I don't shoot rifles competitively, except amongst my friends, and most of them are better shots than I, so consider me a rank amateur. That said, my longest shot on a prairie dog to this point was last year at 487.5 yards (according to the laser rangefinder) with my essentially stock Ruger SR556E (Vortex 4x12 44mm scope, and a Geissele 2-stage trigger), which for that rifle I would consider that a long range shot.
Longest shot I ever made was with my bone stock Savage M110 in .30-06 at just over 600 yards on a 10-inch steel plate. Put four rounds in about a 4-inch group, after using three rounds to find the plate, using 165 grain Core-lokts (prone from the bed of my truck, using a sandbag). Not long range to some, but I was very happy with that result.
Longest shot I ever made with my Henry AR-7 was 150 yards on a clay pigeon, and that in my mind was at least 50% luck. Somehow I hit the thing right in the center, and I've never been able to repeat that performance.
Oh, I voted 600 yards.