The thing isnt the recoil so much as the rapid fire. If you've fired a civvie .223 you kind of understand, but Military grade 5.56mm fires at a higher pressure than its civilian counterpart. The SAW's weigh about 15 pounds a piece, with a 200 round drum of linked ammo weighing in at 7.5 pounds a piece.
So you're holding up about 23 pounds in each hand, and dealing with full automatic recoil. Its understandable.
And no, thats Army, not a Marine.
ANY rifle caliber fully automatic weapon (ie M60, M240B, M249 SAW) is better fired from the prone, and is surprisingly accurate when done so. Go ahead and do so otherwise, but don't expect to hit much.
82nd ABN Div
CSC Scania
Iraq