additional wonkiness. Did you know, for example, that you can infinitely sustain ammo on the TrollHammer Torpedo with scrounger using the outcast engineer’s guaranteed crit talent? Is that appropriate? Crits are a weird choice to govern ammo sustain.
This is also consistent with my thoughts on the matter. Scrounger is basically mandatory on some weapons and careers, and on those weapons / careers, +10% crit is almost also a requirement, which I think is problematic. To quote myself:
It’s important to re-center though and recognize that changing scrounger in isolation - just changing how it works without adjusting anything else in the game, would cause a ton of problems. A change to scrounger would need to be accompanied by a lot of tweaking elsewhere, which makes it a good candidate for a larger balance pass.

it somehow doesn’t sit right with me that the best option for ammo sustain … on shotguns , is a trait requiring headshots.
This is a good point, and I’ll add that headshots in this game can be insanely difficult due to stagger and AI pathing finicks, and going for headshots usually reduces your overall hit rate which mitigates a lot of the ammo sustain you get from conservative shooter. Not to mention that a lot of weapons don’t even need headshots to kill in one hit, and some weapons aren’t even really capable of headshotting consistently even if you use an aimbot (blunderbuss!)
BUT conservative shooter isn’t really the focus of this discussion and neither is the crit meta (6/14 traits are crit mediated!), but they are both very relevant tangents.