Not sure how other people deal with them but for me I place a 8x8 grid of turrets with 2 tiles space between and I lose 14-15 turrets with each worm.
I gave all 64 of them cores and basically none of them levelled up, I gave up and went back to default turrets while sprinkling the remaining cored turrets I have until they get natural selectioned away.

I was thinking adding a similar modifier to asteroids but of course having it be more like 500% base gain. After all there are only a limited number of worms that turrets can reasonably handle by themselves so you cannot exactly train them at that rate for long, plus factoring the natural selection part.

Hello, thanks for the feedback. I'm aware that the XP currently is not correctly handled, it's based on kills and damage.
In a future update I want to revisit it to scale on a per mob basis for vanilla, or some sort of formula that evaluates a monster difficulty to scale the xp reward, to be compatible with modded mobs as well.

Let me know if you have any ideas or suggestions for it!

Oh that would be wonderful, I did notice it was based on damage and kills as a bigger chunk of my turrets's xp spread from the demolisher was channelled to whoever seemed to get the last bullet in and honestly; for demolishers I am not sure if any xp should go to the one who get the last shot in as it should be considered a "team effort" and no one member of that team should be given more just because of luck... well definitely for gun turrets! Not sure about railguns, but I'm sure you can cross that bridge if/when you get there.

Perhaps a "close call" xp boost, % of health lost over the last X ticks multiples xp by Y?, Shield damage should not count, maybe have a floor before this set in, start applying this after 40% health lost? I'm sure you'll find a good balance :)
Doesn't sound too complex for performance's sake but would need to see that in action to verify that!