Would a magnet retained barrel be not be difficult to get the barrel straight on? Threads pull the barrel tight to the gun while magnets would just hold it in place.
So if the magnets would pretty much have to be nearly perfect tolerances just to keep the barrel from wobbling around and effecting accuracy. I would imagine it would have to be so tight that it would be difficult to pull apart just because it is so tight. As the parts wear it would begin to lose accuracy.
I say it would be cheaper and far easier just to keep taking the 5 seconds just to unthread the barrel.