I purchased the Trijicon RMR Dual illumination sight with the 12.5 MOA green triangle. It is a small optic but it's made rugged. You can rack your slide aganst an object with the RMR in a pinch. Be careful not to drop it on concrete or you might damage the cover for the fiber optic, or worse, you might break the capsules that contain tritium gas.It is very easy to install if you have the correct mounting plate for the rear sight slot on your pistol or your pistol is a model pre-milled for an RMR that comes with a mounting plate for an RMR.The mounting instructions were included in the box with the optic. I used a sight pusher to remove the rear sight from my pistol, a Sig P220, applied a drop of oil in the rear sight slot, then started the RMR mount in the slot with a few gentle taps from a plastic hammer. I then used a sight pusher to center the mounting plate before attaching the RMR to the mounting plate. Trijicon includes a hex wrench and screws (with the directions under the insert in the box) to attach the RMR to the plate. It took me about 15 - 20 minutes to install and, believe me, I am no gunsmith.I zeroed the optic indoors using a laser-bullet at 7 yards. I used a small vice to hold the pistol in order to zero the optic. I wrapped the pistol with a silicon pot handler-masking tape, a shop cloth or a lot of other things would also work well to prevent the vice from scratching the finish on your pistol. Once the RMR was attached to the pistol adjustment is simple and intuitive. You can feel and hear clicks as you turn the adjustment screws on the sight, and the optic is marked to tell you what each click of the adjustment screws means in terms of MOA.I tried the RMR at an indoor range and it was dead-on from the first shot at 5 yards and no adjustment was necessary at all ranges from five yards to the most distant shot available at the particular indoor range, 25 yards. My Sig P220 is chambered in .45 caliber.I have had to qualify with several small arms since 1970 when the U.S.army required me to be proficient with the M16 and M14 rifles and the M1911 pistol. I have had to qualify or requalify many times for P.O.S.T. Annual certification. I am not a match -grade shooter with a pistol. The RMR made an obvious quantum leap in the evolution of my pistol marksmanship. It is a sigle plane optic. Keep both eyes open so your dominant eye can focus on the tip (top) of the triangle reticle. Put the tip of the triangle where you want the bullet to hit. It is intuitive and fun to use. In about 10 -15 minutes I shot 100 rounds, 90 of which were at 25-yard targets. Every 16 rounds, I stuck a new shoot-and-see target on the face of a life-sized photo of a bad guy brandishing a handgun up range at me. See Photo of the last stick-on attached. (As you can see I am not a match-grade pistol shot at 25 yards, but the barrel was hot.)I am blessed with excellent distance vision in my dominant eye, but at 64, I normally have to wear reading glasses to see the front sight on a pistol when I practice and qualify. However, the 1 power Trijicon RMR makes that unnecessary. In fact my reading glasses made the triangle difficult to see (I don't think as small and faded as the one disgruntled wag put It in a negative review, but if that person wears reading glasses, it may explain one of his problems). Without my reading glasses on, the green triangle was big, bright and obvious, and it gets bigger as I extend my arms out to hold the pistol out in front of me. Don't let the 12.5 MOA triangle fool you can still aim small, miss small with this sight. Zero the pistol at 7 yards to the point at the top of the triangle and thereafter at all practical ranges for your weapon, no hold-over will be necessary.Also, notice that when Looking through the optic at a post target 100 yards out, the bottom of the triangle reticle appears to be the same width as center mass on the post target. This will take some of the guess work out of ranging a shot at 100 yards.The down side of this optic is apparent when you are in a dark area looking through the dual illuminated (tritium and fiber optic) RMR sight into a brightly lighted area. The reticle becomes barely visible because the bright background you see is brighter than the tridium illumination and the fiber optic needs ambient light to be seen in the RMR when you cannot see the tridium light. This reticle washout will only happen when you look into bright light while the scope is in darkness. The upside is you will never need batteries for the dual illuminated RMR.Aside from reticle wash out, when I hold the pistol out away from my eyes, the green triangle is big, bright and easy to see in all other lighting conditions including in pitch- black darkness or brightest sunlight.I mounted the RMR on a pistoI that I use for hog hunting, home defense, and range shooting. If If you carry professionally i.e, are LEO SWAT Team member or military spec op I you will do well to get the battery on/off model RMR 2 Gen with the adjustable red dot so you have the most manual control of reticle brightness and maximum confidence in the sight. For me co- witnessing the fixed sights through the RMR is all that was necessary when I encounter a lighting situation that causes reticle washout. So, I had a gunsmith mill the slide down on a sig P 226 and mounted the dual illuminated RMR So I can co-witness my suppressor height front and tridium-fiber optic night sights through the RMR. Worst case, when I have lighting that makes the RMR's triangle reticle fade, I can see or co-witness the fixed sights through the RMR for the same sight picture as I have been trained to see for 45 plus years with pistols that did not have an RMR. The key is that I need no cheaters to co witness, because of the 1- power RMR optic.FN, Glock, Walther, Kimber, sig Sauer, and Smith & Wesson Now mafrketc pistols that are pre- milled at the factory to drop-in mount the RMR, but confirm before you buy that the fixed sights are suppressor height.This sight is best used with both eyes open, which makes it easier to see, easier to zero on a moving target, and easier to acquire a sight picture on a stationary target without loosing peripheral vision.Five-star gear and the best of its kind.If you find this review helpful, hit the “helpful” button below to Let me know that I did not waste my time reviewing this product and make me more inclined to review other products.