The 20mm shell used in CIWS is ineffective at more than 1.8km. Against aircraft you could probably extend it a small amount but 20mm is just too small these days. Light projectiles shed velocity too quickly and lack payload.
Modern combat aircraft do not have poor protection… an Apache probably has better protection than a Shturmovick. The A-10 even more so. Modern fighter bombers have the protection of being able to fly higher and faster and still detect and engage targets with better sensors and communications with ground forces.
And 1.8k is all the range a CIWS needs. Other AAA or antiair systems kick in beyond that. And with the CIWS that is the reason DU is used to get the weight of the projectile and rate of fire. And for the last statement, that is the exception and not the norm. There is the famous instance of an F-4 deciding to scare the manure out of a Thai guard on the ground and in turn the scared guard fired an M1 Garand with .30 caliber ammo and forced the crew to eject. Most combat aircraft today can’t take anywhere near the smacking that aircraft of WW2 could. Another prime example was the first Gulf War in 1991. A US Army SpecOps team was emplaced secretly along a main road from Iraq to Kuwait and got discovered by some kids. They called for some help when the regular Iraqi Army arrived in trucks. The F-16 that arrived would not come down for fear of a 37mm AAA in the area. They did not get help till some A-10s arrived.
At about 1.5km a 20mm cannon shell has less energy and penetration than a 50 cal SLAP round. A heavier projectile of larger calibre might leave the barrel at a lower velocity than a 20mm but it will shed velocity slower and if the range is 2km or more would probably actually get to the target sooner than a 20mm round.
There are 20mm round and their are 20mm rounds. If you are talking about the 20mm round used by the HS and Oerlikon cannons that is most likely true. But not the ones used by AA of today and from the late 1950s on. A German Army study into the subject of cannons for armored vehicles looked into the subject. The reason why the 25mm and 30mm was adopted by many NATO nations for armored vehicles was not penetration as many believed but the HE content of the shells. That was why the US Army went with the 25mm cannon over the 20mm (ie the Germans stuck with the 20mm considering it good enough against non-MBT targets). And I think the US Navy was smart enough to weigh the pros and cons of going with the .50 caliber (ie it is offered in both a three and six barrel gatling version by GE) or the 20mm. They decided on a the 20mm for good reason. Weight with muzzle velocity has an effect especially if it has a high rate of fire.
Most systems actually tend to use HE to engage targets. The intention is to set off the incoming missiles warhead rather than punch holes in it. The Soviets, whom arguably have the most experience with highly supersonic Anti Ship missiles use a mix of 30mm in HE-Frag and light guided missiles. Muzzle velocities are not huge… in fact range between 860 and 960m/s, compared to the Phalanx at something like 1,200m/s.
Ahhh…the big debate. The big one between Bofors and others. The USN after WW2 based on their experience with the Japanese sucide aircraft when this route. Developing auto 3 inch and 5 inch cannons firing proximity fuzed rounds. But with ASM and high speed aircraft you have problem: ie the slew rates of cannons slow dramatically with caliber. And with large caliber proximity rounds their is no assurance that you will explode the warhead of the incoming missile or aircraft and it will still after being disabled come crashing into you. The USN (and the Dutch and the Spanish, etc) want a close in weapon that will smack the warhead so hard that it will explode it.
The GAU-8 is enormous and very heavy. A lighter single barrel model firing the same ammo fitted to, say an M60 tank chassis so it can move with the troops with a rate of fire of maybe 800-1200 rounds per barrel and fitted with 2-4 barrels (1200 rpm for 2 barrels or 800 rpm for 4 barrels) would be ideal. The fixed barrels would improve accuracy. The fact that they were not gatling barrels means they wouldn’t need a huge power supply to wind them up… they would fire at normal rate of fire from the first shot instead of having to wind up… and the barrel length you could use would make them even higher velocity than the aircraft model… you could also use Sabot rounds for even better penetration. Of course that would mean you’d need a digital fire control computer as the trajectory of the AP and HE rounds would be completely different. With dual feed guns you could change ammo types at the flick of a switch.
It is not the rate of fire (ie although that is one of them) but the fact that the cannons rate of fire can easily be changed, they have a chance to cool increasing accuracy and more important with the Gatling principle a jam does not make the cannon inoperatable. It just keeps firing but not that barrel. And the USN and USAF thought of SABOT rounds for the Phalanx and the A-10s cannon but decided that a better option was a standard AP with a core surrounded by an aluminum body. And it seems to have worked. Whether right or wrong they believe that 35mm and higher is best for sabot rounds.
Jack E. Hammond
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