Why is it considered a fault to cant the barrels on a target?

Tuition
JOHN BIDWELL says:
In its simplest form canting the barrels causes the gun to come off the line of the target or bird, usually resulting in a miss.

And losing the line in this way can often affect the swing of the gun, thereby compounding the problem.

I say ‘usually’ result in a miss but there are times when it pays to deliberately cant the barrels in order that the muzzles can remain in line with the bird.

Certain targets lend themselves to this approach particularly rising, quartering away, birds and also quartering driven.

Generally I prefer to see a shooter move his feet to address targets of this sort but times do crop up when it’s not possible to change stance between shots – for instance if you find your feet stuck in mud, or the second shot has got to be taken quickly with the gun already in the shoulder.

This is when canting the gun with both hands and dipping the shoulder a little can help you out of a predicament. That said, I don’t think you should make a habit of shooting this way unless it’s absolutely necessary!