Adding to what Slam said about the realities, I can speak to the series, a bit.  Perry Mason was based on a series of books by Earle Stanley Gardner.  Gardner had been a practicing attorney, though he did not have a law degree.  In his time frame, it was not a pre-requisite to take the bar exam.  He started it, but had to give up and finished on his own, then passed the California bar exam.  He worked as a defense attorney, especially for minority clients and started one of the early organizations dedicated to fair trials for minorities.  In the early 1920s, he began to write mystery stories for the pulp magazines, particularly those about a private investigator firm.  In 1933, he introduced Perry Mason, who was a defense attorney, but the stories were a mixture of detective mystery and court room drama.  There were 82 Perry Mason books published between 1933 and 1973.  The books spawned a radio series, then the tv series, in 1957.

Because the stories were a combination of mystery and court room drama, you saw Mason do more investigating and questioning, outside the court room, then cross examination within.  However, he did employ the services of the Paul Drake Agency, to investigate his cases, with Paul Drake played by William Hopper.  Paul usually does some of the leg work in each episode, though it varied from episode to episode.

Much of what you see in the courtroom is played for drama and law schools have traditionally advised students to forget what you see in Perry Mason (and later dramas, like Law & Order).  Mason would bring surprise witnesses, that would never be allowed, pulls stunts in the courtroom that might get him held in contempt (or disbarred, in the extreme), to get the dramatic confession.

Elements of the novels were adapted into episodes; but, the general consensus (I've never read Gardner) is that the books were superior to the tv series.  He loses 3 cases over the course of the series.

Perry Mason had a huge influence on how people in the US perceived courtroom cases, often to the consternation of real lawyers and judges.  Similarly, shows like CSI have affected the way jurors deliberate cases, when there isn't elaborate forensics evidence, but considerable eyewitness testimony, according to studies.  Also, shows like Judge Judy, where civil cases are done, have been heavily criticized for the antics of the judges in them.

In the US, you don't have to "swear" an oath, you can "affirm," as swear has been known to cause offense, and the "So help me God..." bit can also be deleted.  You also have different types of courts, between municipal courts, state courts and Federal courts, due to various jurisdictions.

In regards the ballistics stunt, the prosecution would present the forensic ballistics tests and the defense might present an expert witness to dispute those findings.  They might have tests done by an independent lab, to offer in rebuttal.  I don't have first hand experience, but, I suspect it is more likely that the interpretation of test results will be questioned more than the actual results, much like other data.  More often than not, what data tells you is based on who interprets the data, which is why Hell is being locked in a room with two economists to debate the same set of government figures.

For its era, Perry Mason is probably a bit more accurate than similar drama.  The books were definitely better, because Gardner had the experience that the tv writers lacked.

Now, Night Court was 100% accurate!