Relative to a 50bmg barret, its quite light

Barrett's M82 .50 BMG Semi auto is already prohibited in Canada.

Regardless of that, I will replace a Barrett M82 with a GM6 Lynx in your argument, because giving you the benefit of the rifle being semi auto doesn't really help you that much.

An M305 is more deadly to police officers than a GM6 Lynx. A GM6 is practically unusable in a fast firefight, whereas an M305 is perfectly usable (although arguably less than optimal compared to a lighter 5.56 rifle). .50 BMG's high recoil leads to an inability for fast followup shots, the low ammo capacity means you are not able to sustain any meaningful amount of fire, and the high weight and length makes it a deathtrap in close quarters. A .50 BMG rifle is only heavier in the sense of literally weighing more, in terms of armament in a firefight a lower caliber rifle with a higher magazine capacity will be much more effective.

Have some real examples in history of officers being outgunned by semi automatic rifles.