Hopefully you have an attorney. Its a strict liability charge--if in possession, the hammer is coming down unless you can raise an affirmative defense. I suspect ( I am NOT a CA attorney) you would have to PROVE you contacted the agency and PROVE you intended to bring the weapon in for destruction. Not enough to merely claim it.
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Your question seems to have two parts. First, what can you expect at sentencing. Second, you are basically asking questions that may be relevant towards an appeal. The jury and the judge, hopefully, heard arguments on all these issues and I doubt my comments here will change the verdict.
Sentencing is controlled by California Rules of Court, Rule 4.414. Your attorney should submit a sentencing brief, wherein he writes suggestions as to how the judge should evaluate each of the factors in aggravation and in mitigation. They are:
(a) Facts relating to the crime
Facts relating to the crime include:
(1) The nature, seriousness, and circumstances of the crime as compared to other instances of the same crime;
(2) Whether the defendant was armed with or used a weapon;
(3) The vulnerability of the victim;
(4) Whether the defendant inflicted physical or emotional injury;
(5) The degree of monetary loss to the victim;
(6) Whether the defendant was an active or a passive participant;
(7) Whether the crime was committed because of an unusual circumstance, such as great provocation, which is unlikely to recur;
(8) Whether the manner in which the crime was carried out demonstrated criminal sophistication or professionalism on the part of the defendant; and
(9) Whether the defendant took advantage of a position of trust or confidence to commit the crime.
(b) Facts relating to the defendant
Facts relating to the defendant include:
(1) Prior record of criminal conduct, whether as an adult or a juvenile, including the recency and frequency of prior crimes; and whether
the prior record indicates a pattern of regular or increasingly serious criminal conduct;
(2) Prior performance on probation or parole and present probation or parole status;
(3) Willingness to comply with the terms of probation;
(4) Ability to comply with reasonable terms of probation as indicated by the defendant's age, education, health, mental faculties, history of alcohol or other substance abuse, family background and ties,
employment and military service history, and other relevant factors;
(5) The likely effect of imprisonment on the defendant and his or her dependents;
(6) The adverse collateral consequences on the defendant's life resulting from the felony conviction;
(7) Whether the defendant is remorseful; and
(8) The likelihood that if not imprisoned the defendant will be a danger to others.
You can help your attorney by giving him suggestions on facts to discuss in brief on each point that the judge will consider. A thorough, yet respectful sentencing brief can make a big difference for you. Good luck.