When someone calls you a Pistol it means they are surprised by you or taken aback by you or shocked in some way.
For example if someone suddenly pulls a pistol on you then it would be a shock and surprise to you so calling someone a pistol means they are shocked or surprised by you.
It could be a good thing to be called a pistol and then it could be a bad thing depending on what you did to shock them.
If you're 89 to 90 years old and you go on a long run and outrun someone in a race then if someone calls you a pistol then they are meaning it in a good way.
If you did something bad and they were shocked and called you a pistol then it would be bad.
Being called a pistol though is almost always seen as a compliment to say that you're full of surprises.