SWFA is a great option. Athlon does have some good options as well and they do track well. In the budget scopes using the mildot style reticle and holding over can be a great way to hit at extended ranges. I run a primary arms r-grid on one of my guns and its great if you can find one. I...
Anybody have any experience with the new Nikon 4K? https://www.nikonsportoptics.com/en/nikon-products//rangefinders/black-rangex-4k-laser-rangefinder.html
FFP Mil/Mil Scope with R-Grid Reticle. Works great! Great condition. Will include scope rings. Price includes shipping in the original box.
$250 or best offer
Also, with increased magnification you increase mirage and lens distortion. If you are only going to shoot out to 500y then 15x is more than enough. A friend of mine routinely shoots a 1-4x out to 500y. With 15x you will be good. Plus for hunting, the 3x min magnification is nice. I vote...
Primary Arms makes a 4-14x for $350 ish. They also have a 3-18x coming out soon for around $500-$600
Vortex came out with a Diamondback Tactical FFP that would be in your price range.
Athlon Optics also has a few options for you.
That is all i can think of right now.
Did you set the altitude in your ballistics app? How did you confirm velocity? Do you know for sure its 500 yards? It doesn't surprise me that you were on at 100-400y, it's over 500 that things get interesting.
But like LaHunter said it might be the scope tracking. Every scope should be...