Yeah, the question would be what is best.
The Antifa types usually make cartoon shields with garbage cans. Although an SS109 would easily penetrate such childish armor, it will probably deflect and be less effective. 55 grainers might blow up. So while an M4 or a Tavor or AUG would be good because of its light weight (Pro Tip: make sure your gas key is properly staked and use a weapon with a 1/9 twist to better stabilize the bullets), I would recommend a 7.62x51 like a Congo style FAL or an M1A equipped with an Eotech or other modern, 0 magnification optic.
For those of you on a budget or in a fascist leaning jurisdiction, a nice lever action in any pistol cartridge would be OK.
To reduce the risk of overpenetration, I would see if Hornady makes a TAP round in your chosen cartridge.
Remember, in order to be legal, you are going to have to absorb some punishment. Make sure to check your local laws. For example, in NY, you can use deadly physical force to stop an arson or an attempted arson. Google your local laws as to when you can use deadly physical force.