I’ve lived in SoCal for a decades and have never heard of a machine gun being used in a crime.
Never.
There is abuse with Form 10 guns sometimes when the new idiot (usually young) constable/sheriff buys a whole bunch of Post sample machineguns "for his personal collection" instead of for his department.
Arkansas has had three (3) Chief LEOs pinched for this. Latest one is in Searcy this week. 2010 there was a Sheriff that had a Vickers and M1917 post sample MG, and an M203/M16 built "for his department". When the chief retired, he took the post sample machineguns with him to his retirement home. THAT is a BIG NO NO. Form 10 guns stay with a 07/02 SOT or AT the department.
Well guys in the North Hollywood shootout had full auto weapons.
I don't know why anyone (even criminals) would want a ghost gun. Simple possession of a weapon with a missing or altered serial number is a felony even if you never used it.
Also, who knows if the gun has been used in a previous crime that could then be pined on you.