Legal Question in Criminal Law in California

"Strikes" are qualifying crimes listed in Penal Code section 1192.7 ("serious felonies") and Penal Code 667.5(c) ("Violent felonies").

If you had been convicted of one of those previously, then you have a strike. No matter what your current felony conviction is for, your sentence would double and you would be limited in the credits you would get while in prison. Two qualifying previous strike felonies on your record and your next felony conviction you face 25 to life.

Is just possession of a firearm a strike all by itself? Not usually, but it can be in certain instances. For example, if there are gang allegations pursuant to 186.22(b) alleged, it can turn a non-strike possession of a firearm into a strike offense.

If the gun was used during the commission of certain felonies, then it can make a difference.

The second part of your question - can they prove it without finding the weapon and based only on witness testimony? That depends on the specific charge and the entire case. Can they convict just on testimony alone? Yes. Would it be more difficult without the actual weapon? Absolutely. Can it be beat on legal grounds and through effective lawyering? Yes.