Clemenza's advice was very important. Forensic procedures can positively identify a gun as the murder weapon--if the bullet is retrieved from the victim, police forensics will fire another round into a tank of water and microscopically examine the rifling lines that the gun's barrel left on the bullet. If that bullet's markings match the ones from the bullet taken from the victim, every court of law will accept that as proof that the gun was the murder weapon. So, if Michael had been caught with the gun in his possession--even if he were arrested far from the restaurant--he would have been found guilty.
The gun is "as cold as they come." It's impossible to trace. If he takes it with him he runs the risk of being caught with it. Since it is untraceable, if he leaves it there is no way to tie it to him.
Yes. Clemenza meant "untraceable" in two ways: It had no "history"--that is, it hadn't been used in another crime that might have left a telltale bullet in another victim, and thus could have associated Michael with a murder he didn't commit; and he'd treated the butt and trigger with tape that left no fingerprints on the weapon. But, if Michael had been arrested with the untraceable gun in his possession, he still would have been nailed for the reason above.
