As previously stated, don't overlook barrel fouling, but the one thing I would look at closely is the barrel bore uniformity. You'll need a 'smith or barrel maker who has an air gauge; if the barrel opens up even .001" from the breech diameter, it will negatively affect accuracy. Turning down the muzzle diameter in order to thread it for muzzle brakes or suppressors can relieve stresses in barrels, especially button rifled barrels and cause the barrel diameter to open up at the worst possible place - the muzzle/crown area. Since a lot of .408 bullets are solids or bore-riders, this aggravates the condition.
Other things that can affect barrel harmonics are devices attached to the end of the barrel. I have only one rifle that screwing on the suppressor does not change POI. Last, barrels vibrate like strings, with nodes of little/no vibration and areas of higher amplitude of vibration. If your barrel was cut and crowned at an area that experiences a degree of vibration, it will not shoot as well as if it was cut and crowned at a node of no vibration.
Scope: What optics are you running? Have you tried a different scope?
1 MOA from this rig at 500-600 yards is nothing to sneeze at, and .1" difference between bipods is essentially no difference. I don't remember if you told me what platform you are resting your rifle on - concrete, ground, etc. Before I started changing things, I'd clean the barrel as clean as possible with SLIP 2000 Copper and Carbon cutter, check to see that the barrel crown is still crisp, and start weighing and sortiing your bullets by weight and OAL from ogive to rear driving ring; don't forget that hinky bullets and brass give you hinky groups. Clean your chamber and dry it with alcohol and make sure your loaded rounds don't have lube on the outside. The final bit of advice is to take a fired piece of brass and try to slip a bullet in the neck. If it won't go it, that's a problem and needs gunsmith attention right away. If it slides in smoothly without noticeble wobble, you're about where you need to be.
That touches on the more common problems that could be affecting your accuracy, but man, 1moa doesn't suck. Do the math on the minute deviations in point of aim that make the difference between 1 moa and .5 moa. Your heartbeat is enough to open up groups that much. If I were you, I'd keep shooting and be happy. Good luck and good shooting.
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