This myth may have started with smooth bores. Mainly cannon. They used wads to seal the undersized steel balls, which did not expand in the bore.
Obturation effects all types of metal, but at different rates and some, due to their hardness, the effect is negligible. Lead, is malleable enough to be compressed into the grooves of rifling, whereas steel is not.
If a bullet is not sealed in the bore, the loss of gas pressure will result in a loss of velocity. Also the burning gas will act like a torch and burn into the metal. This will cause an enormous loss in accuracy. If there are blow-by gasses, this will occur on the bearing sides as well as the base of the bullet. If no blow-by gasses, it will only occur to the base of the bullet. Either way, there is a loss of accuracy; However, it would be more extreme in a non-sealed bore with blow-by gasses.
The best way to detect blow-by gasses in the bore, is with a chronometer, due to the loss in pressure versus a sealed bore. Also, accuracy may be used. In the blow-by situation, most of the bullet's surface is effected and an extreme loss of accuracy should be noted. With a sealed bore, only the bottom of the bullet is effected and accuracy will be diminished, but not to the extreme level of a blow-by situation.
Sorry, the engineer in me is coming out.