|Disclosure: This is probably a 40 S&W case. I chose it because the bulge resulted from an incompletely supported chamber. Eye-catching, isn't it.|
All progress is due to "crazy" people. There are thousands of reasons why things are exactly the way they are and to try anything different is to be crazy.
"9mm Major" is a wildcat designation for a 9mm handgun cartridge that is loaded so that the power-factor (the muzzle-velocity in fps) TIMES (the weight of the projectile in grains) DIVIDED BY (1000) is reliably above 165.
For example, a 124 grain projectile requires a minimum velocity of 1331 feet-per-second to be 9mm Major vs. the 1100 fps that one might get from SAAMI compliant 9mm Luger loads.
Oh, the other thing is that this cartridge uses common, 9mm Luger "9X19mm brass".
That extra velocity comes at a cost of higher pressure. SAAMI sets the maximum pressure for 9mm Luger at 35,000 pounds-per-square-inch. 9mm Major reloads run a wide range of pressures with 45,000 psi being common and 60,000 psi a real possibility.
Why would anybody torture equipment and brass by running 30% to 70% over SAAMI spec?
Because loading 9mm Luger to those power levels provides a scoring advantage in IPSC and USPSA shooting competitions due to how "hits" are scored.
What can we learn from 9mm Major?
When you are riding the ragged-edge, very tiny details make a huge difference.
Let me paint a word-picture:
The 9mm Luger case is straight-walled and short. One way to increase the volume behind the bullet is to not seat the bullet as deeply when reloading it. A typical 9mm defensive load (like Hornady 124gr XTP) might have an over-all cartridge length of 1.06" so it chambers and fires in any SAAMI complaint chamber. The SAAMI length spec for 9mm Luger is a maximum of 1.169" and it is not uncommon for 9mm Major reloaders to push that to the max allowed by their magazines....often 1.18"
Just as increasing the over-all length of the loaded cartridge lowers pressure, inadvertent shortening (often called "bullet setback") will cause it to spike even beyond the 45-to-60ksi range.
How can bullet setback happen?
Inadequate neck tension is one big cause. The reloading die for 9mm reduces the diameter of the outside of the neck and the inherent assumption is that the wall thickness is consistent. Well, it may be within a specific brand of cases but it is not consistent from brand-to-brand. Some cases have thicker material in the neck (good retention) and others have thinner material (less aggressive retention).
Another consideration is the geometry of the ramp that the projectile hits as it is stripped forward and out of the magazine. The ramp lifts the pointy end up and guides it to the chamber. If the ramp is blunt then it exerts more aftward force on the bullet when it first strikes the ramp and starts to lift it up.
In a similar vein, if the surface of the ramp is rough it increases the friction and all of the forces produced when hitting the ramp increase.
The material characteristics and geometry of the projectile's tip and how they interact with the surface material of the ramp can come into play. Remember that time you went down the slide at the playground and the shorts you were wearing rode up? Yeah, no joy there.
One thing that I found to be useful when researching 9mm Major is that some equipment sets, specifically barrels, are more accommodating to supporting the demands of reloading to 9mm Major pressures. The ramps are polished. The surfaces are nitrided. The chamber has minimum cut-outs for extractor clearance to better support the brass. Free-bore is generous in length and diameter and the tapering of the rifling is gentle. And so on, and so forth.
If I anticipated that I might someday own a handgun, and if I thought it was prudent to have a back-up barrel in stock for repairs, I would certainly research what the 9mm Major competitors were running and give those exact barrels/part numbers very strong consideration.