a primer is pressed into a case with a fit so tight that it is compressed. A bullet is pressed into a case with a fit so tight that it stretches the shell, and then it's crimped.
You realize that a beer can isn't sealed or welded, it's just pressed, metal to metal, just like a cartridge. The beer can doesn't allow high pressure gasses to escape, and it doesn't allow seepage.
NO factory loaded shell should leak under even bizarre circumstances. You should be able to drop them into a 100 meters of water, bring them back up years later, and fire them.
The military seals and crimps and takes a lot of extra steps, because that is what they do. They don't allow for failures.
When you see the pretty red spots of lacquer on a commercial primer, it is only because the factory wants you to see how seriously they take their work. That dab with a red sharpie adds a couple bucks to the perceived value of a box of ammunition.