I'm not a lawyer, and you'd be best served in contacting one.
However, I see no reason what so ever that the caliber of the FAB-10 system when assembled into a rifle is important in any way.
Just make sure you keep the same mode of operation so as to not go outside the FAB-10's exemption.
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The following is not to do with answering your question:
All of that said, can anyone show me a reason why a perfectly conventional AR that is just missing the mag-release BUTTON is not the same as a FAB-10?
12276.1 (a) Notwithstanding Section 12276, "assault weapon" shall also mean any of the following:
(1) A semiautomatic, centerfire rifle that has the capacity to accept a detachable magazine and 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.
(2) A semiautomatic, centerfire rifle that has a fixed magazine with the capacity to accept more than 10 rounds.
978.20 Definitions
The following definitions apply to terms used in the identification of assault weapons pursuant to Penal Code section 12276.1:
_
(a)
"detachable magazine" means any ammunition feeding device that can be removed readily from the firearm with neither disassembly of the firearm action nor use of a tool being required. A bullet or ammunition cartridge is considered a tool. Ammunition feeding device includes any belted or linked ammunition, but does not include clips, en bloc clips, or stripper clips that load cartridges into the magazine._
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(b)
"flash suppressor" means any device designed, intended, or that functions to perceptibly reduce or redirect muzzle flash from the shooter's field of vision. _
_
(c)
"forward pistol grip" means a grip that allows for a pistol style grasp forward of the trigger._
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(d)
"pistol grip that protrudes conspicuously beneath the action of the weapon" means a grip that allows for a pistol style grasp in which the web of the trigger hand (between the thumb and index finger) can be placed below the top of the exposed portion of the trigger while firing._
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(e)
"thumbhole stock" means a stock with a hole that allows the thumb of the trigger hand to penetrate into or through the stock while firing.
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NOTE:_
Authority cited: Section 12276.5(i), Penal Code. Reference: Sections 12276.1, 12280, and 12285, Penal Code.
So, then would an AR-15 with just a spring loaded mag-catch mechanism, BUT NO MAG RELEASE BUTTON (that would require you to drop the mag by pressing in with a punch or a round of ammunition, but would still allow you to quickly ATTACH a mag) be a legal long-gun?
How come this is not done?