Through the 1970’s to the 1990’s, the automatic assumption, within the US Department of Defense, was that the M-60 family of tanks; and its 105mm tank gun, were obsolete and had to be replaced. For the purpose of this entry to the Bustle Rack, let us analyze the gun only. We can leave the design of the M-60 family of tanks, and the design of the Soviet tanks, to other entries.
As an ammunition design expert and a terminal ballistics expert; I am decidedly unconvinced that the 105mm gun was, or even is now, obsolete. The proper name of what we are discussing is the M-68 105mm L/52 Tank Cannon. M-68 is the US Army designation. The L/52 means that the barrel is 52 “calibers” long. In this usage, the word calibers means the diameter of the bore. So, the barrel is 5,460mm long which is approximately 18 feet.
Compared to the 120mm Smoothbore gun now in use, or really to any other tank cannon, the 105mm has 3 great advantages that no other gun can match. They are 1) ammunition stowage, 2) accuracy, and 3) variety of ammunition.
Stowage – The rim diameter at the base of the 105mm gun cartridge case is 4. . for the 120 mm smoothbore it is 5. The length of the loaded cartridge for both guns is roughly the same (3 ft., 3.5 inches). The length of the cartridge is driven by the diameter of the turret ring and by the room the loader has to move the cartridge into position and load the cannon.
The practical effect is enormous. On the M-1 Abrams tank armed with the 105mm, the onboard stowage of ammunition was 48 rounds. The same tank, with the current 120 mm gun is 40 rounds. One may think that such a decrease is not really meaningful.
A tank carries a mix of different cartridges for the main gun. With only 40 rounds, the SWAG Estimate (SWAG – Scientific Wild Ass Guess) of how many, and of which type round, to carry onboard is critical. If the tanker consumes all of the ammunition of one specific type, his tank now becomes very limited in effectiveness until he can get to the rear and “re-arm” (tanker talk for replenishing the onboard stowage of ammunition).
But go to another tank, such as the US M-60A1; and the on-board stowage is 63 rounds. That is a very significant number. Remember Alphin’s First theorem: “There is never enough ammunition.”. In researching my dissertation (see CV, Special Assistant to Commandant, US Army Armor School) I read the after action report of every US Army tank battalion in Africa and Europe in World War II. The expenditure of ammunition was a constant and universal problem. I found dozens upon dozens of photographs (all in the National Archives) of US tanks going into battle pulling one or two caissons (a type of trailer designed to carry ammunition) behind them.
Now let us discuss accuracy. The 120mm gun is a smoothbore. this means that it has no rifling (see essay on spin stabilization). So the only way to keep subsequent shots exiting the barrel the same way is by the crush fit of the projectile in the bore. And that never works because gravity has the cartridge sitting in the low point of the chamber at the moment of discharge. Thus the current doctrine is that the maximum effective range is 1,500 meters. And, truthfully, that is it. The error from one shot to the next, is so large that one cannot hit an enemy tank at much beyond 1,500 meters.
Now, look at a rifled gun. The spin induced by the rifling keeps the projectile centered in the bore of the cannon so that it has a much more uniform exit from the muzzle. This increases accuracy. I should mention that this is not a new revelation. Look at the difference between the smoothbore flintlock musket of the American Revolution to the “Kentucky” rifle. And in the Mexican War, the Mississippi Regiment, raised by a guy named Jefferson Davis, used the M-1841 flintlock rifle to great effect.
When I entered the Army, the M-60A1 tank, with the 105mm gun, was rated for a maximum effective range of 4,400 meters; which was the maximum graduation on the dial of the rangefinder. But the gun was phenomenally accurate. I regularly hit tank hull targets at ranges up to 5,400 meters, on the first shot. I just used the firing tables, the mil scale in my binoculars, and over-rode the servo-mechanical “computer”. I trained my men to do the same. I normally eschewed the prescribed tank gunnery firing ranges, and selected ones that overlooked the artillery impact zone. Thus my men could fire at those long distances.
Part of this capability was due to the zero procedure. The rifled gun, with American engineered ammunition, is more accurate with a fin stabilized (meaning it has fins on the back, like an arrow) projectile. A fin stabilized projectile does not have Magnus drift or rifling drift. So the projectiles go to exactly where the tube is pointing. It was easy to zero the tank (the act of bringing the sights and the impact of the projectiles to the same place) with the fin stabilized HEAT-TP-T cartridge (an aluminum duplicate of the High Explosive Anti-Tank projectile, see another essay). It was normal, when zeroing the tank at 1,200 meters, to fire a 3 shot group that could be covered with one’s hat. Such accuracy means that all the holes are well within a 50 centimeter circle at over 5,000 meters.
The job of the tank is to place direct fires upon the enemy. The 105mm Gun has, by a wide margin, the largest variety of different projectiles to fire. For a tanker, the ammunition for the main gun falls into the category of anti-tank or the category of antimateriel/anti-personnel.
For anti-tank there are:
For anti-materiel/anti-personnel there are:
For target practice and for zeroing there are:
There is still one more advantage for the 105mm. The HE, WP, and APERS are all a steel cased projectile with a fuze well in the nose. This means that the crew can install different fuzes to do different jobs.
Therein lies another great advantage to the 105mm Gun. A fuze has to be “bore safe” meaning that it is not delicate and only arms itself after being fired. In the rifles gun, the projectile instantly goes from stationary to rotating at some 2,500 revolutions per second, as it leaves the chamber end enters the rifling. This is heaven for a fuze designer. This centrifugal force allows the designer to do all kinds of different things (see essay on fuzes), which cannot be done on a fuze for use in a smoothbore gun.
For an ammunition and ballistics expert, the answer is clear. With the greater number of rounds of main gun ammunition stowed in the tank, the far superior accuracy, and the incredible variety of available ammunition, the 105mm Tank Gun is not obsolete.