Next to Thug Splicers, Leadhead Splicers are the most common enemy, and they always carry a gun (obviously).
To "research" just simply switch to your camera and take pictures of them.
As with other Splicers, try and get pictures of them mid-attack, or in a group, or fighting other enemies, for higher bonuses.
Equipping photo enhancing tonics will also help you net this faster.
Fill the research bar 5 times for the achievement (also each time you fill the research bar you will receive a damage bonus towards Gun Splicers).