Yeah "hired guns" (as in experts who will render whatever opinion they're paid to render) are common enough, but it's usually pretty easy to shoot them down. I was in a federal criminal trial once where the defense brought on a hired gun. The prosecution team had stressed because they didn't arrange an expert pathologist to prove a very basic element of the crime charged. The element was so basic that it was essentially easily assumed by everyone, but it was mistaken oversight by the otherwise on point prosecution team. You can't assume anything in court. Long story short, the defense's hired gun was so focused on pushing very old and dated research that wasn't really relevant to hopefully create reasonable doubt that he didn't catch the prosecutor baiting him into testifying to prove one of the basic elements of the crime FOR THE PROSECUTION!
Not quite the same as your scenario, but quite close.
At my job I am considered an gun "expert." I am a certified armorer for several platforms, and one of two people at my agency that can teach firearms or who can repair our firearms. That said... I could be called an "expert" by some... but I certainly don't consider myself an "expert." Especially amongst many members of TFL. Maybe a peer to many, but a pupil to many others. To answer the original question... it depends on the definition of "expert." But, to the general populace most of us here would probably qualify as an expert.
I'd generally agree with that assessment. I've been on a few TV shows as a subject matter expert, as well as working on cases of gun failures, shooting reconstruction and self defense shootings as an expert. I've been hired by SHOT, NSSF, half a dozen manufacturers and several attorneys for my expertise.
But, there are a LOT of areas of expertise within the broader topic of firearms. Ballistics, Shooting, Ammunition, History, Design, Manufacturing. Within those are bunches of others.
Sadly, there are "experts" who have some credentials, but lack the real depth of knowledge. In most cases where I am pitted against an LE expert, they are more functional practitioners than true experts. And sure, there are many who will say what their client needs, as their advocate.
I had a recent video call with two attorneys and their client. When the attorney said, in reference to me, that they needed a strong advocate, I stopped her. In more words than this, I basically told her that she is the advocate for the client, I am the advocate for the truth, no matter how it comes out. While I am guessing the other attorney over ruled her in retaining me, she was not happy with my answers. I won't let attorneys move me off of the fundamentals very far, which usually bothers them.
I've been called to the stand three times in my career to testify for the side opposite of who hired me. Some attorneys won't hire me for that reason, but for me, it is a tip of the hat to my integrity that I advocate for the truth regardless of who pays my bills.
A hobbyist certainly could have the depth and breadth of knowledge to be a subject matter expert in some areas of firearms. Just because someone has not been paid professionally in a field does not prevent them from being an expert. A colleague of mine comes from a family that designed and built amusement park rides. Combined with his engineering education, he is one of the foremost experts on amusement park rides failures and accidents. I have another colleague whose family business was fireworks shows, he is a foremost expert on firework display incidents. In most cases, (my opinion) there is a combination of education and experience that makes someone an expert.
An expert witness is a person with specialized knowledge, skills, education, or experience in a particular field who is called upon to provide their expertise in legal proceedings to assist the court with understanding complex technical or scientific issues.
While that definition uses "or", to me, there needs to be more "and".
If what an "expert" opines upon is common, or the common man would know or understand, then that is not expertise.