Depends on what you are shooting in the 12 gauge. Birdshot is effective to about 40 yds, but can travel about 500. Buckshot (larger pellets) is effective to about 50 yds, but travels about 800. Slugs (one big piece of lead) is effective to about 100-150 yds, but can travel about 1500 yds.
a 12 gauge shotgun can shoot about 30 yards accuracy
No, you can not shoot anything but 12 gauge shotgun shells through a 12 gauge shotgun. Never use any ammunition other than what the firearm was designed for.
Depends on the ammunition. Rifled 12 gauge slugs are accurate to about 150 yards
yes
It starts to drop the instant it leaves the barrel
A 12 gauge shotgun cannot fire 20 gauge shells.
12 gauge by far
Depends on your parents
Yes.
what do you normally shoot out of it
Yes
Yes. Be careful if it is choked full.