Have you loaded the rifle using the charging handle and cycling the round into the chamber. Fire one round where a round is fed into the chamber. Extract that round to inspect for a change in cartridge overall length or damage to the bullets.
If the cartridge overall length is changing this could affect each rounds pressure, different pressure equals different velocity. At distances of 600 yards this could contribute to stringing. Are the bullets contacting the front top edge of the magazine during the feeding cycle? What barrel length, gas system, gas block (fixed vs adjustable) buffer weight and spring are you using? Are the bullets being marred by contacting the barrel extension?
Have you measured the first round from the mag for overall length, loaded the round then extract the round to check for change in cartridge overall length? The bullets/cartridge overall length in the magazine could be moving during recoil and through the feeding process changing the cartridge overall length. Out of curiosity what magazines are you using? Have you noticed this same stringing with different type of bullet or different reload or factory ammo.
Have you verified there are no clearance issues between the gas block and hand guard. Is there a muzzle device on the barrel?
I’d also want to make sure the barrel nut is tight and the front of the upper receiver is lapped. Have you verified the scope is torqued to correct specs and the mount is secure.
This is where I would start. If I assembled the upper I’d know whether the barrel nut was tight and the receiver lapped. I know Alexander Arms used to use Loctite on barrel nuts in the past but if someone else assembled the upper it is worth checking. It appears something is changing during recoil, you just have to isolate what is changing.
If the cartridge overall length is changing this could affect each rounds pressure, different pressure equals different velocity. At distances of 600 yards this could contribute to stringing. Are the bullets contacting the front top edge of the magazine during the feeding cycle? What barrel length, gas system, gas block (fixed vs adjustable) buffer weight and spring are you using? Are the bullets being marred by contacting the barrel extension?
Have you measured the first round from the mag for overall length, loaded the round then extract the round to check for change in cartridge overall length? The bullets/cartridge overall length in the magazine could be moving during recoil and through the feeding process changing the cartridge overall length. Out of curiosity what magazines are you using? Have you noticed this same stringing with different type of bullet or different reload or factory ammo.
Have you verified there are no clearance issues between the gas block and hand guard. Is there a muzzle device on the barrel?
I’d also want to make sure the barrel nut is tight and the front of the upper receiver is lapped. Have you verified the scope is torqued to correct specs and the mount is secure.
This is where I would start. If I assembled the upper I’d know whether the barrel nut was tight and the receiver lapped. I know Alexander Arms used to use Loctite on barrel nuts in the past but if someone else assembled the upper it is worth checking. It appears something is changing during recoil, you just have to isolate what is changing.