Brad johnson from dragondoor.com done this...
Looped 2 trouser belts over a chinning bar. Graved a pair of COC 2 (i think) and turned them upside down so that the spring was in his palm. He then closed them on the belts looped over the chinning bar so that he was gripping the belt with the butt of the COC's
He then chinned.
Phenomenal