Youtube is the very best source of girls with guns videos. Short videos of about anything. The only problem is that -if you only save the link to the video- it can be removed by the owner.
Showing their guns
Girls and gun holders
Try also this page: Weapons Cache, they have videos of girls firing guns.
There are web services like http://catchvideo.net/# where you can enter the video link (from Youtube, Dailymotion..) and download it to your PC.