Uncle Sams Heavy Metal: The 120mm Tank Guns
Posted Dec 24th 2013 | By:
Ever since the US military went to war in France in 1917 against the Kaiser, we have been in love with the tank. These armored and tracked fighting vehicles have evolved over nearly a hundred years until today the M1A2 Abrams stands at the top of the food chain. This Thor of the tank world relies on the 120mm M256 gun to as its warhammer and we are taking a closer look.
Back in 1974, the West German Army was working on a new gun for their Leopard II main battle tank. Built with all the lessons learned during World War Two, coupled with Israeli, Pakistani and Indian lessons of the tank vs tank wars of the 1960s with British/US L7 105mm guns, the Germans decided they wanted a larger, 120mm cannon for their new tank. You see, the Soviets knew the same lessons and were building their new T-72 series tanks to be as 105mm resistant as possible. This led to the Rheinmetall GmBh L/44 gun.
This 120mm smoothbore gun weighed in at 7300-pounds and was 17.3-feet in length. Put into service with the West German army, the US military chose it in 1985 to equip their new M1A1 Abrams tanks. We call it the M256 gun.
The M256 Today
Built in the US at the famous Watervliet Arsenal in New York, which has been in the heavy artillery biz for 200 years on the dot this year, the M256 has been the standard US tank gun ever since the Regan administration. Built to the same specs as the German original, it gives the 67-ton, 32-foot long M1A2 its main weapon. Using a set of hydraulic and hydro pneumatic retarders to absorb the recoil of the weapon, an M1 can fire up to six rounds per minute through its chrome-lined barrel, even as the tank is moving without the use of a muzzle brake. This is important, as older tanks do not have the capability to shoot accurately on the move. A thermal shroud helps insulate the barrel while a bore evacuator clears out any burning powder remnants before the next round enters the breech.
The loader's job on an M1A2 Abrams. This tank is on the range in Kuwait, but they still achieve an impressive rate of fire. Note the recoil of the 120mm gun that could really ruin your whole day if you step in front of it, the target monitor, and the general layout inside the cramped turret. In addition, there is a misfire drill in which the round doesn't go off as planned so watch for it. The loader and the gunner work hand in hand with one working the gun and the other targeting it.
Inside the M1, the gun is targeted, aimed, and fired by the member of the crew duly named the gunner. Seated to the right hand side of the turret, the gunner uses a complex sighting system with daylight optics and a 10-x magnification with an 18-degree field of view to pick out his target. When night or smoke shuts out this sight, he can switch to a thermal (heat) sensor to see with "Predator' vision the bad guy's tanks. This is all tied to a laser rangefinder and a fire control computer that calculates everything from wind velocity to lead angle measurement to the cant of the tank itself as it moves over ground and barometric pressure to come up with an instant fix on the target. Then with a flick, the round is sent on its way out to 4,000 yards. This combination of accurate fire control and all-weather capability proved the M1 a winner on the battlefield.
Lance Cpl. William Laffoon, tank crewman with Tank platoon Alpha Company, Battalion Landing Team 1st Battalion, 9th Marine Regiment, 24th Marine Expeditionary Unit, braces himself after firing a 120mm round from a M1A1 Abrams battle tank during a live-fire range in Djibouti, Africa March 30
To keep the M256 accurate, it has to be manually bore-sighted to ensure its calibration. The lifespan of the gun's breech is 4000-rounds, but the barrel is more like 200-rounds before it has to be replaced.
The M256 uses a number of different ammo types. This includes the 46-pound M829 Armor-Piercing, Fin-Stabilized, Discarding Sabot (APFSDS) tank round. This uses a small dart made out of depleted uranium held in place by a sabot shoe that strips away upon firing. This 10-pound dart is the 'silver bullet' the military uses against enemy tanks and can penetrate well over 26-inches of steel armor. That's two feet of steel...plus two inches.
(The Sabot, pronounces 'say-bow' will go through another tank like butter...)
When the going gets tough and the bad guy's foot soldiers are roaming around, the M256 can fire a 30-pound M1028 Canister round. This 120mm-wide shotgun shell is filled with 1100 tungsten steel balls that are about .32 caliber. Yes, that is like 120 9-pellet 12-gauge buckshot rounds all fired at once, but with much more power. Oh yeah, and they are tungsten, not lead, which means they will penetrate like you just wouldn't believe.
(That's a three foot long shotgun shell fired from a tank)
A platoon of M1s on the training line at Fort Knox back when the armor school was located there.
Then there is the 50-pound M830A1 High Explosive Anti-Tank Multi-Purpose Tracer (HEAT-MP-T, or MPAT) round which can level buildings, shoot down low flying helicopters, and just generally give the bad buy a really hard time. The M908 High Explosive Obstacle Reduction (HE-OR-T, or MPAT-OR) round is a 38-pound three charge explosive shell that can turn a concrete wall into so much dust and powder. Then there is the...well, you get the point. Uncle likes to keep lots of round available for these big boys to choose from.
Typical velocity of these rounds is something like 5500fps, or about twice as fast as a 7.62x51mm NATO round, with many hundreds times the impact down range. The average M1A1/A2 can carry 42 rounds of various types.
Does it work?
The M1/M256 combo is a winning team that has been turning terrorist hideouts, rogue-state's Soviet made tanks and tank like-objects, and insurgent foxholes into life size easy bake ovens for the past twenty years. The first real-live shooting tests of the M256 cannon came in the First Gulf War. There, US Army M1A1's encountered the Iraqi Republican Guard forces armed with the vaunted T-72 tank.
(The tell-tale sign of a T72 hit by a sabot. The small entry wound, the huge exit wound, the turret blown off, and everything sucked out of the tank through the resulting hole. The M256 gun with a sabot is what you term a straight up and down tank assassin.)
At the Battle of 73 Easting, units of the 2nd Armored Cav Rgt and the 1st ID routed the Republican Guard in an afternoon. Not a single M1 was lost, while the desert was littered with blazing T-72 hulks. In the Second Gulf War, when the 3ID entered Baghdad in the "Thunder Run," they did so with the M1 and its 120mm cannon in the lead.
Today the M1 with its powerful 120mm gun is still seeing service with Army and Marine units around the world and will continue to do so for the near future.
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