US Marines arrived in Syria in March to support the effort to retake Raqqa with artillery fire.
The Marines, from the 11th Marine Expeditionary Unit, came with M-777 Howitzers that can fire powerful 155 mm shells. The 11th MEU returned to the US in May, turning the operations over to the 24th Marine Expeditionary Unit.
US-backed Syrian Democratic Forces said they recaptured the city in mid-October, and, according to Army Sgt. Maj. John Wayne Troxell, the Marine fire supporting them was so intense that the barrels on two of the Howitzers burned out, making them unsafe to use.
Troxell, who is senior enlisted adviser to Joint Chiefs of Staff Chairman Marine Gen. Joseph Dunford, said last week that US-led coalition forces were firing on ISIS in Raqqa "every minute of every hour" in order to keep pressure on the terrorist group.
"What we have seen is the minute we take the pressure off of ISIS they regenerate and come back in a hurry," Troxell said, according to Military Times. "They are a very resilient enemy."
The M-777 Howitzer is 7,500 pounds — 9,000 pounds lighter than its predecessor. It is highly maneuverable, and can be towed by 7-ton trucks or carried by MV-22 Osprey tilt-rotor aircraft or by CH-53E Super Stallion or CH-47 Chinook helicopters. It can be put in place and readied to fire in less than three minutes.
Its sustained rate of fire is two rounds a minute, but it can fire four rounds a minute for up to two minutes, according to its manufacturer, BAE Systems. While it's not clear how many rounds the Marine M-777s fired or the period over which they fired them, burning out two barrels underscores the intensity of the bombardment used against ISIS in and around Raqqa.
"I've never heard of it ― normally your gun goes back to depot for full reset well before that happens," a former Army artillery officer told Military Times. "That's a s---load of rounds though."
The M-777's maximum range is 18.6 miles (though it can fire Excalibur rounds accurately up to 25 miles, according to ). Video that emerged this summer showed Marines firing 155 mm artillery shells with XM1156 Precision Guidance Kits, according to The Washington Post.
The kit is a type of fuse that turns the shell in to a semi-precision-guided munition that, on average, will hit within 100 feet of the target when fired from the M-777's maximum range. The XM1156 has only appeared in combat a few times.
The number of rounds it takes to burn out a howitzer barrel depends on the range to the target as well as the level of charge used, which can vary based on weight of the shell and the distance it needs to be fired.
If the howitzers were being fired closer to their target, "the tube life might actually be extended some," the former Army officer told Military Times. Open-source imagery reviewed this summer indicated that Marines were at one point within 10 miles of Raqqa.