I think it may be a matter of personal preference and mission perimiters. I've had a Marine sniper tell me he carried an M16A2 and had his M40A1 in the drag bag. I've seen pics of Israeli Sayeret Operatives holding M4s with M24s on their backs. I've seen pics of Delta Force Operators doing the same. San Antonio SWAT snipers carry MP5s (but they're not having to lug 'em across no-man's-land). It probably doesn't happen much with regular military forces, but I guess certain spec ops units must do it (situation permitting of course).