The 6mm CM is not necessarily a new cartridge. I've found information on it from Joe Hendricks as far back as 2005 and I know he's been experimenting with other variations long before that . I'm not exactly sure what year he first chambered one in 6mm CM, but from 2005 to now it has finally gotten to the point where the rifles that were chambered in it have had the time to get 3500 - 4000 rounds through them and prove the longer barrel life.
Here's some quoted info from Joe Hendrick taken from the pdf attachment earlier in this thread.
What our research and experience has shown is that barrel life is directly related
to how much you push the cartridge. There is much we do not understand about
barrel life, but our experience has proven that what we are doing works and works
very well.
When you shoot a cartridge with less capacity you have to shoot faster burning
powder to obtain equal velocities to a cartridge of the same bullet diameter with a
larger capacity, or you are giving up velocity while still pushing the cartridge as
hard as you can. (6BR, 6mm-22-250, 6XC).
Shooting the faster burning powder does three things to your barrel.
1) First; the faster burning powder burns at a hotter temperature. Steel, what
all barrels are made from, is more malleable at higher temperatures. What
this means is that the hotter the temperature inside the barrel the more
metal that can be removed by the softer copper bullet. This is especially
important in rapid fire strings and during practice where there isn't as
much time between shots or strings. The difference in temperature from
the faster burning powders to the slower burning powders is minimal,
however over 2000 to 3000 rounds it does make a difference.
2) Second with the faster burning powder, the pressure behind the bullet goes
up very fast and goes down very fast. The maximum pressure of the shot
takes place very quickly in or around the throat area of the barrel. This
puts the maximum force on the most important part of the barrel. Anyone
who has ever bent a paper clip or wire back and forth understands that if
you do it enough the material weakens and eventually breaks. Faster
burning powder is doing this your barrel and overtime it weakens. This is
obviously a factor in barrel life.
3) Third faster burning powder burns faster and burns more in the throat of
your barrel. What this means is that the area where the bullet contacts the
lands of the barrel takes all of the abuse. Not just from the bullet, but from
the powder also.
What has been concluded from this is that faster burning powder stress the barrel
more than slower burning powder, by the force of the pressure, by the heat that
weakens the steel, and then by the burning of the powder.
The reasons for longer barrel life:
1) First the slower burning powder burns at a cooler temperature than faster
burning powders. These lower temperatures have a great affect on
extending barrel life. The barrel is not subject to as hot of a temperature
and therefore the bullets do not have as much of an affect on the steel as
with faster burning powders. The barrel's steel is less malleable because
of lower temperatures. You are not subjecting your barrel to the same
temperatures you do with faster burning powder. Keep in mind that the
temperature difference is minimal to you and I but to the barrel it is
making a huge difference.
2) Second the pressure curve is much different with a slower burning
powder. With a slower burning powder the pressure peak is down the
barrel much farther. The pressure curve is much smoother and even
though the peak pressure is about the same it stresses more of the length of
the barrel. What this means is that you are subjecting your barrel to a
lower average stress over the length of the barrel on every shot and the
stress is spread over a larger portion of the barrel's length. Again, think of
the paperclip, if you only bend it a little and bend it in different places it
takes much longer to break it. The lower pressure and the pressure applied
over a greater amount of the barrel adds to the life of the barrel.
3) Third, it burns slower. Therefore the powder is burning as it goes down
the barrel. However, we want as much of the powder to burn in the case
as possible and that is part of the design of the 6CM. The slower burning
powder gives a more even affect to the barrel. The burning powder is less
concentrated in one part of the barrel and barrel ware is more uniform.
