Horizontal 'stringing' like you show in the photo isn't really stringing. Looks to me like you are shooting to 3 different points of aim. After checking the scope mounting, the only way that could happen is if there is a problem inside your scope. Someone suggested a nick on the muzzle. The only way that can affect anything is if the nick creates an intrusion into the barrel. Most nicks create voids, and voids don't have a large effect. Copper fouling near the muzzle can create larger dispersion, but won't create such a perfect horizontal effect, just a larger round group. Try this first, before each shot rap the scope body at 2:30 O'Clock (45 degrees) with something heavy enough to shock the spring (which is generally located at 7:30 on most scopes like the Sightron). The object is to settle the reticle against the spring. Something like a 6" piece of 2"x2" pine or a tough knuckle will generally do it. Might try only 'smacking' it at 3:00 O'Clock because the problem appears to be only horizontal. Any change in the way the group develops will prove the problem is in the scope, as this sort of external shock to the scope should have no effect. Another way to test the scope is to add then subtract a full revolution of windage and/or elevation between each shot, and yet another test is to move your point of impact way off (up/down - left/right, or both) and see if it still shoots that horizontal signature (you might end up aiming well off the target to give this test enough difference in spring tension to show up in the group.) Moving the reticle that far should allow the reticle to reset to the same location after each shot. If the spring has lost it's strength, or the surface between the spring and reticle is no longer slippery, you might find the group shrinks when the sight is adjusted far up/down or right/left. Be sure to NOT force the adjustment to a limit, that often will permanently damage the spring. Between them these tests should get you back on track to a solution. If the problem persists, change the scope. Even a cheap one will allow you to sight well enough to see if that horizontal signature is present. If so, glue the sight screws, base, ring screws, rings etc. with Loctite 722 or similar. Its important to remember that the amount of movement to create that much dispersion is still very very small.
