Don't waste your money ... you got it for steal because the previous owner had the same experience.
I got the same problem, spent a ton of money on all kinds of match ammo and a 1800 dollar scope.
The 716 is reliable, shoots everything I put through it, M80 ball to Federal match ammo.
Groups tightened up a bit after 500 rounds but still getting 4-8inch groups with match ammo ... and that is down from 8-10 inch shotgun patterns.
Your choices are ... send it back to Sig for warranty accurizing, sell it or accept it for the pig it is.
I've had reputable bench rest shooters shoot it, bought several hundred dollars of prime quality match ammo, and finally spent over $2k for high quality scope and mounts.
Not one bit of difference ... yet with the 516 and my other rifles, I can easily crack sub-MOA groups.
I will try the Sig route once and if it doesn't work, I'm selling it for cheap to get rid of it.
For the money I paid, I'm very disappointed... should of spent a grand more and gotten a tier one platform.