What is the effective range for a 12 guage question.  I want facts not just opinions.

I could probably throw a Mossberg about 50 to 75 yards, depends on if I get a good bounce and wind conditions, of course.  

ARE WE TALKING BUCK SHOTS OR SLUGS?

Well, I just spent three days in training with these guys: http://www.morrigan-consulting.com to learn that and much more about the shotgun.

The short answer is: depends on your target, barrel, choice of load, and choke (if any).

For defensive use, I found that 00 (Usually called 'double-aught buck') buckshot is *generally*, although not always, effective to about 15-20 yards.  Beyond that, the pattern opens up such that most pellets are not hitting in a human torso.  In our class, my shotgun (rem 870 Police Magnum with a cylinder bore and 18.5" bbl) would pattern all the pellets onto a 11X17" piece of paper out to 15-20 yards depending on brand of buckshot used.  Beyond that, I had to go slugs in order to keep from hitting non-targets; the pattern of the buckshot would be wider than a torso and this creates a severe hazard for other non-targets downrange!

Your basic 1oz slug fired from a shotgun with sights is fairly easy to put in a torso-sized target in under 2s out to about 50 yards.  Good shooters can extend this to 100 yards.  Some specialty slugs in rifled barrels can go a little over 200 yards.  Guys in our class that had only bead sights had trouble getting them on the paper reliably at 50 yards.

#4 buckshot is lighter and will not reach as far as 00 shot, so its range is a little less.

Your best bet is to go get a B-27 silhouette target and pattern your shotgun with your load and your barrel.  Shoot at 5, 7.5, 15, 20, and 25 yards.  At either some distance, usually 15 to 20, your group will 'Doughnut' and some pellets will be outside the target zone.  This is the limit of usability of this load in your gun.

Hope this helps!

What shotgun, load, and targets do you have in mind?

Accurate to about 100yards with rifled slugs in smooth bore and improved cylinder choke 26" barrel.  Brenekes (1 1/8oz) shoot very well in my Win1300.  I wouldn't hesitate shooting a deer at 100yrds.

You can shoot much farther with rifled bore and sabot slugs.

Bird shot is packs a punch out to 30yards or so.

Shok

Loaded with what and used for what purpose?

I group about 8 inches with rifled slugs through a smoothbore with Improved Cylinder choke and rifle sights, and that's from standing, off-hand with hasty sling.

I am mainly talking about buck shot but would like the info for every thing.  Also if you have the info for 4-10 and 20 gauge.  Thanks

410 and 20?  If you're looking at a light recoil defense gun go for featherlites in a 12 less recoil and probably effective at 10-15 yards when placed appropriately.  410 or 20 slugs if they make them might be ok for 30-50 yds depending on what the target is wearing and placement.

You're getting in the territory a pistol caliber carbine of some kind is better for defense.  Equivalent recoil and better effectiveness, easier training.

If you're talking about pest control 20-30 yds for 410s and 20s with excellent placement on really small stuff.  Cats sized and below, otherwise your getting in the not enough pellets in the pattern to do the job area.

As a follow-up, do be aware that the pellets from buckshot can be fatal out past 100 yards, but ideally, they are for use from about 0-15 yards because beyond that, not all of the pellets will be in your average target (assuming deer, hog, or human size target).  Most Foster slugs are good to 100 yards or so.

I was wondering because a co-worker of mine said she went hunting with a 12 guage for deer.  And she said it was good for 100 yards.  I wanted to prove her wrong. Thanks

Sorry you won't prove her wrong. A 12ga is plenty good to 100. With the right gun/load I would say 150 wouldn't be out of the question.

With slugs 100 yards is easy. 150- 175 is very possible. With Buckshot not nearly that far.

I've killed white tail deer out to 120 yards with slugs.

I sight my 12 bore in dead on at 75 yds.
It's an Ithaca Deer Slayer (smooth bore barrel, improved cylinder full length.)and shoots the standard Forster style slug (Remington.)
I've taken deer at a measured 97 yds. with a bit of holdover.
Based on limited experience with a borowed gun with a rifled barrel, and using slugs designed for them, I would sight in at 100 yds., and shoot out to 125-150 yds after some practice for them.
I keep thinking about trading up to a rifled barrel model, but I don't hunt  as much as I once did, and have never been in a situation where I didn't take a shot based on the slight edge gained from from the rifling.Where I hunt there is no real gain.
But if I were a new hunter, just gathering my own gear together, I would readily pay for the 25 + yd. advantage.
While I prefer using a rifle, I do admit that for Upstate N.Y. a shotgun is not necessarily a bad choice for deer.