Sorry, I was gone with no internet for quite some time, so i didn't have a chance to address the issue.
Basically, it is not that she is incapable of using rifles, but rather she prefers to cause as little damage to life as possible. She understands that sometimes it is inescapable to harm people, and will defend herself, but will use non-lethal/non-harmful methods if at all possible. in light of this, a pistol would make sense, as it has limited range, less power, and less chance of accidentally causing a mortal wound than a rifle or other larger weapon.
Does that make sense? Not that she is incapable or unwilling, rather a matter of preference.