Try this, I use the following method for all my AL jacketed loads since the inception of the .40 cal., it may work with plated bullets as well, I've been told it does.
Resize a case, chamfer the inside of the mouth enough to create a bevel to allow for the bullet to set up straight, don't bell the mouth at all, and then just seat it, don't apply any crimp at all either. Check and see if the bullets are still not attaining enough neck tension. If they still have poor neck tension, it is either thin brass, under sized bullets ( smaller than .355"), or an out of spec resizing die.
FYI, crimping does not produce any neck tension what so ever. It's the resizing of the brass that provides neck tension. The purpose of a taper crimp / FCD is only to remove any belling of the case mouth and could create a loss of neck tension, if, the brass is over belled, and, or, the mouth is over crimped.
You mentioned factory bullets being .357". If they are jacketed bullets, they should measure .355" for the .380 acp.. So I wouldn't personally put too much stock in a factory bullet that measures .357", as that simply isn't within SAAMI spec for that cartridge with a jacketed bullet.
GS