So I make it about 15 minutes, it seems like plenty of time. 
Excellent. A constructive response. Other posters just tried to undermine post 152.
It seems to you believe Sheila was super fast and clinical in her actions. Almost execution like.
You've removed the shower section. Which was optional and takes off ten minutes. However there is then no explanation why Sheila was completely mark free.
You must allow time for Sheila checking whether everyone was dead. Neville's back was burnt three times. The OS saying it was done by the rifle. June was also shot again when on the floor. Time estimate reinstated.
The bible was open a specific page. Which apparently is read before suicides. Bit weird that Sheila would get it, open it and then not read it. The only other explanation is that it was a plant. Time estimate reinstated.
The suicide note is optional. Supporters and the OS will have to retract on the suicide note claim to attempt to fit the 22/38 minutes Sheila had.
Taking the silencer off and putting it away. Then returning upstairs to pluck up the courage to shoot herself. Twice. Will take more than two minutes. Time estimate reinstated.
Neville's fight was after he returned upstairs from phoning Bamber (3.10) or the police (3.26). To then get shot 4 times, run downstairs, put up a massive fight, before being shot again will take 15 minutes. His first upstairs shots may have been minutes after his phone call. Time estimate reinstated.