There is no one "important muscle for punching". A good punch involves synchronizing various muscle groups and has more to do with balance and control than "raw strength". Your triceps are just as involved in a good punch as your shoulders, biceps, chest, forearms, shoulders, abs, hips and legs.
Big guys have more momentum and strength behind their punches but that does not translate to a "powerful" punch. More often then not, a bodybuilders punch is just a big shove. The excessive upperbody weight adds momentum to your punches making it more difficult to recoil, and the muscles tightness doesn't allow you any "pop" or follow through. You are left slower and more open, I know all this from experience. Excessive upper body mass makes movement difficult and slow, it's like trying to corner in a Cadillac, akward to say the least.
Also, one punch knockouts are rarer than people think, regardless of size. Most knockouts are accomplished by a succession of blows. Big guys who don't know how to punch, usually just wind up with a hay maker, telegraphing their punch and leaving themselves open in the process. Doesn't matter if you weigh 120 lbs, or 220, a three or four punch combo to his ribs and face and the body builder won't want no more.
So what is it ? The single shot 10 guage may look fearsome, but in the hands of an unskilled practitioner is no match for the expert with his Ruger 10/22
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