Brinell Hardness Number and handgun bullet performance.

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https://missouribullet.com/technical.php

In short, an optimal hardness exists per a given cartridges chamber pressure. Bullets which are too soft or hard will both cause excessive leading of the barrel, as per the article. I also contend that bullet hardness contributes significantly to bullet's terminal ballistic performance in the context of hollow point self defense rounds.

Here's a chart showing typical chamber pressures for various handgun (and rifle) rounds...

https://glock.pro/attachments/ammunition/2032-saami-pressure-charts-pressure.png

As shown, .38 special and .45 ACP can be classified as typical "low pressure" handgun rounds, with chamber pressures around 17-23k PSI, while 9mm, .40 cal, 10mm, plus any magnum rounds can be considered "high pressure" rounds, with pressures exceeding 35k PSI.

In defense of old-school rounds.

Low pressure rounds such as .38 special or .45 ACP use softer lead than higher-pressure rounds. The softer lead has a lower "expansion velocity threshold" than harder lead bullets would, meaning .38 special and .45 ACP hollow point rounds can perform quite well in terms of expansion, even at relatively low velocities.

In fact with low enough pressure, such as the .45 Colt, the lead can be so soft, a hollow point isn't needed to get expansion. The NYPD used a 200g round nosed lead .38 special bullet with good results. The bullet was so heavy and soft, it could mushroom or deform even as a round nosed bullet. However the round nose would bounce off windshields, and it was replaced by the 158g SWCHP that to this day is a stellar performer. The 158g SWCHP doesn't even have a big hollow point, just a pinhole, yet it mushrooms like a wet turd because of its soft lead.

The benefit of .38 special or .45 ACP, or any any lower pressure handgun rounds obviously isn't the magnum-like pressure and high velocity, rather it's the softer lead bullets they can employ.

Conversely, when these lower-pressure rounds use lead that's too hard, bullet performance suffers. I've seen .45 ACP rounds that hardly mushroom vs one's that practically spatter. The relative differences in bullet performances of low-pressure rounds across different brands can likely be traced to BHN number used in production, with poor bullet performance, such as inconsistent mushrooming, indicating bullets that are too hard.

If you can't stand the pressure...

The BHN has a curious effect with high pressure rounds. For example, 9mm hollow point performance can vary quite significantly between lighter and faster 115g HPs, and heavier and slower 147g HPs. This is because the "threshold expansion velocity" is easily surpassed by the 115g HP, while the 147g HP velocity is just around or below the threshold. An oversized 9mm HP exceeding 147g wouldn't expand much at all, because it wouldn't reach the velocity threshold required to expand the 9mm's fairly hard bullet with the high BHN.

With certain rounds, the BHN and the bullet's inherent velocity conspire to produce a poorly designed cartridge. A 147g 9mm can be considered marginally well designed round, even useful for extra penetration, but the 9mm round doesn't suffer from the inherent flaw of underperforming its own "expansion velocity threshold".

The round that does routinely underperform it's own "EVT" is the .40 caliber auto. Sorry .40 cal fans, the round is just plain poorly designed. A lighter, faster .40 cal bullet would solve the problem, but I'm not sure if that's feasible. The bullet is just too big and heavy and hence not fast enough to make the hard lead compound expand. A 185g.45 ACP HP will spatter compared to a 180g .40 HP.

The 10mm can perform well with lighter 160g HPs, but not as well with 180g HPs, again due to the relative "expansion threshold velocity" of the two bullets.

We know who championed the .40 & 10mm designs. It's almost as if they were trying to design bad rounds, or just didn't have a clue about the BHN or "expansion velocity threshold".

