I built my A2 at the end of 14. It was a very solid rifle. Threw my 55gr fmj loaded over 25gr of Win748 very respectably. Then about 3 months ago it started having windage issues.when initially zeroed, it was 6L of mechanical zero.
Then i started experiencing windage drifting. So bad that i would nearly run out of sight movement to the left. Then it would be moving the other way. Or i would add 8 clicks and poi would not change, then all of the sudden whammo big change.
Not really elevation, but always windage issues.
Everyone chimed in. Folks blamed the sight. I could clearly see the sight move when i dialed. They pointed at ammo. This phenomenon happened even when using Hornady 68gr match loaded over 25.8gr of 748.
After blaming everything, even my glasses (really had me questioning myself). Last night i took off the handguards and while holding both sights, i could twist the barrel. The index pin would bob back and fourth in the upper index slot.
I have never been so happy! Some how in 7-800 rounds the ole girl settled in and was finger loose.
So after a few minutes of open heart surgery, she is now at 40 foot pounds.
Looking at my notes, the last zero i had was 10 L. So to try to get my zero as close to mechanical zero as possible, i shifted my barrel to the rightside of the index slot.
I will rezero next saturday. Next job will be installing a DCM freefloat tube. Really not looking forward to messing with that front sight.

Glad to hear you found the culprit.

I had a similar thing happen to me on a 9mm carbine, but it was instantly apparent what happened because the handguards rotated with the loose barrel nut.

There is a certain amount of movement expected. So i just passed it off as normal. But when i took the handguards off it was pretty obvious.

Ok so i am getting this rifle ready for Camp Perry 2018. I plan to add a DCM free float tube to it.
The official bullet of the Match is the Hornady 68gr match. So i am thinking of swappingvout my A2 upper reciever for a flat top that i can mount a scope on. So basically i am using my barrel and bolt with a scope for load development.
Then once the load is dialed in for that barrel, go back to A2 iron sights.
Any of you guys ever do this?.

"Official bullet of the Match"

What's your classification, or what class do your scores match? You might be better off using "standard" loads and practicing, like Konrad Powers.

An optic is helpful in diagnosing wobble and reducing it, which should transpose back to irons.

I was told by the DCM office that the 68gr Hornady is the most popular bullet used in SR matches today. When i spoke with Hornady they confirmed this and provided load data which was 25.8gr of 748. That just might be the magic load but not a gurantee from my 1/9 tube. With out accuracy testing, how would i know if my best performance takes place at 25.5 or something else? So picking some other load and never checking it, kinda seems like throwing money away.
Do not be confused, the scope is only to settle in on a specific load. Once i find a match for this bullet and barrel, the scope goes away.

I would've bet on the 75gr Hornady or 77gr Sierra being most popular.

1+ MOA in position refinement is easier than 0.25 MOA in mechanical refinement.

Eta: were you getting a detachable carry handle or were you going to put the barrel back in the A2 upper after load testing?

Goes back to A2.

Torque everything to the same value as on the flattop.

I am using 40#s now. I also plan to introduce the free fliat tube at that time.