That picture was partially faked. The rifle was never on the man the hand that is on the ground in front of him was also placed.AS was the theory by a TV show I watched that the photographer wanted to make it more dramatic.
"There stands Jackson like a stone wall! Let us be determined to die here and we will conquer!"
-Brig.Gen. Bernard Bee, Henry House
The man is missing his left hand. The hand that was placed there is a left hand and was probably his. If the photographer had to walk some distance to retrieve it then that speaks more to the force that removed in the first place. His disemboweled state may well have been due to scavenging by animals/hogs but I doubt they chewed his arm off. The rifle I'm sure was placed where it is.