ArtP said:
If the penalty was a misdemeanor, I would take the risk
In California most firearm related misdemeanors, and many non-firearm related misdemeanors prohibit you from owning firearms for 10 years.
If you then proceed to possess a firearm within those ten years you are a prohibited person in illegal possession, the same statute as a felon in possession.
Which results in a felony charge.
(2) Commencing January 1, 2000, manufactures or causes to be
manufactured, imports into the state, keeps for sale, or offers or
exposes for sale, or who gives, or lends, any large-capacity
magazine.
This can be charged as either a misdemeanor or a felony under California statute.
Given that discretion the prosecutor will typically choose to charge you with a felony, but not always.
There is no law on possession:
(2) Commencing January 1, 2000, manufactures or causes to be
manufactured, imports into the state, keeps for sale, or offers or
exposes for sale, or who gives, or lends, any large-capacity
magazine.
Technically you can possess it all you want. Losing it, leaving it on the front lawn unattended, throwing it out into the middle of the street and leaving it there, leaving it in a stall at the gun range, or finding one someone else has lost or forgotten someplace is not a crime.
No matter when it was made or imported. However making it or importing it into the state after 2000 is a crime. Selling one is a crime.
If you bought it in the state it is the seller who is committing the crime. Though the purchaser could probably be charged as being part of a "conspiracy to commit" the crime or an accomplice if they can locate the seller.
So if you happen to find a high capacity magazine in California you could technically consider yourself lucky and go buy a pistol to go along with it.