Defensive-handgun projectiles are designed with consideration for feeding, energy transfer, expansion and penetration, as well as accuracy. It’s a delicate balancing act.
Back when I first got into this business, slinging guns across the glass at a shop in suburban Atlanta in the early 1990s, I asked my boss to subscribe to three or four of the most popular gun magazines of the time. This was to build up a reservoir of information I could use the same way I’d used my nerd powers of memorizing tons of trivia in previous jobs selling and repossessing cars and motorcycles. At the time, believe it or not, there was a huge war on about how pistol bullets worked, a war that consumed many pages of those esteemed titles each and every month.
On one side were people influenced by researcher Dr. Martin Fackler, who posited that all pistol bullets did was poke holes, and unless those holes went deep enough to go through something important, they were pretty much pointless. On the other side were people who believed that if a bullet had enough velocity and energy, its very expansion would transmit some sort of shock to the bad guy and incapacitate them.
Here in 2025, the people who followed Fackler have won that fight pretty decisively. We now know that pistol bullets only work by poking holes in important structures in the bad guy, and yet hollow-point bullets—a darling of the loyalists of the “energy-dump” theory of handgun stopping power—remain the norm for defensive ammunition. Why is that? I’m glad you asked (or that I tricked you into asking).
The simplest reason usually given is that, all things being equal, a bigger hole is better than a smaller one. While this is technically true, it’s a drastic oversimplification of things. Most experts today concede that where the hole is on the target matters way more than a couple hundredths of an inch in diameter.
Most vital structures in the human body don’t work any better with a .355-inch hole through them than they do with a .451-inch hole. Conversely, a 10 mm hole through a “love handle” of flab really isn’t any more disabling than a 6.35 mm one.
One thing that expanding bullets do, however, is change the nature of the hole. Hunters had long preferred flat meplats (that’s the nose of the bullet) due to the way they tended to cut a hole with their sharper shoulders as opposed to round-nose bullets that just kind of politely elbowed their way past vital structures with their rounded ogives. Think of using a hole punch and the little discs of paper it cuts out. When a hollow-point bullet expands, it now has a very large, flat frontal area with jagged edges that greatly increase its effectiveness.
Paradoxically, another thing that an expanding hollowpoint does is reduce penetration. To say this is a good thing may seem weird considering how important penetration is to effectiveness, but something to understand is that modern service semi-automatic chamberings do not lack for penetrating ability.
When doing ballistic-gel testing, not only is it typical to put denim layers on the back side of the 16-inch-long gelatin block, but also to put a second “catch” block behind the first. I have seen—more times than I can count—a 9 mm +P or .40 S&W jacketed hollow-point projectile that failed to expand sail clean through that second 16-inch block and bury itself in the berm. All but the mildest full-metal-jacket loads in service calibers certainly will do the same.
That’s something to think about next time you read some adventure story where the hero bravely “uses their body as a shield” for a companion, because with FMJ ammo calibers from 9 mm on up, Our Hero is just concealment; not cover.
So, while penetration is good, over-penetration is bad, because the shooter is responsible for everything the bullet hits. Most modern, premium defensive ammunition is finely tuned to penetrate just enough, but not too much. As evidence, when speaking about Speer’s Gold Dot 124-grain +P jacketed hollowpoint, which is practically a benchmark personal-protection load, retired police officer and firearm trainer Chuck Haggard said that the bullets were almost always found just under the skin on the far side of the bad guy, tangled in the clothes on the far side of the bad guy or on the ground about 5 feet away on the far side of the bad guy. You literally can’t ask for better performance from a handgun bullet unless you could figure out a way to have it magically do a U-turn on the far side of the bad guy and come through again for a second pass.
There are a couple of unusual edge cases where hollowpoints come in handy as well. One is in low-velocity chamberings like .25 ACP and .32 ACP. Hollowpoint expansion in these loads is iffy at best, and downright unlikely when they encounter any barrier more challenging than a light T-shirt. Turns out that bullets—especially jacketed ones—tend to need more velocity than these calibers offer in order to expand robustly.
However, the cookie-cutter nose of the hollow-point bullet, even though it’s unlikely to expand, also makes it less likely to skid off curved bone surfaces like a rib or skull. Further, the lack of expansion is probably a good thing in these pipsqueak chamberings, which need all the help in the penetration department they can get, and can’t afford to trade any of that for greater expansion.
Another beneficiary of hollow-point technology is the in 5.7×28 mm round, which is becoming more common as a handgun self-defense chambering. In the case of full-metal-jacket bullets in 5.7×28 mm, ballistic-gel testing was always complicated by the fact that the long, skinny, fast FMJ loads would veer off in unexpected directions in the gel block on impact. It was not at all uncommon for the 40-grain FMJ projectiles, especially, to penetrate 4 or 5 inches and take an almost 90-degree turn and exit the top or side of the gel block, never to be seen again. With the SS197 load using the 40-grain Hornady V-Max, the Speer Gold Dot or FN’s new SS200 “DFNS” 30-grain hollowpoint, expansion is predictable and the bullet tracks straight, retaining enough velocity to deliver excellent penetration.
Ballisticians must work with different projectile payloads and materials (lead/antimony, copper), as well as different jacket materials (copper, brass or aluminum) as varying shapes and features. Yet, within the constraints of the laws of physics, these engineers at ammo companies have figured out how to harness the abilities and properties of the expanding hollow-point bullet to deliver controlled penetration, and in a chaotic situation, the more factors you can control, the better off you are.
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