David Hambling explains why U.S. Gatling guns are not stopping Iran’s Shahed drones:
The Centurion C-RAM (“Counter Rocket, Artillery and Mortar”) was first deployed in Iraq in 2006 and is a land-based variant of the original Phalanx CIWS (“Close In Weapon System”) used by the Navy since 1980. It is the last line of defence when urgent action is needed to prevent casualties. The land version is a self-contained unit weighing around 24 tons and costing something over $4 million.
As the name suggests, C-RAM was introduced to protect bases against rocket, artillery and mortar fire. It has an integrated radar which tracks incoming projectiles as well as the stream of rounds fired by the 20mm M61A1 Gatling gun to put them on target.
The cannon is the same as that carried by F-15 and F-16 fighters. Its distinguishing feature is its phenomenal rate of fire, the six electrically-powered spinning barrels selectively firing 3,000 to 4,500 rounds per minute – that is 50 to 75 per second — producing a sound like a buzzsaw, often rendered as “Brrrrt.”
While the Navy version fires solid tungsten projectiles, CRAM uses the M940 Multi-Purpose Tracer – Self-Destruct round. This weighs 3.5-ounces/99 gram and consists of a tungsten cone to punch through the target skin, and a body which explodes in a dense mass of fragments inside the target. “Tracer” means the round produce a visible glow, and in operation the stream of projectiles appears as a bright ribbon reaching out towards the target. Automated tracking shifts aim until that ribbon overlaps the target.
The “Self-Destruct” part means that the rounds automatically explode at a range of around 2,300 meters if they miss the target, an effect also highly visible on videos of CRAM engagements. CRAM is a point defense system placed to protect high-value assets. If a drone strikes just couple of miles away the operators can only watch.
CRAM has a magazine of 1,500 rounds. This sounds like a lot, supplying 30 one-second bursts of 50 rounds each. But it is only enough for 10 two-second bursts at the higher rate of fire. It reportedly takes some 30 minutes to reload CRAM manually with 15 boxes of ammunition each weighing around 60 pounds.
Each M940 round costs $168, so a 150-round burst costs around $25k, comparable to the price of a Shahed.
Because of the way that CRAM lights up the night sky and how much noise it makes, it is hard to keep its operation secret.
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What we do know is that the reported success rate against rockets and mortar shells in Iraq was reportedly 70-80% with an average of 300 rounds per engagement. These are relatively convenient targets because, although they are moving fast, they come in on a very predictable trajectory and they descend from high in the sky making them easy to pick out on radar.
Tackling drones may be more difficult. Being made of composite material rather than metal, they may have a small radar reflection. And Shaheds can fly at extremely low level, sometimes at under 100 feet with a flight path that takes them between buildings. The level of background clutter will make radar tracking challenging.
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Unlike rockets and artillery, Shaheds do not need to fly on a predictable path. Some of the Russian versions automatically carry out evasive maneuvers when they sense a threat. As the videos show the stream of rounds can be seen and potentially evaded. This dodging would at the least increase the number of rounds needed for a kill. Russian Shaheds are also accompanied by numbers of low-cost Gerbera decoys to distract and deplete defenses. Iran does not yet seem to have either capability.
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The U.S. Army only acquired about 20,000 rounds of M940 this year, which one weapon could burn through in five minutes of firing.