Yes an expert sworn in as such stating no other firearm could have fired the round that killed the person in question is pretty hard for a defendant to counter when the defendant has already stated the firearm in question was his .
Actually its not as difficult as you might think, and that's why experts are trained to say "is consistent with" and not "absolutely came from the crime gun".
The difficulty is convincing jurors who don't know the reality of ballistic ID, but do know what they've been "taught" watching a lifetime of tv dramas where ID is always simple and absolutely positive.
IF(and I emphasize "if") the crime gun has some unique tool mark or scratch that no other gun has, and transfers that mark to the bullet THEN a case can be made for an absolute match. But if NOT, then they can only go by the regular rifling marks. Also be aware that tool marks and scratchers can change over time with rounds fired and cleanings. This is seldom the case when the crime gun is recovered shortly after the crime, but if it is not, if say the gun is recovered a decade or more later, it may still have individual marks but those marks might have changed (been worn) through shooting and (poor) cleaning and not be a clear positive match to the marks left on the crime bullet.
IF (again that word) all they really have is the basic rifling, all they can positively state is what twist it is, and which maker(s) use that twist. Say the slug in question is a .38 and shows it was fired from a gun with a 1-14" twist.
Colt uses that twist. SO that bullet is consistent with having been fired from a Colt, not a S&W that uses a 1-18 3/4" twist.
However, EVERY Colt .38 has that same 1-14" twist, so you need something more specific to link the bullet to a specific Colt revolver.
Also keep in mind that, in court while the facts matter, and what can be proven matter, in the end what matters most is what the jury believes.
Means, motive, and opportunity are the requirements and it can be a "tactical error" for the prosecution to try and make it an absolute when leaving it "consistent with" can be enough for a conviction.
remember that tv crime show writers can do anything they want and make it seem plausible when actual reality is something different. And most people who are not expert on these matters (and if they are, they probably won't be seated on the jury) have been conditioned by TV shows their entire lives. Some folks will go with that, no matter what some "suit" says in court....