Breechplug
My question was if a finger tight BreechPlug would allow blowback and thus cause a loss of velocity from it.
Here are my thoughts... the finger tight plug will allow some blow back back around the breech plug, even with BP grease. How much I am unsure of as it varies in each gun depending on how tight or loose your particular BP is. I have always maintained it is best to stop the blow back at the flange of the BP and the barrel flange inside the bore. If you get it stopped there then you really do not have to worry about it coming back down or out the BP threads. In my case I use teflon tape on the face of the BP and the threads to prevent this.
As to effecting accuracy - unless it was a bad blow back leak I can not imagine that you would ever notice it in the accuracy department. Just remember blow back will contrubute to loss of velocity but again I can not imagine it affecting accuracy with a normal person doing the shooting.