I doubt anyone is going to have much experience with the 7.62x39 version of the Sig 556, because I dont think its out yet.
One big draw for me is that it is suppose to take standard AK mags! Its on my 'look into' list, thats for sure!
Well somebody better tell Gunbroker.com and their merchants to quit selling yet even more potential vaporware then!
I'd certainly like to see them start popping up on gunbroker. The x39 version is out but I have yet to see the x51.
They'll have to take off that auto switch for me to own it too
Long range shooting/fun/pinking for now. Part of my SHTF defense for later.
I can't speak for the long range part for the Sig 556R but Im very sure you will have alot of fun shooting/pinking and alot of it for the price of the rounds. I also have no doubts that the 7.62x39 will hold it own in SHTF moments giving that you have trained yourself how to properly use the firearm. So that means lots of muscle memory and practice making perfect.
I don't know your idea of long range, but I have confidence in the 7.62x39 out too 300/400yards, but thats not from experience with this rifle thats from my own personal experience with the SKS and AK47. The AK has a 1/9.45 twist rate. Effective to 400 meter or 440 yards. Your Sig556R has a 1/9.5 twist and a better quality barrel then the thin AK47 barrel. So I can only speculate that the Sig barrel with not have the down fall of the "barrel flex" that most if not all AKs have, witch is a main factor in the inaccuracy of the AK47 at longer ranges.
If your not aware of the "barrel flex" problem search it on youtube. You will find a few videos on slow motion that will allow you to see it.
If you want longer ranges you will need the faster more stable round such as the (7.62 nato/.308WIN)
7.62 x 39mm is med power 30 cal round and not generally known for long range capability or accuracy. NO where close to a .308/7.62 x 51.
Even a high dollar Sig is not going to turn this cartridge into a long range, nail driving, power house, that was never the intent of the round.
I don't have any experience with the 556R, but you might find some good information here:
SIG 556R - M4Carbine.net Forums