Scary Instructor
A .380? Steeeerike 1!
A Taurus? Steeeerike 2!!
Not knowing that FMJ is the only round to use in a .380 since they're under powered and JHP doesn't expand properly? Steeeeerike 3!!!
The expansion of a jacketed hollow point bullet is a function of construction design and materials. The performance of the bullet is a function of the cartridge design, powder load characteristics and delivery mechanism. The results are a function of the target media, distance from source and environmental factors encountered during delivery. You can design a JHP that expands perfectly but couldn't pass through a cotton t-shirt or one that can pass through a car door, a leather vest and still reach vital organs and be one-shot-stop effective. You aren't doing a very good job instructing.
However, most commercial off-the-shelf pistols designed for a standard .380 load are made to be small for concealed carry. Being small, they have a tendency to thinner chambers, barrels and construction mechanisms so increasing powder loads and burn rates could cause the pistol to explode in your hand. However, it is easy to construct a pistol strong enough to take as much c4 as you can load in a brass case and shoot the projectile through steel - it just won't be small or convenient. The average difference between .380, 9mm, .38 special and .357 magnum is not really very significant for mass of material. Thus the charge propelling the projectile controls most of the reaction. Since the bullet lengths vary a bit, there is a small variance in bullet weight, but is not the primary factor in stopping power. Muzzle velocity is a primary factor, since the diameter doesn't change much in this family, the twist is fairly similar for this diameter using Greenhill's numbers. The rifling is a factor between calibers when the diameter changes significantly since more twist is required to produce stability and that energy is spent spinning the bullet rather than propelling it. The weight of the bullet is of course a factor, both in maintaining momentum from the kinetic energy leaving the barrel and in the energy transfer at impact. This is where the design is so important. The bullet must be designed for the amount of energy remaining and the momentum carried forward at impact.
So, you need to evaluate your environment. A CCI/Speer JHP at 88 grains leaves the barrel at 1000fps with 196 ft./lbs of energy and penetrates ballistic geleation about 17 inches. This is a good round for winter when people are wearing lots of heavy clothing. However, the .355” diameter bullet only expands to about .36 inches, which is almost nothing as mentioned by the “instructor”. However, a Winchester silvertip JHP at only 85 grains still leaves the barrel at 1000fps with 189 ft./lbs of force, but it expands to a full .3”, nearly doubling its size, however, in only penetrates ballistic gelatin about 6.5 inches, so you’d want this load for summer when people are wearing very little and thin clothing.
My recommendation for a general load is to take the best of both worlds using something like Federal Hydrashock JHP. A 90 grain bullet at 1000fps develops 200 ft./lbs of energy, expands to about .58” and penetrates ballistic gelatin about 10.5” That’s about as good as you can get without beefing up the gun, and if you do that, then you might as well increase the caliber as well. A .380’s principle advantage is lightweight carry, small size for concealment and still maintains a realistic chance to stop an attacker in close combat. If you want to hit them while they are rushing you, you probably should consider a bigger gun.