I use the "2x4" method. First, I punch the pins. Then, I put the rifle muzzle down on a thick piece of carpet.
Then, I take a long 2x4, and longwise, strike the base of the GB from the top. I don't swing it down, but rather "punch" down, as if I was setting a post in a hole. Usually takes about 4 or 5 times to come off.
Does that make sense?