Taken in part from the Steyer M40 review on 1911.com:
Some manufacturers of 9mm pistols simply rebarrelled, modified the breechface
and put in a stiffer recoil spring to develop their initial .40 S&W pistols.
Since the .40/10mm bullet is bigger than a 9mm, the only way to get the .40 to
feed reliably was to create an intrusive feed ramp, and possibly an oversized
chamber to match. Thus the .40 S&W "Unsupported Chamber" was born. This was a
quick and dirty fix by some manufacturers to get to market fast.
Other manufacturers either started from scratch or went through the added
expense to redesign their 9's to safely handle the .40 with a well-supported
chamber that still feeds reliably. These types of .40 pistols are therefore
safer to use, whether you shoot factory ammo or reloaded ammo.
If you want to find out if your .40 has a well supported chamber, then do
this: Measure the diameter of the lower, most expanded part of some fired
brass. After the first measurement, rotate the brass slightly and measure it
again because the brass sometimes measures greater from a certain angle
because of the 6-o'clock chamber opening affect. If your brass measures .431
or greater, then your chamber is entering into unsupported territory. Also,
put a round into the barrel and look at the 6-o'clock position of the chamber
opening. If the thin part of the brass wall is exposed, or too much brass is
exposed, you have an unsupported chamber. "Both" of these measurements are
important to determine if your chamber is well supported or not. The greatest
brass expansion occurs when shooting full power loads.
The diameter of fully supported chanber brass usually measures around .427 -.429 for full
power loads. The Glock .40 can expand brass as much as .431 .433, which is a
Generally speaking, ammo and gun companies don't care about reloading safety
and case life. Some of the newer reloading manuals have strong warnings about
reloading for pistols with unsupported chambers, especially concerning high-
One positive side effect of Glock's famous unsupported chamber and their
marketing omnipresence, is that some ammo companies have beefed up their .40
S&W brass so it has a better chance of surviving when fired in a Glock
A "Miss" is the ultimate overpenetration!
You can never be too rich, too skinny, or too well armed!