IMO I don't think a gun was used in LB's case. There was absolutely no evidence of a shooting in the house in LB's case. I think DM murdered her via over dose or strangulation. I assume he originally intended on using a gun but not in the house where other people where present (his father, MS, MM) and bullet and fragments would cause damage. Plus IIRC he hadn't acquired bullets at the time of LB's murder for the gun he had recently purchased from MWJ. Perhaps he had another plan in mind, maybe of taking her out to the farmland and murdering her with a gun, but the "opportunity" arose where she contacted him just prior to him picking her up at the station, he brought her back to the house and he found another method of killing her without leaving a mess or damaging evidence.
If the creep had it in him to stand over his father and shoot him or put a gun to TB's head, he also had it in him to use other methods to murder. I think he took LB's text to him serious and carried out what she suggested. Sorry I cannot find the text, but she text him something like "I want you to hurt me as you F me." The replacement of the mattress was because she leaked bodily fluids after death, perhaps vomited from an over dose. We will never know what the sick b@st@rd did to LB or what his plans were, but we can be pretty certain he planned to murder her, carried it out and included MS in her disposal, and he is one very warped and sick minded, evil individual. For that he will pay spending the rest of his life in a prison cell....thankfully. All MOO.