Iirc, you shrunk biter collision_box a long time back to improve pathfinding. At some point (I can't find the relevant post), the devs made a change to biters to where they can now overlap each other for the purposes of pathfinding.

I'd like to suggest removing your change to biter collision_box, as it is no longer needed and makes it near impossible to hit them with shotguns.... Or at the very least, have an option for it like spawners do.

Does UNchecking this option make what you want?
https://imgur.com/BGSuY2G

No. As the setting name and description says, it affects only nests and worms (and yes, I've tested this to be sure).

try the enable the swarm mod option

Ah-ha! Yes, that does what I want. Thank you, and apologies for missing that.

Maybe a minor suggestion here, then, can you update the setting name and description a bit? The other setting, "Shrink nests and worms" is rather obvious what it's doing, whereas "Enable the swarm" made me think it was some kind of AI setting. Also, the description says "collision_mask", whereas I believe you should mean "collision_box"? I only pick on that distinction because collision_mask is an actual valid property, and that doesn't affect what I was looking for.

I can add it to the todo list.
As you said that setting and the biter collision_box change was a long time ago.

Thanks! :)

Turning off this option after gamesave was created leads to critical error in a few seconds after gamesave load.

The mod Rampant (3.1.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Rampant::on_tick (ID 0)
'prototype': string expected, got nil
stack traceback:
[C]: in function 'find_non_colliding_position'
Rampant/libs/ChunkProcessor.lua:155: in function 'processPendingUpgrades'
Rampant/control.lua:1029: in function <Rampant/control.lua:974>

So the recommended setting now should be to turn off the following:

Attack Wave: Shrink Nests and Worms
Attack Wave: Enable The Swarm

Have I got that right? I should uncheck these two options?

So the recommended setting now should be to turn off the following:

Attack Wave: Shrink Nests and Worms
Attack Wave: Enable The Swarm

Have I got that right? I should uncheck these two options?

Yeah, that's what I did. The first setting of course only affects nests and worms, as the name implies, while the second one affects biters and spitters. Makes it easier to hit them with the shotgun with those options off, so it doesn't feel (even more) useless.