I have the idea but i didn't get the positioning right. we should totally work together on this 
Now what my finding is: When I opened the original model in the somimp it had an arrow above the hand which is probably telling the game where to hold the weapon.
the only thing that remains is finding how to exactly create that triangle. Considered your extra bone and decided: maybe it's not a bone object after all.
<EDIT>
I think we're doing it all wrong.
I had a look at original hand's skeleton and it isn't anything like ours.
It's late night now so I'm gonna go to sleep and resume my experiments tomorrow. wish me luck guys.
Now what my finding is: When I opened the original model in the somimp it had an arrow above the hand which is probably telling the game where to hold the weapon.
the only thing that remains is finding how to exactly create that triangle. Considered your extra bone and decided: maybe it's not a bone object after all.
<EDIT>
I think we're doing it all wrong.
I had a look at original hand's skeleton and it isn't anything like ours.
It's late night now so I'm gonna go to sleep and resume my experiments tomorrow. wish me luck guys.