From US News:

Two military gear blogs, citing multiple insider sources, credit the highly reliable HK416 rifle, an M-16 type weapon, with the “double tap” of 5.56 mm bullets to bin Laden’s head.

While the military isn’t talking about what SEALs from United States Naval Special Warfare Development Group, formerly SEAL Team Six, carried in, two sites—Military Times Gear Scout and Soldier Systems—said the gun used was the German made, Delta Force designed Heckler & Koch rifle used by several militaries.

…

HK, which didn’t have any immediate comment, describes the gun on its website as the perfect SEAL weapon: it shoots an effective 5.56 mm round, has a gas system that doesn’t foul the weapon like some other rifles, can shoot 20,000 rounds with “minimal degradation of accuracy,” and fires just fine after being submerged in water and not completely drained.