My Dog Is A Jack Russell Terrier I They Are Know To Be Naughty But Is This Acceptable?

My dog has always been naughty but he has never bitten but does show his teeth. If he shows his teeth then we put is muzzle on for 5 or 10 minutes so he knows he was in the wrong. I went to pull him always from the fence yet again the other day as he was barking at a car or something and he really snarled at me but didn’t bite. We are getting a bit fed up with his behavior now but I would never want to put him down even though my husband says he will end up having that done if he carries on. I don’t slap him, just tell him no, then put his muzzle on if he is bad. Its not all the time and he has got better over time but still I worry about his behavior. He has never been nasty to others, its just me mainly and my husband sometimes when he is told off but has never been nasty to other people. If he had bitten then it would be a different story but he doesn’t like me pointing at him and saying no as he gets all defensive as if I am going to beat him or something. I feel bad for even shouting at him if he has done wrong as I don’t want him to fear me but do make sure that he knows if he has done something wrong by raising my voice slightly and often make him have time out in his bed in the kitchen to see if it will sink in. Does anyone have any ideas as to what I could do better to try and make the problem resolve in some way?. Hes a rescue dog but Ive had him for about 5 years now so it cant be that hes a rescue dog as he has been with me long enough now. I totally trust him around other people but know that he can be a pain with me and my husband. He does have a very lovable side too and it doesn’t happen that often but do wonder if anyone has any tips. He has also been castrated as the vet recommended it to calm him down and that was done years ago. We also have a female dog but she is very good and never gets angry.