The more I think about this.
There is a chance he was building (or built) a .510 DTC. (at least I hope he was, as he will get off if so.) It will take some expert examination of the chamber to determine if this is the case.
Probably not that unusual to have incorrect markings on the barrel, especially if he converted a previously made upper.
The .510 DTC is almost indistinguishable from the illegal .50 BMG. That is how shooters in California get the exact same performance from their big guns as the shooters in the free states do.
If he can prove that his rifle is chambered in a legal round, he should be good to go.
This is not something unheard of in California, that round was developed for California shooters after the law changes and is popular there. Going through the process of building that rifle the LEO should have had plenty of chances to build it legally. He should have know this.
The .510 DTC is about 1/10th inch shorter than a .50 BMG, so a regular .50 BMG round will not chamber in those rifles. The rounds are made from using all the components of a .50 BMG by shorting the case and the shoulder is a little different. But you use the same bullet, same powder charge, etc. So the performance is almost identical.
Also California does not see the .50 BMG as scary or dangerous enough to prohibit others living outside California from bringing in their .50 BMG rifles to compete in rifle matches, thats legal.
.
Its possible that he was calling it a BMG to his 'buddies' but it actually wasn't. But that wouldn't get him out of the other charge under PC 30605 (Which isn't a major charge - Not exceeding 1 yr and not exceeding $500 fine. Where-as PC 30600 for the BMG is up to 8 yrs unless you have more than 2 and then you can be charged separately for each rifle.)
I don't know enough about it to say with any certainty other than it seems the .510 DTC is on the fringe of being legal in CA
CA PC's
30525. As used in this part, ".50 BMG cartridge" means a cartridge
that is designed and intended to be fired from a center fire rifle
and that meets all of the following criteria:
(a) It has an overall length of 5.54 inches from the base to the
tip of the bullet.
(b) The bullet diameter for the cartridge is from .510 to, and
including, .511 inch.
(c) The case base diameter for the cartridge is from .800 inch to,
and including, .804 inch.
(d) The cartridge case length is 3.91 inches.
30530. (a) As used in this part, ".50 BMG rifle" means a center
fire rifle that can fire a .50 BMG cartridge and is not already an
assault weapon or a machinegun.
(b) A ".50 BMG rifle" does not include any antique firearm, nor
any curio or relic as defined in Section 478.11 of Title 27 of the
Code of Federal Regulations.
ETA:
https://oag.ca.gov/firearms/regagunfaqs#3
What is considered a .50 BMG rifle under California law?
A .50 BMG rifle is defined as a centerfire rifle that can fire a .50 BMG cartridge and is not already an assault weapon or a machinegun as defined by Penal Code section 16880. A ".50 BMG cartridge" means a cartridge that is designed and intended to be fired from a centerfire rifle and that meets all of the following criteria:
a.It has an overall length of 5.54 inches from the base to the tip of the bullet.
b.The bullet diameter for the cartridge is from .510 to, and including, .511 inch.
c.The case base diameter for the cartridge is from .800 inch to, and including, .804 inch.
d.The cartridge case length is 3.91 inches. (Pen. Code, §§ 30525, 30530.)
4.Can assault weapons and .50 BMG rifles still be registered?
Generally, no. The public registration periods for assault weapons and .50 BMG rifles ended several years ago as follows:
Category 1 AW: must have been owned by 12/31/1991 and registered by 03/31/1992
Category 2 AW: must have been owned by 08/16/2000 and registered by 01/23/2001
Category 3 AW: must have been owned by 12/31/1999 and registered by 12/31/2000
.50 BMG: must have been owned by 12/31/2004 and registered by April 30, 2006
However, Penal Code sections 30625 and 30630 provide an exception for specified peace officers with written authorization from the head of their employing agency.