You gotta be careful with a crew served/belt fed type of build.
Prior to 2020, such builds did not fall under California's "Assault Weapon" statute because they did not fall under the definitions of "Rifle, Pistol, or Shotgun" as those terms were used in the statute. But in 2020, as a result of SB118, a new class of "Assault Weapons" was created which includes centerfire weapons that are not rifles, pistols, or shotguns and which include prohibited features. Here's the new class:
"A semiautomatic centerfire firearm that is not a rifle, pistol, or shotgun, that does not have a fixed magazine, but that has any one of the following:
(A) A pistol grip that protrudes conspicuously beneath the action of the weapon.
(B) A thumbhole stock.
(C) A folding or telescoping stock.
(D) A grenade launcher or flare launcher.
(E) A flash suppressor.
(F) A forward pistol grip.
(G) A threaded barrel, capable of accepting a flash suppressor, forward handgrip, or silencer.
(H) A second handgrip.
(I) A shroud that is attached to, or partially or completely encircles, the barrel that allows the bearer to fire the weapon without burning the bearer’s hand, except a slide that encloses the barrel.
(J) The capacity to accept a detachable magazine at some location outside of the pistol grip.
(10) A semiautomatic centerfire firearm that is not a rifle, pistol, or shotgun, that has a fixed magazine with the capacity to accept more than 10 rounds.
(11) A semiautomatic centerfire firearm that is not a rifle, pistol, or shotgun, that has an overall length of less than 30 inches."
I can't think of a potentially feasible crew served weapon build that would not incorporate at least one of the above evil features.
Belt fed designs are highly problematic under both California's "Large Capacity Magazine" statute and the "Assault Weapon" statute. It's clear that the authors of the statute failed to consider belt-feeds when writing the statutes, but here's the result of what they wrote:
1) A "Large Capacity Magazine" is defined as one that requires disassembly of the action in order to REMOVE the magazine (the use of the word remove is quite notable in its distinction from the INSERTION of the magazine). The law also clearly defines the word "Magazine" in a way that includes ammunition belts (refer to Penal Code section 16740). A lot of folks have suggest that the law could be satisfied by altering the weapon such that the action had to be disassembled in order to load a belt, but that doesn't work. So long as the belt can be removed without disassembly, the law is violated. With a common "disintegrating belt" system, the belt is removed from the weapon by firing all of the rounds.
2) A "Large Capacity Magazine" is not one that holds more than 10 rounds. A "Large Capacity Magazine" is one that with the capacity to hold more than ten rounds. Those extra three words make a big difference. One could comply with the statute by using a fabric belt holding only ten rounds, but such belts have pretty much gone by the wayside in favor of disintegrating link belts. But if one were to make a 10 round disintegrating link belt, it would still have "the capacity" to hold more than 10 rounds simply by adding more links to the belt.
Note - As with my prior posting, I'm writing from my enforcement experience with California State statutes. I'm not well-versed in the NFA and will leave commentary regarding the NFA to folks with more experience in its content.