Quote:
Originally Posted by Raellus
I'm no physicist, but doesn't the muzzle velocity have a lot to do with the powder charge in the round? Might a 40mm buckshot round have more power in it, and therefore more muzzle velocity upon firing, than a standard 40mm HE round? Plus, I reckon that a 4omm HE round is heavier than a handful of OO buck. Wouldn't the weight of the projectiles have something to do with the relative muzzle velocity of a buckshot round v. a 40mm HE round? I'm not trying to be snarky here- these are serious questions.
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There's an irony that this thread should come up just as I was researching the 40mm (now) M576 Multiple Projectile Round which was mistakenly listed as the "multipurpose round" in several FMs until the 1980's (along with the HE's mistaken 370m effective casualty radius). The Buckshot round actually contains 20 rounds of NUMBER FOUR BUCKSHOT (.24") inside a special sabot with 6 vent holes in it that allow the sabot to fall away without causing the shot to spread. Its velocity is 880ft per second and this presents a fairly anemic shotgun round when you consider that a standard 12 Gauge shell (2 3/4"), which is 18.5mm in bore diameter, holds 27 #4 Buckshot Pellets traveling at up to 1600ft per second.
There is a reason for the anemic loading at HALF the velocity of a normal 12 Gauge loading though. The 40mm M79 and M203 have either an Aluminum or Composite (metal) barrel. They also use a High-Low pressure system that limits rounds to 35,000 PSI upon launch (note that the M320 can handle MUCH higher pressures and longer cartridges). Some 12 Gauge loadings can hit 50K PSI. Thus, the round is "downloaded" to avoid rupturing the barrel or otherwise damaging the weapon. I thought that a better pressure containment could have been used INSIDE the cartridge (a double liner perhaps?) but the developers didn't do this. It could be due to recoil (the M576 aught to be fairly tame to shoot) or because they used the space inside the cartridge for the sabot (which acts as both a "full choke" AND a "flight control wadding" to control load expansion). It does compare favorably to a standard 12 Gauge loading from a 20" Full Choke barrel in terms of terminal effects (pellet density and pattern). The round keeps 13 of 20 pellets on a man-sized target at 40meters. The use of #4 is also not unusual in military circles as the Russian KSG-23/TOZ-123 (the TOZ is the civilian smoothbore while the KSG has a rifled barrel) also uses #4 Buckshot in their LIGHT loading (the Heavy Loading uses 0000 Buck by weight of projectile).
It should also be noted that the XM576E2 variant which contained 27 #4 Buckshot pellets spread the width of the 40mm Cartridge (and only a SINGLE PELLET DEEP) performed poorly. It could only keep an average of 6 pellets on a man-sized target at 40 meters and lost HALF of its pellets before 20 meters. This caused the Army to discontinue the E2 variant.