I've just mounted an RMR to my Heym .500 double. The RMR mount comes with a locating pin that one can use a small hammer and gently tap the pin into the underside of the mount that assures the sight won't slide along the rib.

Fortunately the Heym rib has a locating hole on the top of the rib parallel to the front of the mounting grooves machined into each side of the rib.

Unfortunately, the right claw on the mount can't fully bottom in the groove on the rib because the locating pin locates the sight just a tad too far forward so that the very front end of the claw touches the rib just past where the right side groove stops.

Fortunately I have a teeny tiny file that I used to file off the front 3mm of the right side claw on the mount and presto! the mount now fits perfectly and the locating pin assures that the sight will not move forward or aft under the heaviest of recoil.

The sight works absolutely fantastic and I couldn't be happier!!!

The locating pin of course is optional however one may under heavy recoil experience movement of the sight without it and in addition, return to zero may possibly be inconsistent upon removal and reinstallation of the sight.

Now I need to have the grooves and locating pin hole machined into the rib of my Merkel so I can mount an RMR on it as we'll.