This is, unfortunately, a misconception.

Sure, a bullet the same size or smaller will 'fit' in a given barrel, but a depending on its weight and dimensions it will, at best perform equally, and at worst destroy the barrel.

But a bullet isn't the whole story; the rest of the cartridge has to fit snugly for the action of the gun to lock properly. Again, best case - the gun doesn't fire. Worst case, the action blows up.

This is a pretty simplified overview, but unfortunately the assertion above is a (pretty absurd) myth. Sorry :(