I wqas wrong, it seems, about the 12.7 mm Russian round being larger diameter than the US .50. I thought they were about 51 or 52 caliber. So, the cartridge probably would be legal for importation, and the cartridge would also be legal in California. The .50 Browning cartridge is not illegal to own in California - only guns for it.
I don recall how specific the law on that was - that is, whether it referred to .50 caliber rifles, or to Caliber 50 US Browning MG rifles specifically. I wouldn’t test the law here by bringing one of the rifles, if they exist, into the state. I have a feeling this ammo must be meant for the MG shooters. I am surprised that the market would support an importation for that purpose, but live and learn. I guess those of us here, safe and sound from all the big, bad Machine guns, in the People’s Republik of Kalifonria don’t have much of an idea about how many MGS are being legally and safely fired in other states. Yes, safe and sound - only 100 or more murders in Oakland, right across the Bay from me, including four police officers, in the last year or so. But then, its been at least a couple of years since they had a riot over there, so live is good in the Bay Area.
Mexico has very strict gun control and I just saw something on the news about one of the Texas border towns, on the south side of the border, have something like 7,000 murders in the last year. Gun control is soooo effective.