As others have noted, CA is extremely hostile towards AR15s and the like. One person uses one of those ugly neutered ARs in a mass shooting, and they be banned or have more restrictions put on them. Not worth the trouble and I personally wouldn't be interested in owning a neutered AR.
I wouldn't go M1 Garand either. Ammo is more expensive, not as readily available, and not ideal for home defense. I'd rather go with a Ruger Mini 14 and keep several loaded magazines on hand before going M1. With a Mini 14, your firearm would be covered under Ruger's excellent customer service, spare parts would be more readily available, and in a SHTF situation, ammo would be easy to find. The Mini 14 accepts factory magazines with capacity ranging from 5-30 rounds of 2.23/5.56..
Just for kicks... The Kel Tec KSG is 100% California legal, holds 14 rounds + 1 rounds of 12 guage, is extremely compact, but still has a 18.5" barrel. With its compact size and higher capacity, it's perfect for home defense and close quarter combat scenarios. 12 guage ammo is easy to find even during panic buying ammo shortages and is perfect for home defense and SHTF situations. There are a wide range of 12 guage ammo to choose from, and if need be, you could hunt and take down game with it whether it be birds, deer, etc. The KSG can be had for around $600-$650.
On top is a 18.5" barrel, 6+1 capacity Remington 870 with a pistol grip; A 14" barrel, 5+1 capacity Mossberg Shockwave with a bird grip; A 18.5" barrel, 14+1 capacity, Kel-Tec KSG with a full shoulder stock. (There is also the S&W M&P12 and IWI Tavor TS12)