I really want to get a SBR in 7.62x39.  Something not an AK.  Perfect choice would be the sig 556xiR pistol variant and then SBR the thing.
I've been trying to do a lot of research on them in 7.62 but it seems every review/video shows them having nothing but issues. Most of the videos are older reviews.
I continue to buy Sig pistols but it seems their rifles are hit or miss which is why I never owned a rifle caliber from Sig.
What is the latest word on them?  Good to go or stay away?

I would assume the RRA would be able to take any standard forearm, the Mutant who knows.  The Mutant is listed on their website at $1600 + barrel work + foream replacement.  Just takes it up beyond what I can get a factory 556xiR pistol for and just SBR it.  No barrel work required.  
Not really digin' the RRA.

Did you purchase it as a rifle or pistol?  Looking to order very soon.

Well I bit the bullet and just decided to go ahead and order one.  Some people were telling me that the rifles were the only ones that were problematic.  Pistols shouldn't have the gas port issue.  In any event I contacted the shop to get a born on date and the one I ordered was made at the end of January 2015.  If they did have problems, I'm thinking Sig would have them all fixed for production in 2015 so I'm hoping to get a good one.
Once I get it and run with it, I'll report back.  If all is well, going to form1 it right away.

jwise got back to me and his was a rifle that he had the barrel cut down.  from what I'm hearing sig just didn't put the right size gas port on the rifles.  guess they are all fixed now.  pistols i don't think had the issue. I am hoping to have my pistol no later than Tuesday of next week and will head to the range to test it out.  If all goes well, I'm going to form1 it right away.
Also I posted the same question over at the akfiles and people that have them love them.  if they got early rifles and sent them back to sig to correct, they all work fine now.

When was it made and when did you get it?  When did you send it back to sig last?  What is it doing, short stroking?

That sucks.  All I plan on using is Wolf so if I have problems it is going back as well.  I did ask for a build date before I purchased and supposedly it is 1/31/15.  I won't be arriving until next week so I'll report back.  I know people have stated that sig wants you to use that mill com 25 grease in the rifle.  did you try using that?

Got mine today, pistol version.  Hope to hit the range.  I cleaned all that grease out lubed with mobil 1 oil.  It is what I run on all my rifles. I also cycled the bolt 100 times for the hell of it.  Gun was made on 1/31/15.  I will report back after my range session.

Range report:

Cleaned all the grease out.  Ran Mobil 1 oil.  No oil on the gas piston or spring.
200 rounds of Wolf
1 tapco 30 round mag that came with it and 3 20 round hungarian steel mags I took.  Other metal mags were really tight.

Ran flawless up until about round number 120 on a full 30 round tapco mag where it failed to pick up the second round in the mag.  Don't know if it was a mag issue or what.  No other problems finishing up the rest of the 200 rounds.
Spent casings were coming out from a dribble to flying out at 2 or 3 oclock and various heights.  Nothing seemed to be consistent.  Aluminum handguard got very warm but I was shooting from a rest and wearing gloves.  I didn't try any other gas settings.
At 25 yards I blew the center out of targets with iron sights.  I'm more a red dot person but didn't buy anything yet.

Only negative thing is there is a little wobble between the upper and lower and the aluminum handguard isn't rock solid. That seems to slide a hair as maybe the hole for the front pin in the handguard is larger than needed.  Gas tube also has some movement in it but I think from what others have said, that is normal.

Next thing is to Form1 it and get a side folder stock.

ETA:  The extractor was caked under the lip with that red coating they use on wolf ammo.  Maybe that is why I was getting such variations with flying casings.