Own a SAM site for home defense, since that's what the founding fathers intended. Four A6 Intruders break into my airspace. "What the devil?" As I grab my powdered wig and S-300. Blow a golf ball sized holes through the first intruder, he's dead on the spot. Draw my MANPADS on the second intruder, miss him entirely because he uses flares and nails the neighbors Mi-8 Hip. I have to resort to the S-200 mounted in the backyard loaded with grape shot, "Tally ho lads" the grape shot shreds two intruders in the blast, the sound and extra shrapnel set off car alarms. Fix ZSU-2-23 and charge the last terrified rapscallion. He loses fuel waiting on the F5Fs to arrive since multiple 23mm wounds are impossible to stitch up. Just as the founding fathers intended.
You can if it's made before whatever arbitrary cutoff date would make it c&r weapon. You can also aquire one if you have a NFA class III license but I don't think there are any sellers in that space. From my understanding when I last looked. I don't think they commonly enter the civilian market.
shilka is full auto, is above certain cutoff (1/2"?) that classifies weapons as "destructive device", and each round is actually explosive, which is also tightly restricted