COL. GEORGE CHINN AUTHOR , WEAPONS EXPERT ,MARINE OFFICER AND GOOD GUY TOLD ME THAT HE INVENTED THIS CONCEPT. THAT CAN BE ARGUED BUT THE FACT IS THAT HE WAS IMPORTANT IN THE DEVELOPMENT OF THE M79 AND THE MK19 GRENADE MACHINE GUNS.
The attached photo is a good illustration of how the high-low concept works. This is a standard U.S. 40 mm H.E. frag for the M79 and 203 launchers. This particular specimen was sectioned for me from a live round by the late , great Wayne Markov of Norton Ohio.
In this design the initial explosion is contained momentarily in a thick walled chamber which has been designed with rupture ports. The ports in this design are covered by a brass alloy liner. When the pressure reaches a level sufficient to rupture the liner it bleeds out into the cartridge case proper with reduced pressure but with sufficient impetus to propell the projectile.
WHO CASES ?
This high - low design allows the use of far more sophisticated projectile designs which would otherwise be damaged by high pressure.
This allows for thin skinned projectiles and complex fuzes with less sturdy parts reducing weight and increasing the explosive content capacity.
WHY NOT JUST USE A SMALLER CHARGE ?
RANGES AND VELOCITIES JUST DON’T COMPARE.