k8ysv,
A failure to feed is serious business, particularly in a normally reliable piece like the Thunder 380, and needs looking into.
The
magazine needs to be checked:
Depress the feeder platform (aka 'follower') with a stick, then lock it down by passing a small screwdriver (or a hard wire) through the witness holes above it, on either side of the magazine tube. Check the upper third of the tube's interior for roughness or protrusion, using a cotton bud to 'feel around'. If you find something, knock it off with rag or appropriate abrasive fabric around the end of another stick, and lightly lube.
You could also do a more thorough check, by disassembling the magazine first. (Read
this, there's a portion pertinent to disassembly/reassembly of your mag, but
Eye protection at all times!). Aside from the mag tube interior, the sides of the feeder platform should be checked for roughness/protrusions. Ditto the spring itself.
When unwanted friction delays the next cartridge from rising into proper position in time to be soundly kicked by the slide into the breech, you can wind up with a malf as you describe. The spring and the feeder platform have quite some wiggle room inside the tube, and so the points of contact between the feeder platform and the mag tube can vary from time to time. You thus get malfs sometimes, and sometimes you don't.
-------------
You said you racked the unfired cartridge out and it fell out the magwell.
The nick on the bullet may
not have come from the feedramp. When racking out an unfired bullet, you need a lot of speed (and immediately) for the ejector to do its thing-- otherwise the bullet nose hangs up on the forward edge of the ejector port. An instinctive, follow-up semi-rack then drags the bullet back to the magwell so it can rattle/fall out.
I sometimes rack out an unfired cartridge in SLOW MOTION.
This pulls the cartridge back slowly, and then it falls out the magwell unharmed by improper contact with the ejection port lips. Of course, this is probably a bad habit to form --unsuitable to a defense situation.
Get a snap-cap/dummy (with a profile similar to your carry ammo) to practice racking an unfired cartridge out. It is easier to do this with the hammer down-- overcoming the resistance of the hammer is akin to spinning car wheels on brake before release --you get maximum slide velocity upon overcoming the hammer's resistance.
------------
Thanks for the report, and let us know if the magazine repents and redeems itself.
hth