What Gun Killed Osama bin Laden?

Tom Beckstrand is a former Special Forces soldier who has served in both Iraq and Afghanistan. On Sunday night Navy SEAL Team Six took off from Afghanistan, crossed the Pakistan border and dropped in on Osama bin Laden's massive compound hideout. The SEALs engaged in a violent firefight, killing bin Laden and three of his men. After extracting the body and sweeping the house for information, they were back in the air after just 40 minutes. None of the SEALs were injured or killed during the mission. Navy SEALs are trained in a variety of weapons and right now there is no way to tell for sure which gun was actually used to kill bin Laden. However, based on my military experiences and the details of the mission, there are three guns that stand out as the most likely candidates for the job: Colt's Mk 18, the Knight's Armament SR-25 and the standard issue M4. Here's an in-depth look at the guns SEALs carry into close-quarters battle. In my opinion, one of these guns took out the world's most wanted man.
The MK 18
In all likelihood, this is the gun that killed bin Laden. The Mk 18 is a short-barreled version of the M-4/M-16 issued to our nation's military. The Mk 18 was developed specifically for Navy SEALs to provide them with the most compact carbine chambered in 5.56mm. For missions like the bin Laden assault, 20 of the 24 SEALs carry these rifles. By the Numbers:
Weight: 5.9 lb
Length: 26.25 inches (Stock retracted)
Barrel Length: 10.3 inches
Rate of Fire : 700-950 rpm
Muzzle Velocity : 2,600 feet per second
Caliber : 5.56mm x 45mm NATO
Magazine : 30 round
The Mk 18 is the Navy SEAL weapon of choice for close-quarters battle (CQB) because it features a 10.3-inch barrel. CQB is exactly what happened when the SEALs entered bin Laden's compound and started shooting it out with him and his men at distances ranging from about 1 to 20 meters.
The Mk 18 took the place of the MP5 submachine gun in the 1990s because it enables operators to shoot a rifle round in a submachinegun-sized platform. The change gave our men a huge jump in ballistic horsepower, moving from the diminutive 9mm to the 5.56mm.
Close-quarters battle calls for speed, surprise and violence of action, and the MK 18 allows for all of these. The short barrel of the Mk 18 makes it easier for soldiers to rapidly move through doorways and hallways and around each other.
The assault element that killed bin Laden took two helicopters to the objective and the plan was for the SEALs to fast-rope from the helicopters onto the target. The 26-inch length (with stock collapsed) of the Mk 18 makes it a great weapon for working within the cramped quarters of a helicopter and while sliding down a rope.
The Mk 18 was developed for the Navy by their Crane Division, a branch responsible for weapons development and other tasks. The Mk 18 upper attaches to the standard M4 or M16 lower, giving it the capability of a 3-round burst or full-auto fire depending on which lower is used.
The Mk 18 was designed for use with the 62-grain M855 load, but also sees considerable use with the Mk 262 77-grain load. Both loads experience considerable velocity loss with the short barrel, so it's a weapon best used exclusively for CQB or at ranges of less then 100 meters.
SR-25/SR-25K/Mk 11
If the Mk 18 wasn't the weapon used to take out bin Laden, there's a good chance it was an SR-25. The SR-25 is a semi-automatic sniper rifle similar in design and appearance to the M-4/M-16. Our Special Operations forces have many rifles with the standard 20-inch barrel, but there are a few in select units that feature the shorter 14.5-inch barrel in the "K" model. The K models are used as urban sniper rifles that see dual use in both assault and sniper roles. By the Numbers
Weight: 15.3 lbs
Length: 45.4 inches
Barrel Length: 20 inches
Rate of Fire : semi-automatic
Muzzle Velocity: 2,571 feet per second
Caliber : 7.62mm x 51mm NATO
Magazine : 10 / 20 rounds
The SR-25/Mk 11 is slightly larger than the M-4/M-16 and is chambered in 7.62x51mm; it features a 20-round detachable box magazine. The rifle operates on the same direct-impingement system as its little brother. They were likely used by 4 of the 24 men on the mission and probably featured the shorter 14.5-inch barrel. The optic most likely used on the SR-25K is the Nightforce 2.5-10x32 with a red dot sight in an offset mount. This combination allows the shooter to effectively engage targets at distances from 1 to 800 meters.
The standard 175-grain load that the military uses clocks around 2,500 feet per second from the shorter SR-25K barrel. While that velocity is slower than what's offered with the standard 20-inch barrel of the SR-25/Mk 11, it is still sufficiently lethal out to several hundred meters (unlike the Mk 18), although the longest shots taken on the bin Laden raid probably didn't exceed 100 meters.
The Mk 11 doesn't shoot as quickly as the Mk 18 because of its heavier caliber, but the larger caliber is invaluable for longer and/or sniper shots. The adjustable trigger of the SR-25 enables the operator to set the trigger as he sees fit.
The SR-25 has been in the Special Operations inventory for more than a decade. It was adopted as the first semi-auto sniper rifle and it remains popular with our Special Operations forces. On missions like the Osama assault, sniper rifles still need to be maneuverable and lightweight. The SR-25Ks likely used on the operation to take bin Laden allowed the SEALs to run and gun through the buildings on the objective and later provide sniper overwatch to protect against any potential counterattack. Photo by: Lance Cpl. Patrick M. Johnson-Campbell
The 12-inch railed fore end of the SR-25 leaves plenty of space to attach lights and lasers. With an infrared laser mounted to the rifle, the night-vision-equipped SEALs could put an invisible dot on their targets and shoot them in complete darkness, a terrifying prospect if you're the enemy.
The SR-25 comes standard with a quick-detach suppressor. While the SR-25 suppressor is long and heavy, suppressors are crucial for any rifle that will see nighttime combat operations. Suppressors decrease the noise and light signature associated with firing a rifle, especially at night. The suppressor also traps the muzzle blast and makes it easier for the sniper to spot his own trace during daytime shots. Spotting your own trace is the ultimate solution for almost instantaneous follow-up shots. Photo by: photo: Cpl. Patrick M. Johnson-Campbell
M-4
The M-4 is the standard issue weapon to our men in uniform. It is also the ancestor of the M-16 and is the result of shortening the barrel and attaching a collapsible stock. It is possible that a few of the SEALs that assaulted bin Laden's compound took their standard M-4s with them instead of the more mission-specific Mk 18. The longer barrel of the M-4 would give the SEALs better terminal ballistics, but would also make the rifle more unwieldy. The biggest difference between the M-4 and the Mk 18 is the amount of barrel forward of the front sight post. The M-4 has about 4 more inches of barrel. By the Numbers
Weight: 5.9 lb
Length: 33 inches
Barrel Length: 14.5 inches
Rate of Fire : 700-950 rpm
Muzzle Velocity: 2,900 feet per second
Caliber : 5.56mm x 45mm NATO
Magazine: 30 round
Beckstrand's Bio
Tom Beckstrand is a former Army Special Forces officer (Green Beret) and graduated from Ranger School and West Point. He served four combat tours as a Special Forces team leader. Two tours were with an unconventional warfare team in Afghanistan and the other two tours were as a sniper team leader in Iraq.

Details continue to surface about the raid that took out bin Laden. But for gun nuts, there's still a big unanswered question: what gun was used? We take a look at the two most likely candidates.