Even if Congress were to pass a new version of the so-called Assault Weapons Ban, any weapon or part of a weapons system already manufactured would remain legal to own.
There is a clause in the U.S. Constitution, Article I, Section 9, Paragraph 3, which states - "No bill of attainder or ex post facto Law shall be passed."
A bill of attainder punishes specific individuals and/or groups. This is tantamount to conviction without a trial.
Ex post facto, literally means "enacted after the fact". In other words, what is legal today, can't be legislated illegal tomorrow. Your AR15, and its 30 round magazines which are legal today, can't be taken away from you tomorrow, just because Congress passes a law. They would only become illegal if manufactured after the law went into effect. There are a lot of AR-15's out there manufactured prior to 1994, the last time Congress passed a restrictive gun law. Those guns were legal to own after the law went into effect. Colt, for example, just couldn't manufacture AR-15's with flash suppressors after the law was passed in 1994.
To search your home for illegal items (contraband, drugs, illegal weapons, etc.), authorities must have probable cause that a crime has been committed, and they must attest to this in front of a judge. The judge has the option to sign a search warrant for execution by the police. Search warrants have to be specific. In other words, the police can't be on a hunting expedition. For example, if the search warrant calls for searching for drugs, the search area must be specific, and is limited to that area.
If the police search an area not in immediate control by the suspect, that area is usually off limits. If the police specify the living room in the warrant, they usually can also search the garage if the garage isn't accessible from the living room.
The above are generally true. However, there are specific instances where courts have interpreted search warrants differently in similar cases. The law is a funny (odd) thing at times.