[EDIT] Addressing the original question:
"What is the weight of the stock recoil spring in the Thunder 380?"
My very rough estimate would be 13 lb. For a straight-blowback, at least in my opinion, it's recoil-spring is quite under-powered. I think they wanted to avoid the difficult slide-racking that guns like the PPK can have. That's also why they went with a spring that is much longer than neccessary for the frame - which is also why a Walther PP (a much longer gun) has spring that fits in the Bersa and has the right amount of coils.
I have a post on here somewhere awhile back, talking about them. Not a post I started just commented. What I had trouble with was snappy recoil. It hurt the top web of my hand, and others that shot it as well, not just me. It wasn't hammer bite, because it has a large beaver tail. It was from the beaver tail being too long, actually. And a softer recoil spring than it needed.
So I liked the thing enough I wanted to help it. So I trimmed the beaver tail down shorter, flush with the back of the hammer when cocked, with a cutoff wheel. I then nicely contoured it with a file and sander to blend it all back in to look factory.
I also tried a factory replacement recoil spring in case my factory one was faulty. Nope, they were the same and both in my opinion too soft. So no aftermarket ones are available, so I found an application close enough to work. The Walther PP "larger" 380 has an aftermarket Wolfe spring available that is heavier than the factory Bersa one. Its slightly shorter and smaller diameter, but it works. And it softens recoil a fair bit, with still being reliable. This lessens the shock to the frame from the slide, and will help the frame last longer.
These two mods were definitely helpful to reduce the recoil, and make it more like what I felt it should be given the cartridge and weight. And its not that I'm that smart, I'm just posting this because I could find literally nothing to mention about these mods or reducing the recoil from these things.
Plenty of BS out there though if you ask about reducing recoil "its only a 380, blah blah" well I'd say most of the tools posting that never actually shot a Bersa side by side with a different make 380 to compare, let alone figure out how to fix it. I'm talking about making them suitable to run a few hundred though at a time for practice, like you should be able to without causing nerve damage over time hahaha.
No promises but I'm pretty sure its this one, the heaviest 18lb one that they offer that I used. And there is a total lack of aftermarket thin grips for these things that would be really nice too. Some plastic checkered or aluminum ones would be sweet.
The Concealed Carry model is the replacement magazine I believe that has the shorter base plate, but still works and holds one extra also.
Great question.so you tested 15# - 18# springs, which would you say was comparable to the original factory spring?
Or maybe he didn't want to admit he was straight copying multiple parts of PP/PPK componentry, so he pointed everyone to the Beretta as a Red herring. You can place the parts side by side, and the the whole firearms, and the abundant similarities between the Bersa and the Walther are pretty overwhelming - something you don't encounter when placing the parts/guns side by side with the Bersa and the Beretta.You can guess all you want, but directly from the mouth of the gentlemen that designed and manufactured the Bersa, it is based off of Beretta Designs. The Walther Recoil Spring is not the same.