However, in the years that followed, experience and experiment have shown that the standard 180gr bullet weight is not the best choice for .40S&W handguns. Because of the relatively small cartridge case and long bullet, this particular combination does not maximize the .40’s potential.  

The official industry pressure specification for .40S&W is 35,000 pounds per square inch, just like the 9mm.

A 10mm brass case is approximately 0.992" long, while new .40 brass is only 0.850" long; the difference is 0.142 inches. Since the size of the 180gr bullet remains constant, there is significantly less space inside the .40S&W case than the 10mm case when loaded. That means there’s much less room for error, since pressures build more quickly in that small space. Also, the 10mm was designed for a peak mean pressure higher than the .40 … which means the 10mm brass is engineered to handle greater pressure than the .40 case.

However, because of the deep-seated 180gr bullets, there is very little extra case volume left after powder and bullet are added to the case. Even the smallest variation in bullet seating or powder volume drastically affects the volume of space inside the case where the chemical reaction occurs which builds the pressure which sends the bullet down the barrel. These minor variations, therefore, make it very easy to get an overpressure situation with a 180gr bullet.  The table below shows how dramatically peak pressures increase when the bullet is seated too deeply.

Standard OAL for the .40S&W is 1.120" ... table data from "Handloading" by Charles E. Petty, American Handgunner Jan/Feb 1998, p41.

For this reason, most factory .40S&W 180gr ammunition is loaded a little on the weak side. In order to keep a given load below SAAMI specification for mean pressure, the rounds have to be loaded below their optimal performance level. Why? Because factory ammo is subject to these same minor variations. If companies produced ammunition which was, on average, maximum pressure, every once and a while a round would be significantly OVER pressure. Because such over pressure rounds are unacceptable, the average round has to be "dropped down" a notch in power so there is a wider envelope of safe operation.

This "reduced power" problem is easily seen when the 180gr .40 is compared to the 165gr bullets in the same caliber. While experience tells us that, for any particular caliber and pressure standard, heavier bullets have more momentum (as measured by an IPSC Power Factor) than lighter bullets, this is not the case for the .40S&W – an average 180gr load moves at around 975fps and as a PF of 175.5; an average 165gr load at 1,130fps has a PF of 186.5, a VERY big difference denoting significantly greater momentum (as well as energy).

Some "average" Power Factors:

As a side note, the full-power 165gr .40S&W has about the same momentum as most factory .45ACP ammunition out of a barrel of the same length.

Rate of twist affects how quickly the bullet spins as it leaves the barrel.  A 1-in-16 twist means that the bullet will spin one full rotation in 16 inches.  So, a 1-in-14 twist (bullet rotates once in 14 inches) is "faster" than 1-in-16.  Barrels are designed this way because bullets are spin stabilized, just like a football when you throw a good spiral.

Some folks in the ammunition industry have mentioned to me that one problem with the .40 and 180gr bullets is related to the 1-in-16 barrel twist used in these guns. The experts have been able to perform their own tests with alternative barrels and, with the 180gr bullets, have achieved greater accuracy and velocity (one source safely and consistently made around 1,050fps with a 180gr bullet with a 1-in-14 twist) when using something other than the 1-in-16.

So here you have a chamber design which is not as supportive as it could be, and a load (the full power 180gr .40) which has a tendency towards major pressure fluctuations. Add to this mix brass which has been aged prematurely due to the extra work at the web and it’s easy to see that a particularly unlucky brass could be the unlucky home of one of the high-end pressure spikes and result in a kB!

For more information about the kB! phenomenon, see The kB! FAQ located at at this site.

Most manufacturers have begun producing 165gr loads for the .40S&W now. While some load them light for "reduced recoil" (such as Federal’s 165gr HydraShok and Speer’s 165gr Gold Dot), other companies are squeezing the maximum potential from the bullet by pushing it to the neighborhood of 1,100 to 1,150fps out of a standard 4" barrel. As mentioned above, this results in more momentum and energy downrange as well as less risk of pressure fluctuations. And because of the reduced variation in pressure, 165gr loads tend to be the most accurate in .40S&W handguns, as well.

In my humble opinion, the 165gr is the proper choice for people who normally choose the "slow and heavy" bullets for defensive use. The FBI apparently agrees, as they broke their long standing tradition of using the heaviest bullets available when they approved two .40S&W rounds for use by agents, both of them 165 grainers.

The 165gr is really the optimum choice for .40S&W shooters.  It tends to be more accurate, have greater muzzle energy and momentum, and it significantly reduces the dangers associated with possible bullet setback (a bullet can, through normal handling, seat itself more deeply just by being loaded into the chamber of a gun, etc).  I think you'll see the 180gr loads become less and less popular as time goes on, and within a few years the 165gr will be the standard for the .40S&W, while the 180gr will be all but extinct.