OK - After writing my last post, I saw this and this today...
The second article - from MSNBC was decent and helps fill in some holes.
The first, however, confuses the issue. The unfortunate thing about folks who don't know about guns asking gun questions, is that they are often on a deadline or don't know what other questions to ask.
Most of the Slate piece is good enough until the author starts talking about the mechanics of shooting. His characterization that snipers don't usually engage targets at ranges of 100 feet is incorrect. Although they are trained to engage at ranges beyond 500 yards, most shots - especially those taken by law enforcement - are typically within 100 yards.
I also would correct his description of the flightpath of the bullet and the effect of sighting in at 300 yards. With most .30 caliber-class rifle cartridges, sighting in at 250-300 yards results in the bullet striking the bullseye or a hair high (1/4-1/2") at 100 yards. What this means is at the range encountered by the SEALs, they would have been able to aim dead-on with the cross-hairs.
The comment regarding the angle is a moot point. Shots at angles above or below a level plane should be adjusted for as if the target was at a location on that level plane directly above or below the target.
In other words, think back to your high-school trigonometry class and draw a triangle with a line from the shooter to the target (the hypotenuse - H), a line from the target to the plane on which the shooter rests - at a right angle to the plane (the opposite side - O), and a line from the intersection of that line and the shooter's plane to the shooter (the adjacent side - A). Instead of adjusting for the distance to the target (H), adjust for the distance to where a vertical line from the target would intersect the shooter's plane (A). At 100 feet, this is negligible, even at a 20-30 degree angle. At longer ranges, you would need a ballistic calculator (such as this one for the iPhone from Knight's Armament Company) to nail down the appropriate solution to attain first-shot hits.
OK - I have officially spent too much time wonking on this topic...