HALIFAX -- The lawyer for a man who accidentally shot and killed his son as an intruder broke into their home argued Thursday that Michael Paul Dockrill should not face the mandatory minimum sentence because he's already living "in a prison of grief."

"He's suffering greatly. He feels guilty all the time," said defence lawyer Brian Church as sentencing arguments wrapped up in Nova Scotia Supreme Court. "It's a tearful situation for his family."

The 56-year-old Dockrill was convicted in April of criminal negligence causing death for shooting his son Jason, 20.

Church said the mandatory minimum sentence of four years for gun crimes, enacted as part of the former Harper government's 2008 criminal justice reforms, is unconstitutional in this case because it would be a cruel and unusual punishment.

He told the court that Dockrill's son sold marijuana, and had given his father a rifle a few days before the incident after being threatened.

The court heard there was a break-in at their Halifax-area home on June 12, 2011, and that Dockrill had intended to shoot the intruder, but realized afterward that he had fired at his son.

Church said Dockrill, who he described as a family man, should be given a suspended sentence with probation.

But Crown lawyer Tanya Carter said Dockrill should receive a sentence of five to seven years, because although he did not intend to shoot his son, he did intend to shoot someone -- the intruder.

"This tragedy didn't have to happen," Carter told the court.

Carter said Dockrill mishandled a loaded weapon, and the mandatory minimum sentence sends a message that care needs to be taken when using a firearm.

"I think the four-year mandatory minimum is a clear statement from Parliament, which is highly appropriate in our society, that we have to take much greater care and responsibility when we're using firearms," said Carter outside of court.

"Shooting the wrong person is certainly a huge concern, so I think it makes it more complicated, more unique... but also more serious."

Church said if Judge Josh Arnold agrees with his arguments, the case could be precedent-setting in Canada.

The mandatory minimum sentence for gun crimes was introduced by the former federal Conservative government as part of a sweeping omnibus bill.

Arnold will deliver his sentence on Feb. 26.