This was a movie I saw at family video when I was about 6 or 7 years old. It was live action Japanese with English Dub. I think it was from the late 1980s-1990s. It was apparently about a female assassin who had a blue robotic arm that was either protecting or trying to kill someone.
“The Machine Girl”, a.k.a “Kataude mashin garu” (2008)?
https://www.imdb.com/title/tt1050160/
This movie is older than that. The woman’s robotic arm did not turn into a gun.