Hammer weight

I have done some searching on the site and have seen that some people use bronze bushings to add a little weight to their hammers. I get the concept, and understand where to place the bushing. But I am wondering if they rub or if they are hard on the inside of the frame. If I use one of these bronze bearings, do I risk doing any damage? Should I put shrink tubing around the outside of the bearing to protect the frame, or should it be fine?
Thanks,
Chris