I just put a CST 6-8 lift on my dually and am now going to get some 315 75r 16's to put on my American Racing D-8's.

My question is about weight ratings. It says on the BFG's I am looking at that they handle 3150 lbs. Does this mean that 1 tire can handle 3150 safely and does this mean with 4 in the back (dually) I can handle 12600 lbs? total rear weight? Is this a burst rating or what?

That means they can haul that load safely at maximum pressure. So yes with a dually that would be X4. This is not a burst rating, just a safe rating. Burst rating could be 10 times that for all we know.

You might want to factor in the rating of all the other individual components also (rims, springs, axle rating, BRAKES, etc.). This would get you closer to a true rating.

Usually dually ratings are lower than single ratings.

The tires in a dual set-up should Never touch. If they rub, even a little, the sidewall temp will raise and could cause failure, usually at speed.

For what it's worth.

Idahofox

Are you sure about the size? 315 75r16 sounds awful wide to put on a dually. IIRC most dually owners stick with 215 or 235 size tires, because of the rubbing issue that idahofox mentioned. About the weight rating, Rocklin is correct that dual applications have a lower weight rating than singles. And of course you "should" stay within the limits of GVWR and GAWR.
Cheers!