Triple Se7en
The grease/anti-seize gets pushed backwards a bitas you screw the plug in. Same with the tape.
Actually if you apply the correct thickness of tape the correct way it will not move backwards at all. If everything is right it should screw just like a lubricated bolt. If the tape catches, tears, or wads up you need to change your approach. I always end polishing the breech plug threads and the threads that the BP screws in to prevent this.
Besides, no one I know uses that much overlap at the breechplug flashhole-end - that would bend it down enough to cover the hole. Then, the primer flash would pentrate that tape anyways - even if it did.
You are absolutely correct here the discharge of the prime BLOWS the tape away even if it is covering the flash hole.
The way I check my tape job is to check the bore from the muzzle with a light - If I do not see tape all the way around on the breech plug then i did not get it installed evenly or correctly - i really do not care if the flash is covered or not. Pop one cap or primer as I think most of us do before we load and the tapeis blown away - if you are using shotgun primers it will blow it away even if it is loaded - It will not stop the load from going off. The excess tape is blown back up against you BP and forced into any place that the gas my try to escape backwards... it does really seal the the breech threads...
Excess grease can contaminate your load, but those that are using it know and understand that and they take the steps to prevent it - althought the greas thing is not for me. I worry about contamination and the mess bothers me.