You have to seal the bullet, too.
I have been working my way through ammunition exposed to smoke, fumes, and water during my house fire. Some of it is shootable, some of it is shootable with an occasional misfire, some of it has many or all misfires and I pull them apart to salvage the bullet and brass.
I note that there are fewer misfires in reloads with lead bullets than reloads or even some brands of factory load with jacketed bullets.
I conclude that the soft lead and bullet lube seal the case mouth better than a copper jacket.
(The red lacquer on Sellier & Bellot ammunition at primer and bullet appears to do exactly nothing.)