You can change between the 4 reticles using the selector switch on the back of the sight. You can mount this on a TAR-21 as long as you have a picatinny rail on top.
The Browning Buck Mark Reflex Sight uses a brightness dial located on the top of the optic. When you set the brightness to "0" the optic will be off. All brightness adjustment is done manually as well as it's on and off functionality.
It is possible.
As long as you have a picatinny rail on it, there should be no problem with that setup.
Yes, this optic can be mounted onto any rifle with a picatinny rail.
If your rail is a picatinny, yes it will work.
This sight is designed for rimfire rifles and should work well on an AR platform.
Yes, these are new optics.
This sight is desgned for rimfire rifles and pistols. I believe the MCK is for centerfire pistol calibers and would probably be too much recoil for this sight.
This sight is designed for rimfire rifles and pistols. I would not recommend this for a slug shotgun.