Respecting your views fully, I have another speculative vibe on the shooter.I bet this inexperienced killer would have gone to a gun range probably less than a week prior to build confidence and bc it seems like they were a big time planner so I'm sure they would have wanted to practice prior.
The shooter is not inexperienced. Rather they are just not formally trained. So, no drilled in two handed shooting techniques. Though almost nothing is truly rare in the age of the internet, .22 revolvers are just not that common. A casual purchaser could walk through more than a few pawn shops, gun stores and even gun shows and not readily see one.
So... my vague vibes point me towards:
A "daddy's tom girl" (shooter really seems feminine to me). Relatively big boned, but light on her feet. Softball produces a number of those types of girls.
Daddy introduced "Sally" to .22 plinking with a revolver at a young age- and she got pretty good at it. Sally .22 plinked to please her father (always wanted a boy). But... she never developed a personal interest in guns. So, no gun purchases in larger bores.
One day, Sally gets enraged by something (she may have a history of rages over 'disses, "stolen" boyfriends, jealousy over sports team positions etc.) Sally retrieves the old .22 revolver from a closet or sock drawer, then fires quickly and accurately, killing the victim.
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