The (felt) recoil on a mounted M2 isn't that great. The gun itself (barrel and receiver) weighs 85 pounds or so. Since these guns were fired "free gun" as opposed to using a traversing and evelating mechanism on the mount, they weren't capable of the precision available with a T&E, but certainly more useful for going after fast-moving targets. AAF Gunners in WWII went to Aerial Gunnery School, where they learned to lead targets and fired at aerial targets pulled on a wire behind another plane.
Pa Bruegger was a Navigator on a B17, and had two .50s at his beck and call, 1 on either side of the nose. As has been stated earlier, it's not so easy to hit moving targets with these things and the only "Fritzes" Pop got to shoot at got home to their beer, bratwurst and sauerkraut. Bombers being hit by "friendly fire" wasn't unheard of, because they flew in close formation (sometimes 1,000 planes at once).
In the early days of the war there were no fighter escorts over Deutchland. The best defense the bombers had was flying a tight formation, so there were many guns facing in all directions. In a unit that flew tight formations, most bomber losses were to flak, not fighters.
A lone B17 was indeed a dead duck, because it would be swarmed by multiple ME109s and FW190s. When Pop's plane got hit, and they fell out of formation, the tail gunner reported six or so bogies on their tail, but since the plane was burning merrily, the fighters didn't waste ammo on them!