Why 25mm?

A repost of one of my earliest entries here. I’m going to post a few of my earlier posts, since most of you weren’t around to see them the first time. I’ve also taken the time to made a few corrections.


The Bradley has a 25mm automatic main gun mounted in its turret. It also has a two round TOW missile launcher and a 7.62mm machine gun next to the main gun. 25mm is an oddball size ammunition. The US has previously tended to use the same calibers over and over. Examples would be 20mm, some 37mm, and lots of 40mm weapons. So why did the 25mm come in to use with the Brad?

The Bradley family of vehicles was developed in the late 60′s and throughout the 70′s largely as a response to the Soviet BMP-1 infantry fighting vehicle, and primarily with the defense of Germany and Western Europe in mind. The Red Army was huge. Even considering that the US sector of the defense was fairly narrow, units would be facing massive numbers of Soviet tanks, BMPs, and BTR wheeled armored personnel carriers (APC’s). The M-113 was armed with only an M-2 .50cal machine gun. That’s a great gun, but it was insufficient to defeat BMPs and BTRs. Our Army’s tanks would have their hands full just trying to defeat the awesome numbers of Soviet tanks. Clearly, the next vehicle would have to have an anti armor capability. In addition, a prime infantry mission is to suppress enemy infantry and keep them from employing their own wire-guided anti-tank missiles against US tanks and infantry vehicles. It was a foregone conclusion that the next vehicle would have an auto-cannon. This was hardly new. Many M-114 scout vehicles had carried an M-139 20mm cannon. The question was, which gun?

The M-139 was a very attractive option. It was already in service, there were lots of them in the inventory, there was a ready supply of ammunition and a mount already existed for them.

There were several drawbacks to the M-139, however. Maintenance had been difficult for M-114 units, and the gun lacked range and a good armor-piercing round. Also, the exposed action of the gun was vulnerable to dirt and moisture, causing a high failure rate. Surely the Army could do better.

About this time, Hughes came up with the concept of a “Chain Gun”. Rather than using recoil or gases from the firing of the weapon, an electric motor would drive a bicycle chain in a continuous loop. A cam mounted on the chain would fit into a slot on the bolt carrier of the weapon and provide the power to feed, load, fire, extract, and eject the ammo for the weapon. Best of all, the system was scaleable. Chain guns have been made from 7.62mm up to 35mm, and could conceivably go larger. The design was virtually jam free (15,000 rounds between failures), fairly lightweight, the rate of fire could be adjusted just by changing the power of the motor, and could accept two different types of ammo from two feed chutes. So the Army had the gun design it wanted. The question now was, what size.

Everyone who comes here should know that you need to bring enough gun to the fight. But what most folks don’t realize is that in the Army, you also don’t want to bring too much gun. You want just enough to get the job done. Too much gun means more weight, more space needed (which almost always means even more weight), more space needed for ammo, and fewer rounds carried, and it generally costs more as well. It also leads to a larger muzzle blast, making it easier to spot.

After quite a few live fire tests of various sized guns (often on Soviet vehicles captured during the 1973 Sinai War), the Army settled on the M-242 25mm gun. To the best of my knowledge, this was the first 25mm in Army service. Ever. When I first started working on Bradleys in 1990, I was curious how they settled on that, and not just the bore size, but the velocity and range characteristics. A look at the potential battlefields of Europe gave me the answers.

The M-242 originally fired two types of operational ammunition and two types of training ammunition. There was an APDS-T (armor-piercing, discarding sabot-tracer) round, an HEI-T (high explosive incendiary-tracer) round, a TPDS-T (training practice discarding sabot-tracer) round and a TP-T (training practice-tracer) round.

The APDS-T round had an effective range of 1700 meters, or just over a mile. When fired, the sabot fell away, leaving a 12.7mm (.50 cal) slug of tungsten to travel to the target. It penetrated the armor by kinetic energy, with no explosive charge. Given Soviet vehicle design, 3-5 hits should disable a vehicle, it’s crew, or start a fire from onboard fuel and ammo.

The HEI-T round had a range of up to 3000 meters, or a little over a mile and a half. Upon impact or at 3000 meters, the round would explode. The bursting charge was high explosive with a effective radius of 5 meters. The charge also had an incendiary component to start fires.

Mounted coaxially (that is, wherever the main gun pointed, it pointed too) to the main gun was a M-240C 7.62mm machine gun with an effective range of 900 meters. This fired the standard 4 ball/1 tracer mix.

These ranges actually have a basis in doctrine and desired effects on the then current Soviet forces. 1700 meters for the ADPS-T round matched the average field of fire in Western Europe and outranged the BMP’s main gun by about 800 meters. It didn’t need to shoot further since there were few places that you could see the enemy that far away. The HEI-T round self destructed at 3000 meters- The same range as the Soviet AT-3 Sagger anti-tank missile the gun would be used to suppress. Basically, it was like tossing hand grenades a mile and a half, two hundred times a minute. You didn’t even have to kill the missile crew, just rattle them enough to make them miss. Given that a Sagger could take up to 30 seconds to travel the full 3000 meters, you could put quite a few HEI-T rounds in the missile crews direction.

The coax 7.62mm gun’s 900 meter range also just happened to match the maximum range of the Soviet RPG-7 anti-tank rocket launcher.

It came as quite a shock to me to realize that the Army had actually put quite a bit of thought into just how to arm the Bradley. Once I realized that, I started seeing a lot of other weapon systems where design decisions made a lot more sense. A lot of the doctrine of the day became clear as well. Just wait till I give you the lesson on AirLand Battle Doctrine in the 1980′s.

3 thoughts on “Why 25mm?”

  1. Why Two-Five Mil . . .?

    Because it makes a helluva MESS . . . and everyone knows it.

    And Two-Six Mil would be strange.

    That is all.

    1. Now I just wish The Big Army™ would put that much thought and effort into the Infantryman’s assault rifle and cartridge . . .

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