Judge Rowlands said there was “absolutely no reason” to take the gun out on a social evening, visiting pubs, and left in a vehicle where anyone could have seen or stolen it.
He said Fitzsimons had been “less than truthful” about what caused the gun to go off and had been prepared to sit alongside and handle a gun when he had been out drinking.
The accused and Mr Colwell were all gamekeepers on the large Boduan Estate near Pwllheli.
Detective Chief Inspector Gerwyn Thomas of North Wales police said after the case : “The tragic death of Peter Colwell was totally avoidable and has left his family devastated.
“Those issued with gun licences for their work or for their leisure activities have a great responsibility to adhere to the strict licence conditions at all times for their own safety and the safety of others.
“This case demonstrates the catastrophic consequences that can follow when people show an arrogant, casual disregard for basic gun safety and the privilege afforded them of holding a gun licence.
“These men have rightly been given prison sentences and I hope they will spend the time to reflect on their most irresponsible behaviour.”
Solicitor Michael Strain, speaking on behalf of Ben Fitzsimons outside court, said: “Peter Colwell was a friend and colleague. We went out for a beer after work to relax and enjoy ourselves around the pubs of Abersoch and Llanbedrog on the evening of the 5th February 2017.
“The tragedy which then unfolded, resulting in Peter Colwell’s death, is something that will be with me for the rest of my life.”