To me, snug means to not add very much torque at all to the breech plug once it's fully inserted to be flush with the bottom of the breech plug hole. I might back it out once or twice just to make sure that it's all of the way in, but after that, additional torque isn't really necessary since it won't back out on it's own. The plug has too many threads for that to happen.
I apply even less torque to conventional nipples. They're usually only barely snugged and I've never had any back out on their own. And nipples have a considerably lesser amount of threads than a breech plug does.