It depends on a few things like if I'm saving up for something specific, if I need it for something, and it's price. I won't spend $1200 on an impulse buy, but I also won't worry too much about research on a $100 gun. I'm also a sucker for a good deal. If I have the money, and it's a decent gun, I'll probably buy it.
Most of the guns I have were not impulse buys. I did at least some research on them or at least knew that I wanted them beforehand. A few were impulse buys. So far I'm happy with all of them.