Kinda sounds like the mag isn't completely locked. Have you tried giving it an extra bump or push firmly when you load a new magazine in the gun?
I remember shooting a S&W semi auto a long time ago. It was a rental gun, and not very well cared for. The first mag I loaded ended up on the range floor, because it wasn't completely seated. Range officer had to call a cease fire and it was quite embarrassing. I never forgot that, and try to remember to check the magazine before I start shooting.
the wife and kids were out of the house for a bit so i could look over things a bit more (i need to order some 380 dummie bullets so i am more safe when exploring this problem).
I think the mag is seating too well --
I laid the mag alongside the gun to see where the magazine latch lines up on the mag -
the mag will not pull out if tugged gently by the floor plate, so i believe it is seating well.
when lined up with gun, it looks like the catch is a bit too low - so when the mag is forced in, it gets locked in, but the top round is too high - too far toward the "top" of the gun. I believe that when the slide is moving backward, the "next" round is getting pressed too hard against the slide, and this connection is pressing the mag back out.
the mag eject spring is pretty healthy - the mag really pops out when the mag ejector button is pressed. so the mag just has to be forced down just enough to lose a good hold, then the power of that spring takes over, and shoots the mag out.
it is holding, but the mag may be pressed one way or another so that it is not quite squarely seated, allowing it to hold, but having it barely be locked in.
I am going to file the top of the cut-out where the mag-hold/release thingie catches in that cut-out. a little at a time - then see if this effectively allows the mag to get locked at a lower altitude in the gun - not so high up.
i have a monday deadline, and i like to focus and pay attn when doing stuff like this, so i am not going to be able to get to this for a couple days.
i will send a follow-up note.
if anyone has tips on this type of task, please post. i am sure the mag has to be hardened steel, since i have never known a mag to deform and become unusable across time, despite great force all the time from the follower spring pressing rounds against feeder lips, and so on.
i will wipe mag down well as i file so no metal dust gets in the gun.
well, i spent some time looking at this last nite.
I had read that the grips are not screwed in, but are in with a cam that must get in the correct position to allow grip to be shoved off.
Well, this gave no hint of working. I searched the interweb for how to remove a bersa thunder plus grip, and found nothing, no yuotubes or anything.
I eventually figured out that on the thunder plus, the grip does actually simply screw out with a normal-acting screw. very easy.
Then, i could see what the mag was doing. it really looked like the catch for the mag simply did not articulate out far enough to hold on. I swapped it around to the left-handed orientation.
[this one IS a cam; turn counter-clockwise almost 1/3 a turn, and shove it out of its channel; to return, get both cams turned to the far position counter-clockwise as you look at it from screw end, get in place, then turn the full one turn until it feels seated; don't force it at all.]
once the mag release was left-handed, it worked perfectly. I loaded a few rounds, and could rack round after round with the mag not just dropping out like it was. no drops at all.
when i had the mag release button gizmo out, it had always felt gritty, so i got it with cleaner, then lightly lubed it, then made sure to get lube off the acgtual tab that is supposed to hold the mag. it feels better.
But I wanted it rightly, not lefty.
I took a nail and gently bent the top of the mag catch hole out a bit so it might hang up on the mag catch better. this worked.
i plan to order a replacement mag release and see if the one i have just does not stick out far enough - i don't like the idea of depending on the bent magazine - the mag release catch ought to be out there far enough to do the job.