T249 Vigilante – The Biggest Gatling Gun Ever Built

Impressed by the A-10 Thunderbolt’s 30 mm Gatling gun? Well that’s nothing compared to the T249 Vigilante. It was armed with the biggest Gatling gun ever: a six-barrelled 37 mm rotary cannon that could fire 3,000 half-meter-long rounds per minute!

This incredible vehicle was developed in the 1950s as a ground-based air defence system to protect mobile forces. It was equipped with a tracking radar to target aircraft, and also had a ground based mode for horizontal targets.

With the equivalent firepower of almost one thousand Stuart light tanks, the Vigilante was no joke.

Development

The concept of attacking forces on the ground with aircraft matured massively during the Second World War, resulting in the development of specialised self-propelled anti-aircraft guns (SPAAGs) to counter them.

While these worked well then, they were hopelessly outdated by the much faster aircraft of the 1950s. At this time, the US’ most potent SPAAG was the M42 Duster, which had two 40 mm cannons mounted on a modified M41 hull.

It was powerful, but also slow, cumbersome and incapable of tracking newer aircraft. It was clear that something new was needed.

The M42 Duster was most famously used against ground targets. Image by 270862 CC BY-ND 2.0.

The US addressed the problem in 1952 with the Light Antiaircraft Development Program. The program would be conducted in three stages: the first was a fitting a radar system to the M42 Duster, creating the Raduster.

The second was a Gatling-style air defence weapon named Vigilante, and the third was a guided missile named Mauler.

These are all very interesting projects, but it’s the Gatling gun we are focusing on today. Its existence was spurred by the realisation that even with upgrades, the Raduster would still be an inherently outdated system. Thus, it needed a replacement.

In 1955 the US Army looked for self-propelled Gatling-style gun that could meet their requirements in a relatively short amount of time. They wanted a weapon of unprecedented size and performance: 37 mm rounds and a rate of fire of 3,000 rpm.

For comparison, the A-10 Thunderbolt II’s famously large gun is “only” 30 mm!

It wasn’t clear if building a gun of such large caliber and high rate of fire was even possible – reliability, cooling and ammunition supply were all going to be major problems.

The projectile was to be twice as heavy as that fired by the GAU-8 Avenger, shown here.

However, the ongoing development of the T171, which would eventually become the 20 mm M61 Vulcan, provided plenty of experience on large caliber Gatling-style guns and proved it was feasible.

Using this knowledge, designers from the Springfield Armory began work on designing what was essentially an M61 Vulcan scaled up to fire 37 mm rounds.

T250 37 mm Gatling Gun

This took place over 1956 and ’57, resulting in a monstrously large weapon designated the T250 Vigilante. It had six barrels, which were electronically rotated along with their respective bolt assemblies.

As the barrels rotated, new rounds were loaded and ejected, so each barrel is ready when it reaches the firing position. This enables a very high rate of fire because the cycling process of each barrel is achieved while another is firing.

This high rate of fire greatly improved the odds of hitting a fast-moving target.

The T250 Vigilante gun – just look at the size of that thing!

The ammunition fired was the 37 x 219mm T324-E22 high explosive round. The casing was adapted from 40 mm Bofors ammunition, and was tipped with an 20 cm (8 inch) long projectile that weighed 1.65 lbs (.75 kgs) – twice as heavy as the A-10’s 30 mm ammunition!

All in, the round weighed 3.4 lbs (1.5 kg) and measured 15.5 inches (40 cm) in length!

The T250 was first fired in September 1958, but showed a number of reliability issues. It also couldn’t achieve the desired 3,000 rpm rate of fire.

So an improved bolt assembly was designed, and the barrels were lightened down to 63 lbs (29 kg) each, finally allowing the gun to achieve 3,000 rpm. The T250 would not fire continuously though, but instead would fire in 48-round bursts.

T324-E22 37 mm ammunition. The entire round measured 15.5 inches (40 cm) in length.

This was a terrifyingly powerful weapon – it is hard to comprehend how much firepower this thing had. A good comparison is that 37 mm was one of the US’ primary anti-tank calibers when the Second World War started, with tanks like the M3 Stuart being armed with such guns.

Now, a cluster of six rotating at high speed could fire 3,000 of these rounds per minute at a muzzle velocity of 3,000 fps (914 mps).

During testing though, the gun displayed a concerning tendency to accidentally discharge. This was caused by heat build up in the barrels, which caused the T324-E22 rounds to overheat and fire.

The T250 stands as the largest caliber Gatling gun ever built.

Extensive testing was carried out by the Picatinny Arsenal in New Jersey to investigate this issue. After heating the rounds up, they found that the propellant in the casing could cook off at temperatures as low as 239°F (115°C). During these tests, the gun reached as high as 395°F (201°C).

However it was also found that temperatures only entered into the cook-off range when more than seven 48-round bursts were fired in quick succession. If more than seven 48-round bursts were fired and ammunition was still in the chamber, there was a risk of cook off.

To avoid cook offs, Picatinny Arsenal simply recommended not surpassing seven bursts in quick succession, or to ensure all ammunition is expended quickly. If more than seven bursts were fired, and there was still ammunition remaining, they recommend that “care should be exercised to insure that the muzzle of the weapon is not pointing in the direction of friendly emplacements”.

