You can pull a gun out. Roleplay is a very individual thing. Everyone does it different. So do what you want, but just make sure you can justify your actions, because sadly people bitch even if they win, because they want to look like saints.
Most people seem to want a robbery to go like this
"Give me your stuff"
"No, I'm rushing to see my father, he is dying"
"oh fuck...that's rough, alright bud good luck"
How it actually goes
"Give me your stuff"
"No, please, I'm rushing to see my father, he is dying"
"Are you fucking deaf, give me your fucking gear and stop stalling you dipshit or I'll shoot you, then you'll never see him"
Play how you want to play. Just make sure you are on the right side of the rules. If you want to try and shoot em go for it, it's your decision.
With all this being said, please understand that you will still be verbally attacked, regardless of how it goes, people always bitch. Once you've been here a while, you'll get used to it, just don't let it get to you.
Regarding Charlie's answer. What he doesn't understand is it takes 2 to roleplay, one to present the story, and one to digest it and listen to it. The majority who rob people don't care for a story, they want that piece of gear, that weapon, that cash, that vehicle, and they will forever use the argument "Well give me what I want, and you can go about your day and live, we both win".
Anyway. Roleplay it, shoot em, whatever, just try and have fun.