California has strict gun laws and I doubt that you can legally own a 20MM rifle in California. Usually the only guns available in California are small handguns.
Smaller than 20mm.
Anzio Iron Works 20mm
About 3,396 ft/s
Yes, but there ARE restrictions on WHICH rifles, as well as which features are permissible.
18 for a rifle or shotgun, 21 for a handgun.
For a semiautomatic rifle, you must be 18. For a TRUE assault rifle, which is capable of burst or full auto fire, you must be 21. Federal law.
Not with that weapon, but a rifle during basic training yes
mark a pt 20mm above xy planemark 20mm below
weight of ms hollow bar 20mm*20mm
the best is sheridan in 20mm. pumpair rifle Best is a very subjective term that depends on your intended use, pocketbook and skill set.
If both pieces have the correct 20mm weaver rail, yes it will mount fine.
The only thing I've found in the California regs on this is that they must be taken with a centrefire rifle using expanding or soft nose projectiles. So, theoretically, you could. However, you can't own an M4 in California... the only way you can even own a semi auto clone is if it either uses the "bullet button", or is entirely featureless for a detached magazine rifle (no pistol grip, flash suppressor, collapsible buttstock, etc.) AND is built on an off-list lower.