If the cops entered a home on a valid search warrant, then they are allowed to enter and search for whatever it is they are looking for, and only what they are looking for. For example, if the cops are searching for a stolen piano, and they have probable cause
that it is in a particular house, they can get a warrant to search for it. When they enter the home, however, they cant look through dresser drawers or match boxes looking for the piano because it is not possible for the piano to be there.
If the police entered on an arrest warrant, they are allowed to search for the suspect. Again, they cant search through drawers and whatnot for a suspect because it is not possible for a person to be there. They can, however, search the immediate area of an arrested person looking for weapons or similar contraband. They can also search areas where another person might be located that could possibly place the officers in danger.
As far as finding bullets and charging your husband, if they entered the residence on a valid warrant looking for something and they just happened to find bullets in plain view, they could likely charge your husband with their possession. This does not mean they can convict your husband on this charge. They must still prove that he had possession. You would have a good argument that he did not have possession or did not even know they were around. Can easily argue that there were several other people in the residence or who had access to the residence and that it is just as likely the bullets belonged to them. So, the bullets do not really create a search issue, but are more of a fact issue and an issue about whether or not the cops can tie the bullets to your husband.
When your husband gets a lawyer, either appointed or privately paid, that lawyer should be able to find out more specifics about the warrant. That lawyer should be able to tell you whether it was an arrest warrant or search warrant and what the cops were looking for. They will also be able to tell you whether or not they can tie the bullets to your husband based on the facts of the situation.
I hope this helps. Good luck!