I think LE will argue that BM took his 22 short rifle, removed a single bullet from the chamber, put it in his pocket. He then took a dart and syringe of tranq, removed the syringe cap, filled the dart, loaded the rifle with the dart. He put the syringe cap in his pocket.
He then shot her with the dart outside.
She ran to get away. She did not have her phone. She locked herself in her bedroom. He broke down the door.
He finished her off on the bed, smothering her. A struggle occured, which resulted in her scratching his arm and a bullet falling out of his pocket.
The syringe cap ended up in dryer vent when he washed his sweaty, contaminated clothes.
Does it ring true? We'll find out.