Built an A2 upper with a 1:9 HBAR bbl on an olympic arms upper, darn thing shoots to the right, way right. I had to put almost ALL left windage on it to center up at 100 yards, problem was not so bad at 25 yds. Ammo tested was mil spec M193 TZZ from mid 90s.
As far as I can tell the FSB is not canted at all. I also check concentricity of the sight post, which is a square NM style post that came with the bbl, and it seems fine too.
Your last point (over torquing bbl nut) indicating a distorted upper seems potentially the most probably, that would explain rounds printing down to the right.
Without armorers gauges how would I confirm that? How much torque should be applied when installing a bbl on an upper?
I installed the bbl nut hand tight then torqued (free hand) to get the gas tube slot to align with a notch in the bbl nut - not muscleman tight but who knows what the previous owner of this upper did...
Thanks for the fast reply
Don't take this as criticism, but is this the first pistol grip rifle you've fired? Its possible you may just be pulling it.
Well I had considered that, but if I were pulling it I doubt I would be yanking it 12" off target in prone position...
It doesn't take all that much torque to crank the barrel over.
If you hold only the receiver, the flange on the barrel takes torque from the nut and transmits it through the pin to the receiver, SLIGHTLY distorting it.
If you hold only the barrel, torque from thread friction forces the receiver in the opposite direction. What is someone to do?
First of all, take it apart. Grease it up with the correct moly grease. Graphite and aluminum are a bad combo, the graphite attacks aluminum. Moly only.
Now assemble it hand tight. Center the rear sight and boresight it, making adjustments in windage turning the barrel to line up.
Now clamp the receiver in blocks. Get a drill press vise and use this to hold the barrel in blocks. I used hardwood blocks I made myself, coated with pitch rosin for extra grip. Once the barrel is locked from turning, clamp the drill press vise to the bench. I use Bessy Clamps but anything, including lag bolting, works.
Now go to town on the barrel nut to get alignment of the gas tube. Double check boresighting, you should be on the money.
Complex? Sure. Effective? You bet. Mine are within 2 MOA on windage, 4 clicks on the standard sight.
This method is also applicable to Savage 110s, only the issue is headspace. The barrel nut locks the barrel to the action, torquing it leads to decreased head space and can damage the chamber if done with a headspace gauge in it.
BTW, I am thinking of getting into this as a business. Send me the UPPER alone, plus $15 + shipping/handling/insurance and I will assemble within 2 MOA. Even test it, if desired and send the target in the return shipment.
Let me know. I am in San Antonio.