A firearms expert from the FBI testifying on the second day of the murder trial of 16-year-old Paul Wersick, said yesterday that a bullet found in a mattress in a bedroom of Wersick's Wheaton home was fired from the same gun as a bullet found in the murder victim's car.

Evan Hodge's testimony came at the end of a day during which the prosecution methodically continued to piece together the case against Wersick. The youth has pleaded innocent by reason of insanity to charges that he fatally shot Montgomery County tax assessor George Angerman Jr. in a Rockville garage July 10.What emerged from prosecution testimony today about the short, pudgy teen-ager was a picture of a youth with bizarre habits and an obvious fascination with weaponry.

Earlier in the day, Montgomery County police detective Rodney Ingels testified that during a search of Wersick's room, he found hundreds of rounds of ammunition in a cigar box under Wersick's bed, and five discharged bullets embedded in the wall and in his bed.

Ingels said he also found a holster "hidden" inside a "make believe" wooden radio in the youth's room.

In the same mound of material under Wersick's bed-a mound that defense attorney J. James McKenna called a pile of "rubble"-detectives found one spent riflle casing.

In the foyer of the Wersick home, police detective Barry Litsky testified, he found a photograph of Wersick, dressed in a boy scout uniform and neckerchief standing next to three "gentlemen" in boy scout uniforms.

Detectives found the shirt and pants of a boy scout uniform scattered around Wersick's room, but they could not find any boy scout neckerchief.

Prosecutors claim that the murdered tax assessor was gagged with a boy scout neckerchief. A neckerchief was presented into evidence today by prosecutors who said it was the same one that had been used to gag Angerman.

A piece of nylon cord that prosecutors say was tied around one of Angerman's wrists was also entered into evidence.