I don't know if it's still relevant to you, but I've discovered a way to do this without having to weight paint again.
- In the Outliner Window, drag the mesh for your AK to the other armature and set the Object as the parent. You'll see that the AK mesh is still weight-painted to the old armature, but you won't be able to move it in pose mode (yet)
- Make sure there aren't any constraints added to the armature from the AK that you're trying to join to the new armature.
- In Object Mode, highlight the AK armature followed by the armature you want to join it with and hit CTRL+J to join them.
- In Object Mode select the AK and Shift-select the new armature and type CTRL+P and parent the AK mesh to the new armature with empty groups.
That's it. You should now see your AK has the same weight painting it once had but in a new armature! However, you may have to go into edit mode and translate it a bit to get it to line up properly with the bones.
Let me know if you have any issues!
If you join two separate armatures, the child armature (the one that gets joined into another armature) looses all its weight paining therefore you have to skin (apply the weightings) the AK to the armature again.
Also you should delete the parent relation of the ak and the ak armature before joining the two armatures. this way the ak will not disapear.
To remove the parent got to properties -> object -> relations and remove the armature. If you are using an armature modifier, you should remove it aswell