Sheila went on shoots. I'm sure she didn't just stand looking into space,she'd have been watching each and every member loading up ready to shoot their target. When someone sees how it's done on many occasions,the farm too,they DO grasp how it's done. Her father may have shown her,who knows ? Sheila had been brought up on a farm so she WOULD have known how to load a rifle,even if it had been one at a time.