Believe me, if he wanted to bite you it would happen. Dogs, even puppies have deadly serious aim.
It's hard to say without seeing one of these incidents, without seeing the body language of both you and Rufus. If it is only when you are disciplining him, it sounds like it could be a fear/defense mechanism from the pinning, or perhaps guarding the object he is shewing on.
What you have on your hands is the equivalent of a 2 year old child who has learned to say no to it's parents, and is going through the bratty, independance developing stage. I would not pin him, that is a very severe punishment only for very severe "crimes". You have to deal with him as you would a 2 year old child, stay totally calm, don't show any anger, just calmly persist in what you were trying to do as though the snapping weren't happening (like I said, if he really meant to bite you he would), like you would with a child who is trying to hit you. If you respond in anger or severe punishment, you won't get through to the child/dog, you will only raise the tension level that much more, and increase the likliehood that he will make teeth contact one day.
Distraction with a suitable chew or toy (I say "ah-ah, not for puppies--even to my "adult" dogs
and take the thing away or, in the case of a sofa that can't be taken away, give the dog an appropriate substitute, or throw a ball or something if I have one that likes to retrieve and praise when they play with or chase that toy. In other words, I never tell them what they
can't do without immediately showing them what they
can do.
I would recommend two books for you, both by Jean Donaldson; "The Culture Clash" and "Mine!" If you are concerned about potential aggression, and like I said, it is hard to know what is really happening here without seeing it, then you might want to seek out a really good qualified behaviourist to help keep you and Rufus on the right track