TC,

What you are doing, your research is what you need to do before jumping in the the black rifle world. I officially have the affliction, having quite a few, with the heart of mine being a switch barrel (not upper) affair with calibers in 22LR. 556, 6x45, 458 Socom and several 556 barrels.

As has been noted, picking the right twist in the caliber you are pondering is very important. The other two items that are very important are the magazines and the gas system. Some of the variants use propritory magazines, some standard AR mags. If you are thinking of building from parts, then you will need to get educated on the gas system and buffer system. If you plan on buying a complete upper or gun, the maker should have the gas system nailed down.

Generally I reccomend to folks to start with the 556 to get their feet wet and understanding going before jumping into the more "exotic" calibers. But as you have hog on the brain, that might not be the best scenario for you. If you do decide to go 556, you will probably want to shoot heavier bullets designed for hunting, and will need to really watch your barrel twist. The 30 AAC and Whisper are slightly different, with the Wisper being able to be shot in the 30 chamber, but not vice versa. It seems with the blossoming of the 30 right now, it might be better to do that rather than the Whisper.

I almost pulled the trigger on a 30 earlier this summer, but money is tight right now, as it sounds for you. I am guessing you have found ar15.com. Get your pad of paper and pencil out, and start with the notes. If your head is not swimming by now, it will be soon enough ;^)

Craig