Here's what I think the liberals should do instead that I think would be more effective at achieving actual "effect" (reducing gun violence), appeasing the anti-gun lobby, and assuaging gun owners.
Instead of mandatory buy back that can't be properly funded, make it a voluntary buy back at full value of the fire arms.
Here's why.
We know, and Carney isn't stupid so I'm pretty sure he knows, that most lawful fun owners aren't the problem. But, there is a risk that the guns owned by lawful gun owners get stolen and become used in crime. However, "serious" lawful gun owners (the ones who don't want to give up their guns) are also some of the most ardent at safeguarding their guns. Their guns are less at risk of being stolen and used in crime. The real risk are those who take their ownership less seriously (maybe they've "grown out of guns", maybe they've moved to an area where using there guns is no longer convenient, etc). Make an ad campaign targeting these individuals, and offer them full compensation for their guns.
You still get to appease the anti-gun crowd, and you remove (some) of the guns that might actually end up in crime, and you avoid pissing off gun owners with mandatory and sub-market value buybacks.