Elvin
I would suggest attaching some weights on the barrel, you do want the gun to be balance adding weights to the handle may make the problem worse (in my opnion) since you probably have light muzzle now.
My teak-sea has removable metal plates attached to the bottom of the barrel, I think the goal is to have the gun totally level in the water (again in my opnion).
Perharps you could try installing the reel inserts at the bottom of the barrel to balance the gun.
Attachment 11170