According to the story it took 150 Shots to cut the Rope.
He should have used something bigger than a .22 Caliber Rifle. Maybe one of those Evil AR-15 Assault Rifles or a nice Barrett .50 Caliber.
...
Clint Eastwood only needed one shot (well, most of the time).
Josie Wales hit the Rope that the Raft was tied to with one shot.
Of course he could have just walked down to the Rivers edge and cut the Rope with a Knife but it would have been anti climatic.