Please see the video I posted on the general bug forums ((0.18.15) [Krastorio2 Mod Bug] Invincible Biters) for a visual of this, but sometimes the biters attack my gun turrets and my turrets do 0 damage to the bugs. I have to clean up the mess myself with my gun on hand.

Edit: Perhaps the gun turrets have a minimum range set to do damage currently? All the ones stationed further away (but still in range) are doing damage but the ones being attacked 1 tile away aren't doing damage while shooting.

Edit2: Factorio version 0.18.15 & Krastorio 0.9.5

There does seem to be a minimum distance at least in regards to the player avatar when shooting at biters so I guess that turrets also have a minimum distance.

Is this minimum distance intentional? It seems like that's a poor game mechanic - especially since vanilla does not have a minimum distance that I have ever noticed. On my base, I even had a protective wall in place and that 1 tile space was still too close.

Can't remember off the top of my head if this exists in Vanilla, but the minimum range is definitely intentional. If you look at gun statistics, you'll see range of something like "1-15", meaning within 1 tile you can't shoot. I can't speak for developer's intentions but I imagine the idea is to add a little bit of balanced challenge (with enough armor you don't need any strategy, just hold space) and also if they're too close, you can envision yourself holding a gun at them and accidentally shooting your foot or something.

I have this problem as well. Player guns have a minimum range listed, but turrets don't show a minimum. Yet they still can't shoot biters up close. It's enough to make me want to disable biters.