When was Data made bullet proof/resistant?

Question

In Star Trek First Contact, Data and Picard come under automatic weapon fire from Lily. Data comments "Captain, I believe I can handle this," and he drops down to Lily's level of the missile silo, taking several close range bullets after landing. The bullets don't faze Data in the slightest, aside from looking at his clothing now riddled with holes.

During the series, however, it is never suggest that Data was bullet proof. There is even a time when he is shot with an arrow, which pierces his body and damages a secondary processor. The arrow not withstanding, if Data had been bullet proof all along then there was more than one occasion when he should not have been held at bay by someone with a gun (The Big Goodbye and Time's Arrow: Part 2, for example).

Is it ever revealed when Data was upgraded to be bullet proof, or if he had been bullet proof all along?

Answer 1

This seems obvious but the in universe explanation is that it must have happened 'off screen' after the end of the previous movie but before the movie begins. Some of the other characters underwent changes such as Geordi's new eyes so it's not unreasonable to assume that Data made some self modifications too. However there's nothing written down in canon that I'm aware of.

Answer 2

Isn't Data made out of crazy futuristic metal alloys? Hence his super strength and ability to break through solid surfaces, Doesn't he head butt a klingon and say hes made out of some super alloy? I'm pretty sure he was always some what bullet proof, as for Time's Arrow he was sent back in time and couldn't exactly tell people he was from the future and when he is with the rest of the crew he has to stand down to guns because if Bullets went off it could injure everyone else around him. (The Big Goodbye) I'm sure he had his weak spots, like where the arrow landed or maybe that was a mistake on the writers part before they fully developed the Data character.