The Fourth of July 2010 was blood and pain for Sean McMahon.

On Independence Day, five years ago, US Army Specialist McMahon was appointed to test a new M2 .50 caliber machine gun recently delivered to his unit a couple hundred yards outside Forward Operating Base Kunduz, Afghanistan, at the firing range. After McMahon fired his original weapon, his staff sergeant had him swap it out for the new one.

As McMahon pressed the trigger, the turret-mounted weapon wouldn't fire. He removed the ammunition and inspected the gun, preparing it once more for firing. He squeezed the trigger. Again, nothing. Switching from full-automatic to semi-automatic mode, McMahon fired one last time. That's when the weapon exploded. Shell casing tore through his right leg.

"I looked down and it was just my knee and just a pool of blood," McMahon remembers. "And the first thing that came to my mind was, 'Holy crap, I blew my leg off.'"

McMahon was rushed to a nearby hospital before returning to the US for additional treatment. After the incident, McMahon told me his unit refused to use the new M2 machine guns sent over by the Pentagon. His injuries didn't heal properly, and soon blood clots formed in his veins. He would not be able to run or jump anymore. Diagnosed with traumatic brain injury and post-traumatic stress disorder, McMahon retired from the Army in March 2012.

Sean McMahon, 2014. Photo: Damien Spleeters

The M2 machine gun is the embodiment of simplicity in a killing machine. Nicknamed the "Ma Deuce," the M2 is fed with a belt of .50 Browning Machine Gun (BMG) rounds, and has been in use in the US military since the 1930s.

Assuming you have a round in the chamber, and a gunner to press the trigger, the M2's operation is fairly simple. The chain reaction is initiated when the firing pin strikes the primer, causing an explosion in the casing, and pushing the projectile out of the barrel. The recoil caused by the explosion pushes the bolt backwards, extracting the spent cartridge and feeding a fresh one into the chamber.

The bolt is the pumping heart of the gun. If it's damaged, or improperly manufactured, there might be too much room left for a cartridge to be chambered correctly in the weapon, which can result in a rupture of the case, damaging the gun and possibly injuring the operator. That's potentially what happened to McMahon, who is among a growing chorus of Iraq and Afghanistan war veterans who say these sorts of malfunctions, stemming from defective critical parts like bolts, pose a deadly danger.

While US Marines and soldiers like McMahon were fighting the wars in Iraq and Afghanistan, the Pentagon shipped defective gun parts over to them, according to previously unrevealed records obtained by Motherboard. I've reviewed thousands of pages of Department of Defense audits, studies, quality deficiency reports, contracts and correspondence, and court records, and have interviewed dozens of current and former military officials and manufacturers' employees, quality control inspectors, weapons experts, and veterans about the scope of the problem. My research found that tens of thousands of defective machine gun and other firearms parts, including bolts, backplates, firing and extractor pins, and sights have turned up in the field in the last decade, putting soldiers and Marines in harm's way.