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Wanted to share my experience mounting a Vortex Venom on my Ruger Mark IV Hunter. Both products are great but getting things mounted and zero'd was frustrating until I finally sorted things out. After three weeks of frustration, I finally have my RDS mounted to my satisfaction and tested it at the range today and am very happy with it. The bottomline, based on my personal experience, is as follows: When you purchase your Venon RDS, I recommend you order the following three items at the same time. This will help you avoid the frustration I experienced and position you for a successful installation the first time. · Tandemkross Shadow rail #TK05N0014BLK1 www.tandemkross.com · UMTactical shim. “Delrin Shim 1.0 Degree for Vortex Venom…” www.umtactical.com · Longer mounting screws from Brownells to connect the Venom RDS to it’s supplied picatinny mount. Part #080-535-645 6-48x5/16 Filister slot headed screws. Details as follows: 1. I recommend not purchasing Ruger’s own #90623 rail for the firearm. They are in fact NOT Mil-Spec. Specifically, the depth of the top grooves should be 0.118” which is the MilSpec standard(refer to Wikipedia.) I measured it to be 0.100. Both my original purchased in November 2017 and one they recently shipped me as a replacement for it were identical. Thus the crossbar on the Venom mount bottoms out, causing the Venom to not sit flat and rock.. For comparison, I checked the slot depth on the rails on my M&P 15/22 as well as my RUGER Precision Rifle. They all measured between .120-.125”. 2. So I did some research into alternatives and ordered the Tandemkross rail. A bit disconcerting was the fact that the screws that TK ships with their rail have a tapered head while the screw seating area in the rail itself is not tapered. I called TK and they basically said not to worry about it. (Interestingly, she stated they source their screws from Brownells.) That aside, the TK screws were the optimal length and I used them without any issues. I applied blue Loctite to the threads and torqued the screws to 10 inch-pounds using my Wheeler “Fat Wrench” torque wrench. 3. The reason I recommend the UMTactical shim is because I ran out of “Up” adjustment on the Venom itself in order to get a zero at my desired 10-15 yards. On my Venom the “Up” screw will rotate 5 turns for a total of 150 MOA adjustment range total. (Fwiw, the specs state 130 MOA.) Nevertheless, it was not enough. The shim is installed between the RDS and the Vortex supplied picatinny mount that ships with it. Put the thicker end of the shim toward the rear and the thinner end toward the front. I.e. you want to cant the RDS downward in order to raise the point of impact. With the shim in place, I was easily able to zero it without hitting the limits of the elevation adjustment. 4. I purchased the TK Shadow rail in Black. From reviews, their gray color does not match stainless steel. Looks good and matches the Vortex mount and RDS. 5. The screws Vortex supplies to mount the RDS to it’s picatinny mount are T15 head 6-48 x ¼. However they are not long enough to be used with the UMTactical shim. They only grab by a few threads and I easily stripped mine. Hence, my recommendation to use the above mentioned screws from Brownell which are 5/16 in length. Note that the Brownells screws have a slotted head and the diameter of the head is a little smaller than the Vortex supplied screws. Nevertheless they worked fine. I also torqued them to 10 inch-pounds. 6. I shot 180 rounds thru it today and so far everything is still tight. Am finally pleased with the setup! I could not find an exact matching 5/16” length version of the Vortex supplied Torx head screws. I called both Vortex and UMTactical and neither sell them. So if anyone knows where to find a T15 head(.220 OD) Fillister 6-48 5/16” long(the threaded portion) screw, let me know. Hope this helps, regards.