Space Adventure Cobra is a popular Shounen Jump manga from the late 70s, early 80s.
Cobra is a notorious Space Pirate with a laser gun as his left arm. But, he was tired of being chased by the agency in space and decided to lead a regular life by changing his face & being wiped of his memory.
3 years later, he is a regular person leading a regular life. He is tired of his lifestyle. For fun, he goes to see a "Trip Movie" that allows him to see the dream of his choice. When he saw his "dream", he saw his memories of when he was the notorious Space Pirate Cobra. However, he didn't know that was his memories. As he is going home, he meets up with someone who had a grudge with him. There is a little scuffle, and learns that he actually has a laser gun on his left hand.