Cal. Penal Code Sec. 12020(a) prohibits the possession of, among other things, AOW’s (Any other Weapons) and short shotguns and short rifles. Subsection (b) lists exemptions to the application of (a).

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Gunsmith Kit for Add-A-Rail System
Rail Gunsmithing Kit

Contains:
10 Rails
15 Well Nuts
15 Well Nut Small Bushings
15 Well Nut Large Bushings
15 Socket Cap Screws
15 Button Head Cap Screws
12 Flat Head Filler Nuts
15 Large Curved Washers
15 Small Curved Washers
1 Hex Wrench
1 Drill Bit
1 Spanner Wrench
1 Instruction Manual