Actually its been proven that a katana made in ancient times is actually strong enough to block a "pistol" caliber bullet up to a .45 acp. Granted it chipped the blade and deformed the metal a bit so id say a "modern" katana can block maybe 1 or 2 pistol rounds before showing wear. If quicksilver knew where it was gonna hit and blocked with a flat side of the blade, it would be possible