Wisecracker is on a timer.. I'm just relaying his comment
The Colt Single Action Army has no external safety. A later version, called the Colt Cowboy had a safety - but that is not the gun used by this movie. The SAA had a "safety notch" in the hammer, which provided some measure of protection from accidentally bumping the hammer when "half cocked", but nothing like that would allow the gun to fire when the trigger was pulled. It becomes impossible for a bump to cause a misfire, because the safety notch at half-cock position misaligns the hammer, preventing it from striking the primer. You'd have to do a lot more than bump it to set it off, you'd have to beat the hammer on the ground to break out of the safety notch - and then you'd have to hit it at just the right angle to re-align it with the primer.
Unless Baldwin was beating on the back of the gun with a blunt object and incredible luck - there's absolutely no way for it to have gone off without having pulled the hammer all the way back. There would be plenty of evidence of blunt force being applied to the hammer if that was the case.
Once that hammer is all the way back, the safety notch has already been bypassed.
The only way to win with that argument in court is if his lawyer is a hypnotist. Any competent gun expert will be able to show that the workings of the gun contradict the story he's telling.