I had issues with the HK Parts mount shifting zero on my AP-5 with a 507. Seemed like the claws were to wide and one side would not get a good grip. Tried shimming it, but would still shift and move my point of impact significantly at 25 yards and beyond. Damn shame too, I really liked the small size and especially liked the co witness.

I now have the MFI  4 point low mount on there and that thing is locked on there as tight as can be. Much happier with that part of it. With the Aimpoint PRO on a MFI quick change mount it does add bulk and weight. A very good setup  however and pretty much bullet proof.

I may revisit the HK Parts mount and try bedding it and see maybe if I can work out the shifting issue.