When you bore scoped it, do you check each groove all the way down the barrel carefully?  If not, I might suggest doing so.  Specifically look for any possible deformities in a groove, it could be very small where a possible bullet impacted another from something like a squib round left behind and a new round fired behind and you will see the impact sight.  I have also seen a barrel that had about a 1" long groove that was deformed where a jacket on the bullet had separated, and the next round fired swaged out the groove in a small area which resulted in the gun shooting 10" groups due to the bullets getting deformed.  I have also seen a few barrels that had thin rifling on one side of the barrel and deep on the other, which we attribute to either the preparation wash before parkerizing being left in the barrel too long when it sat horizontal and ate the barrel away.  

When teaching Armorer courses, we see all sorts of things.  A few times I have seen barrels that shoot terrible groups as you describe.  I don't see a mention as to what distance you are checking groups, but I would recommend doing it at 25yds and see what results you get.

The other option is to contact the barrel maker and put it back into their hands for any possible warranty replacement.


CY6
Greg Sullivan "Sully"
SLR15 Rifles
DefensiveEdge.com
(763) 712-0123

Barrel harmonic, and with float tube system, barrel has to free float, and to the level that barrel movement during the shot, will not have th barrel or gas block touching anything during flex in the barrel.

From here, double check to make sure that barrel nut has be mated to receiver socket treads, and barrel nut is tighened to plus 35ftlbs, but do not exceed 80ftlbs to index barrel nut channel to upper receiver gas tube channel, center line of barrel extension center of lugs are indexed on center line of gas tube channel/center line of lower take down lug,  and when gas tube is installed, remove bolt from carrier, use just carrier in upper to check gas tube to carrier key alignment, and gas tube gets tweaked over center of barrel to align it to the key.

Last one, brand new barrel, use CLP with chamber and bore brushes to remove protective grease in bore, then run dry patched to remove that mess in the end.  Also as pointed out, use jig with dry brush in the end, to double check the barrel bore rifling for uniformity in rifling it self.  As for installing FS or muzzle brake in the end, do not over torque the device, since if over torqued, will cause distorting on the last inch of bore.

Simply, barrel that is spraying, either has rifling that is over sized and not spinning the bullet enough to stabilize it (sideways strikes on the bullet on target), or barrel contact in float tube, or bent barrel in some form such as last inch or bore, that will cause the POI to shift as the barrel changes temps.  
If you have a lathe, then easy to chuck barrel extension in chuck, use steady rest at gas block, and break out dial indicator to see if barrel is straight, or bent at any point.  Also give you a chance to check FS threads against bore itself, since could be bore is not tram to Treads, and any device added to to barrel, maybe be enough to allow bullets to scrap inside of Device on way through.