California's much-publicised ban on 50 BMG rifles (called AB50, after the bill which lead to it) has lead to a wide misconception of what is and isn't illegal in California. Only shoulder-fired rifles chambered in 50 BMG are covered by the ban. Other than this, any 50cal weapon which isn't an assault weapon, and is in compliance with all applicable federal laws, is not illegal in California. Unless, of course, you live in Los Angeles, which actually does have what seems to be a full ban on the sales of 50cal weapons.
Popular Misconceptions about 50 BMG
There is a lot of absolutely wrong information about the BMG ban out there. I'll address most of the more popular mistakes here:
It's a felony to possess 50 BMG ammunition: That was a part of the early drafts of the bill which went on to become AB50, the 50 BMG ban. This prohibition was removed long before the ban passed, but plenty of folks who heard about the ban while it was being proposed, never found out that the enacted version of the ban did not include this prohibition. The original draft of the bill considered each individual 50 BMG round as a separate felony count.
All weapons in 50 caliber are banned: AB50 calls out the specific dimensions of the 50 BMG round. It does not affect any cartridge other than 50 BMG whatsoever. This has a granule of truth relating to sales only in Los Angeles, but nowhere else I've been made aware of.
Weapons Never Touched by the BMG Ban
A lot of folks seem to think that the weapons below are prohibited in California, but they aren't.
1911s chambered in 50GI: 50 GI is basically just a slightly upsized 45ACP round. There is no similarity to the BMG, and the only reason handguns may be difficult to find in 50GI is California's drop-test list.
AR-15s chambered in 50 Beowulf: The Beowulf is an entirely different cartridge for an entirely different purpose, than the BMG. There are no further restrictions on a Beowulf-based AR-15 than on any other kind of AR-15.
Magnum Research Desert Eagle: The Desert Eagle's 50cal barrel is chambered in 50 Action Express, and is not subject to the BMG ban. It is, however, subject to the California drop-test list. It is only CA-approved for new sale when equipped with the 44 Magnum barrel. No law whatsoever prohibits the installation of the 50 Action Express barrel onto a Desert Eagle originally sold as a 44 Magnum.
Smith & Wesson 500: This case is reminiscent of the 50 Beowulf, but it's commonly encountered in a revolver form factor. It has no relation to the 50 BMG, and is entirely California-legal.
Special Workarounds for the BMG Ban
The 50 BMG ban applies to rifles only. Rifles are statutorily defined as being shoulder-fired. An M2 ("Ma Deuce") or other tripod-based weapon is not designed to be fired from the shoulder, and are thus not prohibited from being chambered in 50 BMG. Using something like the A6 stock for the 1919, however, would turn the M2 into a rifle, and thus illegal. The author desires, at some point, to possess a BMG single-shot weapon designed like an old-style two-wheeled cannon.
Additionally, alternate calibers have been developed which have performance similar to or greater than the 50 BMG.
408 Cheytac: A very expensive round which is fired primarily through the Chey-Tac Intervention Rifle (which is based on the EDM Arms Windrunner platform).
416 Barrett: A pricy caliber which burns up barrels faster due to its higher twist rate, this is a flatter-shooting round with more kinetic energy than the 50 BMG.
460 Steyr: This is another competing round in the BMG energy range, which I don't have much info on.
50 DTC-EDM / 510 DTC Europ: Check the DTC Article for full details on this round and its history. This round is functionally the same thing as 50 BMG, using the same components (with some work on the brass) and comparable if not identical load data.