Like Uxie said, you have messed up some joints...
(i am assuming you are using MS3d)
1. make sure the gun is pointing tward the purple axis.
2. add a joint on the handle (a joint is the size of the space between the blockheads fingers, for resizing purposes)
3. click the select tool and click off of your model so the joint is unselected.
4. click the barrel of your gun and create a joint on it.
5. rename your joints, the one on the handle is "mountPoint" and the one on the barrel is "muzzlePoint"
for#5 go to the joints tab/menu thing.
Hope this helped!, if not, I don't think its the joints that's the problem.