Optics mounting options can be limited for those who use ARs with fixed carry handles. Sure things can be mounted on top of the carry handle, but that leads to a less than optimal chin-weld, instead of a good cheek-weld. The M-21 is a great way to go for those who find themselves in this conundrum. It features a simple goose-neck mounting system that places the sight low, where it should be. Your iron sights are still barely visible in the extreme bottom of the window. The mount is attached via the mounting hole that is in your carry handle. The attachment method is simplicity itself. You place the mount on and then thread the nut on the bottom. It is contoured to fit the bottom of your carry handle perfectly. I was dubious about the resiliency of the mount, but with only hand tight torque applied, I had no issues with loosening or zero shift.
Zeroing of the sight was surprisingly fast. The adjustments were easily made with a coin in the field. The clicks were both audible and tactile. The illumination of the reticle is accomplished by a combination of fiber-optics and tritium. There are no batteries, buttons, or switches to fuss with. I have had the good fortune to shoot this sight in an open field that butts up to some woods. While it was brighter in the open with the sun shining down upon it, at no point in the shadows of the woods did it ever become washed out or hard to pick up. Target acquisition was fast and reliable with all the lighting conditions available to me, even while in the woods aiming out in to the brightly lit field. The blue tint to the lens is a little annoying while shooting with a single eye, but is not noticeable when both eyes are open.
I had zero failures of the sight and was thoroughly impressed with its simplicity and reliability. From the mount to the optic itself I am now a believer in this system for a quick acquisition sight on carry handle equipped ARs.
I have not got to use it yet as waiting for parts for the retro gun build. This looks to be a quality sight, and should have fun when the project is done. Hard to review when not used it yet. The gun will look like an early IMF rifle when done.
I have this optic on 3 weapons now, a Saiga 12ga, and 2 AR platforms one with a binary trigger... this thing holds zero no problem. There can be a bit of washout transitioning from light to dark and vice versa, but the polarizer helps this immensely... if you add the aftermarket light this thing cannot be beat.