I don't agree unless you are someone familiar with guns and she wasn't. I have 5 sewing machines and every one is threaded in a different way. They have to be threaded correctly or they simply don't sew. You wouldn't manage it if you weren't familiar.
Door knobs and microwaves are something you have grown up with. Sheila didn't grow up with or have any experience of semi-automatics and in the throws of psychosis the ability to engage with a new skill would be impaired.
My previous microwave had a stop/start feature and a timer. My new one is far more sophisticated but all I use is the stop/start feature and the timer. Whilst, had she been interested, Sheila MAY have been able to pick up a gun and fired it randomly, I'd bet my life that she wouldn't have known how to load it. Isn't there a story of Jeremy being seen trying to get her to load his new 'toy', at a family function, with no success because she was totally disinterested?