The Brinell Hardness Number or BHN is the standard means of expressing the physical hardness of lead alloy, in this case when used for making bullets. A detailed explanation of the BHN and its implications can be read about in this article... https://missouribullet.com/technical.php In short, an optimal hardness exists per a given cartridges chamber pressure. Bullets which are too soft or hard will both cause excessive leading of the barrel, as per the article. I also contend that bullet hardness contributes significantly to bullet's terminal ballistic performance in the context of hollow point self defense rounds. Here's a chart showing typical chamber pressures for various handgun (and rifle) rounds... https://glock.pro/attachments/ammunition/2032-saami-pressure-charts-pressure.png As shown, .38 special and .45 ACP can be classified as typical "low pressure" handgun rounds, with chamber pressures around 17-23k PSI, while 9mm, .40 cal, 10mm, plus any magnum rounds can be considered "high pressure" rounds, with pressures exceeding 35k PSI. **In defense of old-school rounds.** Low pressure rounds such as .38 special or .45 ACP use softer lead than higher-pressure rounds. The softer lead has a lower "expansion velocity threshold" than harder lead bullets would, meaning .38 special and .45 ACP hollow point rounds can perform quite well in terms of expansion, even at relatively low velocities. In fact with low enough pressure, such as the .45 Colt, the lead can be so soft, a hollow point isn't needed to get expansion. The NYPD used a 200g round nosed lead .38 special bullet with good results. The bullet was so heavy and soft, it could mushroom or deform even as a round nosed bullet. However the round nose would bounce off windshields, and it was replaced by the 158g SWCHP that to this day is a stellar performer. The 158g SWCHP doesn't even have a big hollow point, just a pinhole, yet it mushrooms like a wet turd because of its soft lead. The benefit of .38 special or .45 ACP, or any any lower pressure handgun rounds obviously isn't the magnum-like pressure and high velocity, rather it's the softer lead bullets they can employ. Conversely, when these lower-pressure rounds use lead that's too hard, bullet performance suffers. I've seen .45 ACP rounds that hardly mushroom vs one's that practically spatter. The relative differences in bullet performances of low-pressure rounds across different brands can likely be traced to BHN number used in production, with poor bullet performance, such as inconsistent mushrooming, indicating bullets that are too hard. **If you can't stand the pressure...** The BHN has a curious effect with high pressure rounds. For example, 9mm hollow point performance can vary quite significantly between lighter and faster 115g HPs, and heavier and slower 147g HPs. This is because the "threshold expansion velocity" is easily surpassed by the 115g HP, while the 147g HP velocity is just around or below the threshold. An oversized 9mm HP exceeding 147g wouldn't expand much at all, because it wouldn't reach the velocity threshold required to expand the 9mm's fairly hard bullet with the high BHN. With certain rounds, the BHN and the bullet's inherent velocity conspire to produce a poorly designed cartridge. A 147g 9mm can be considered marginally well designed round, even useful for extra penetration, but the 9mm round doesn't suffer from the inherent flaw of underperforming its own "expansion velocity threshold". The round that does routinely underperform it's own "EVT" is the .40 caliber auto. Sorry .40 cal fans, the round is just plain poorly designed. A lighter, faster .40 cal bullet would solve the problem, but I'm not sure if that's feasible. The bullet is just too big and heavy and hence not fast enough to make the hard lead compound expand. A 185g.45 ACP HP will spatter compared to a 180g .40 HP. The 10mm can perform well with lighter 160g HPs, but not as well with 180g HPs, again due to the relative "expansion threshold velocity" of the two bullets. We know who championed the .40 & 10mm designs. It's almost as if they were trying to design bad rounds, or just didn't have a clue about the BHN or "expansion velocity threshold".

[–] • 0 pt

Good write up.

One thing to make clear — the article references cast lead bullets specifically, which are an objectively different beast than fully or partially jacketed lead bullets in the context of leading.

Within the context of expansion (as well as terminal ballistics) bullet composition, chamber pressures and velocities are critical when speaking of either 1)lead, 2)jacketed or 3)monolithic bullets and is an exhaustive subject that isn’t necessarily within the scope of your post, but is important to acknowledge as discrete topics unto themselves

[–] • 1 pt

I did consider jacketed bullets, but the idea of bullet hardness & bullet performance isn't something I've ever seen discussed. I had to start somewhere, and the Missouri Bullet Company article is the only information about I found.

What is the effect of a copper jacket on lead bullets? I've seen no evidence that copper prevents mushrooming, but it's not like I've ever seen comparisons of similar lead HP vs JHP either.

I've even read an article which implied that harder lead alloys are somehow an "improvement" or at least a better standard. They are better for high-pressure rounds, but not for low-pressure rounds. The article lamented the .45 ACP's relatively low chamber pressure, without seeming to understand the effect of raising the pressure & hence the BHN has on the bullet's performance. High-pressure .45 rounds like the .45AE never caught on, and neither have .40 S&W or 10mm, and I suspect the relation between BHN & "expansion velocity threshold" could possibly have something to do with it.

the idea of bullet hardness & bullet performance isn't something I've ever seen discussed

It is discussed on forums where people concern themselves with

What is the effect of a copper jacket on a lead bullet?

Many things depending on application. To name a few top level applications:

• It can increase the ballistic efficiency of the bullet in flight, sometimes referred to as BC or ballistic coefficient

• It can aid in penetration after expansion depending on thickness of the bonded layers relative to the lead core, such as

harder lead alloys are somehow an "improvement" or at least a better standard

This is a subjective statement based on ones application. Perhaps for the authors of the article it was of merit, but I know of people who load their bullets backwards to punch a more clean hole in paper for scoring. Application matters, and it is often not a one size fits all affair when it comes to terminal ballistics

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