As the title says, this is Peggy's back tattoo. The shell casings each represent a confirmed sniper killshot. She actually has more than that, but she has two twofer kills and she wasn't sure of how to represent that with the casings, and she wound up stopping when they were symmetrical. It's still a work in progress. She had the banner tattooed around the sniper rifle, and she wanted to put some cold sounding sniper shit on there, but couldn't think of what to use, and she thought "One shot, one kill" was a bit trite, especially since her average is actually better than that. (Not including intentional non-kill shots that she's taken, mostly since joining Arc-SWAT, since they try and minimize the body count when possible.)