I'm a long time reader first time poster on this sight.  I've recently finished my first SBR build and received my stamp a few weeks back.  I'm experienced with the AR platform but this is my first 10.5" rifle.  I've never failed to zero a rifle within 15 rds, or qualify with at least a 35/40 (Army and LEO).  Today was my first attempt at zeroing my SBR.  My groups were all over the place with the closest group being maybe 1.5" and the furthest maybe 3".  I run Magpul BUIS gen 2 front and rear, and a Vortex Sparc optic.  The rear Magpul and Vortex were on my old patrol rifle and worked perfectly.  The front Magpul was new, so I thought maybe it was faulty...Shot about 30 rds, failed to zero, went and bought a Troy Fixed Front sight, shot another 30 rds and failed again.  So then I think maybe the cheap PMC 5.56 55gr FMJ BT ammo is the culprit.  Try some Federal and Hornady TAP with the same result.  Tried to zero the optic with a similar result.  So now I'm thinking that it's maybe the upper...which I bought new from Radical Firearms...10.5" 5.56 w/ YHM Diamond Free Float.  I also added a PWS CQB compensator to the front.  What do I need to do to make this SBR group?  My buddy is currently checking the rail and barrel nut to make sure that they are tightened to spec, but I'm stumped?  Suggestions?

It's your muzzle device. Don't remove it. Just simply shoot it without the cone in.  If it's the prob - you'll know in ten rounds or so.

The pig breaks and other krink type compensators are bad about this.

check the muzzle device for proper torque and also make sure the bullets aren't contacting it on the way out, and if it checks out, take a good hard look at the crown. a tiny flaw in the crown can totally mess up your groups.

That's one thing I didn't try today...I wouldn't think that the muzzle device would affect it that bad.  I guess I'll make another trip to the range on Friday to try again...

I'll probably take the muzzle device off and re-install it.  When I got the upper, I bought the PWS CQB from a co-worker who hadn't even installed it.  I did not have a strap wrench so I took it to a local gunsmith to install.  He did not install it with the crush washer, saying "these things don't need the crush washer since the timing of the device isn't important"...I put 90 rds through it this way, the first 90 rds through the upper, making sure the gun didn't have any issues but I didn't zero the upper at the time due to me sending it back to Radical to fix another issue.  When I got the upper back, I had the PWS re-installed at another place, and they used the crush washer this time...I really hope it's not the muzzle device, it does a great job of keeping the flash down and redirecting the noise forward...

At what ranges are you shooting?

Standard 300m silhouette at 25m...

EMAN44,

This happened to me when I first got mine and it turned out my bore was heavily fouled with copper! It was so bad that it took a weekend to clean most of the copper out with solvent. Those patches came out so green it was like my lawn. Try that ans see what happens. My favorite bore solvent is Birchwood Casey's Gun Scrubber.

Impala

My buddy got back to me today...barrel nut and free float are tight, no signs of any nicks or bullet strikes on the inside of the muzzle device...didn't take the muzzle device off though, so I don't know if there are any defects on the crown of the barrel...As for the copper on the inside of the bore, would or could this be present on a brand new melonited barrel??  The round count on the upper is at 260 rounds right now.

I'm gonna take it back to the range tomorrow morning and use a heavier grain round and take the front of the muzzle device off to see if that helps...

Your barrel nut is tight so that's not it.

Your bullets are hitting the compensator. Remove it and try again. Most muzzle breaks only allow .030" clearance for the bullet to pass. That means a build up of carbon can cause contact. It also means that if the threads aren't cut perfectly centered to the bore there is a high probability that the bullet will make contact even when clean.

Iron sights, unless they are loose would not be the problem. The SPARC has a good reputation for an economy red dot sight. That doesn't mean it can't be bad, but your irons would still work great.

Because you are shooting at such short range, your bullets should be stacking on top of each other.

Not the muzzle device either  Shot without the tip of the PWS on, then took the device off completely and shot with the threads exposed (awesome show btw lol).  The tip of the barrel looks fine as well, and the barrel itself after firing 260 rds through it so far looks fine.  I also tried a 62 grain rd today and that didn't help.  Even let one of my partners try to put a group together and that didn't work.  I contacted Radical back to see if they are going to do anything about it...