I can go along with a grip mark because when I've been using scissors for any length of time,especially if cutting a frayed edge on a carpet,the scissors dig in and can cause a " flesh wound " on my index finger.Undue pressure when not used to it can bring about such an injury.
Sheila would have had a grip of steel on that rifle indicating that she was doing what she'd set out to do and showing determination to finish the job she started.