The ammunition was stored in the cylinder to the left of this gun. Only 192 rounds were contained in this magazine

I’m sure that statement would make everyone in the vicinity of the Vigilante much more comfortable!

For actual use in service, the T250 gun was to be mounted on a trailer and on a modified M113 chassis, known as the T248 Vigilante A and T249 Vigilante B respectively.

The T249 Vigilante

Work to mount the T250 on an M113 was carried out by the Sperry Utah Engineering Laboratory, beginning in 1959. The T249 Vigilante was a forward area air defence system intended to cover forces close to or on the front line from low-level air attack.

The gun was mounted in a turret on top of a M113-based hull – the M113 being chosen as a budget friendly, expedient way to make the T250 mobile. The upper portions of the M113’s superstructure were deleted and the entire chassis was slightly lengthened, producing a low-profile hull that measured just 52 inches (1.3 meters) in height.

The turret contained a single operator and could rotate 360 degrees.

The T248 Vigilante towed trailer.

The T249 was fitted with a pulse-Doppler radar system that could spot and track low-flying aircraft against ground clutter.

Thanks to this radar, a complex fire control system was developed to allow the gun to automatically track a moving target across the sky. The operator was alerted to targets either visually on the radar’s indicator, a horn, or a tone in his headset.

Due to the imperfect calculations made by the computer, the operator manually made small final aim adjustments.

The T249 Vigilante self-propelled anti-aircraft gun.

While it had a high rate of fire, its ammunition supply was rather small. 192 rounds were stored in six-round blocs in a rotating circular magazine next to the gun.

At its fastest fire rate, the 37 mm gun would expend this ammunition in less than four seconds, and it all had to be reloaded by hand.

The T249 did have some armor protection, however this was only equal to an M113.

The Vigilante only weighed 9.2 tons, so it could be airdropped.

It was possible to use the system against ground targets, with a setting available to the gun operator specifically for that role. This mode reduced the rate of fire down to 120 per minute, likely to reduce overheating issues when firing lots of short bursts.

Witnessing this thing firing must have been absolutely incredible, although one can’t help but imagine the sheer devastation it would cause on an enemy position.

Cancellation

Despite the overwhelming amount of lead the T249 and its trailered sibling could throw up into the sky, the project came under metaphorical fire in 1957.

Studies that year found that the Vigilante, which was expected to enter service in the early 1960s, would not be able to deal with aircraft from the late 1960s and early 1970s.

With this information, the Army decided that the third stage of the Light Antiaircraft Development Program we mentioned earlier, the Mauler missile, was a much safer investment.

The Mauler, shown here, was regarded as being much more future-proof.

The weapons were developed concurrently, with the Vigilante now seen as a stop-gap to tidy the Army over until the Mauler arrived. A consequence of this was the Vigilante’s priority was reduced, as was its funding, so its development was hampered further.

Evaluations of the Vigilante system were conducted in the early 1960s, but it didn’t show much promise.

While it easily outperformed the M42 Duster that it was meant to replace, it displayed continued reliability issues and it wasn’t durable enough. The ammunition cook-off problem was never properly solved either.

The Vigilante’s sensitive equipment was deemed unfit for military use.

In addition, the Vigilante lacked all-weather capabilities, had a rather low kill probability against the latest aircraft, and would have been a severe logistical burden burning through 37 mm rounds at a high rate.

Overshadowed by the Mauler and displaying poor operational characteristics, the Vigilante project was cancelled in July 1963.

The cancellation increased the Mauler’s priority, but ironically this actually negatively impacted the missile, as its development then became a battle between fast delivery times or good capabilities, which eventually resulted in its own cancellation.

The M163 Vulcan Air Defense System was eventually selected to serve in the role intended for the Vigilante.

Still needing a replacement for the Duster, the Army settled for a cost-effective alternative: an M61 Vulcan mounted on an M113.

Interestingly, Sperry revived the Vigilante system in the 1970s for the Division Air Defense (DIVAD) project, although the gun was rechambered to fire 35 mm NATO rounds instead of 37 mm. This version was mounted on an M48 hull, but it also would not succeed, being beaten by the M247 Sergeant York.

In the end the T249 and its T250 gun stand as an example of the challenges involved in the procurement process, and how interlinked projects can be brought down by a single failed system. It also marks the beginning of a number of SPAAG project failures that denied the US a comparable vehicle to the German Gepard, or Soviet Tunguska.

Perhaps with more time and funding, the Vigilante’s kinks could have been worked out.

One of the surviving T250 guns today. Image by Association of Friends and Supporters of the Defense Technology Study Collection Koblenz E.V CC BY-SA 4.0.

Today this vehicle is rarely discussed, but it has the rare title of being the largest caliber Gatling gun ever made – which we think is pretty cool.

A complete T249 Vigilante still exists in the US, as does a T248 trailer, with both currently located at Fort Sill, Oklahoma. A a T250 gun also survives in Germany, in the Defense Technology Study Collection in Koblenz.