You will find two types: holographic and mini-tube sights. Mini holographic sights such as the MRDS (Miniature Red-Dot Sight) from Insight Technology or Trijicon's RMR (Ruggedized Miniature Reflex) are two of the more popular sights. When it comes to mini-tube sights, Aimpoint's Micro T1, iTAC's RDS, Konus' Pro Atomic, and Bushnell's TRS25 are leaders of the pack.
I have mounted the above sights on handguns, shotguns, large caliber semi-automatic rifles, and numerous AR-style rifles. They have survived literally thousands of rounds and are still being put through their paces today. I have found these mini red dots to be nearly indestructible. They have survived rifles dropping on them, hitting door jams, soaking in downpours, and arctic temperatures; so they should survive a duty day.
Of the mini red dots, Aimpoint's Micro T1 has ratings the others don't. This sight is submersible to 80 feet, making it ideal for waterborne units such as ICE, various wildlife enforcement units, and harbor patrol teams. The other feature that sets the T1 apart from others is its 50,000-hour battery runtime.
Mini Reflex Sights
Mini reflex sights came to be because of EoTech's innovation in reflex sights. The company's 500 series of Heads Up Weapons Sights are so tough they even function if the screen is cracked. The drawback is these sights seem large compared to other optics.
To combat this problem, EoTech redesigned the 500 Series, mounting the batteries horizontally to the screen instead of vertically. The new XPS Series is the result. This shorter sight gives you room enough to also mount night vision, lasers, and magnifiers with it on the rails of your weapon.
I had been using the XPS2-0, which is not NVD capable, so I was happy to start using the XPS3-0, which is. From the time I opened the box I was hooked. A smaller, lighter version of my time-proven 552, the XPS2-0 sight is very much like its larger sibling; it's a keeper.
Insight Technology and Trijicon took this reflex technology and shrank it down. One advantage of the mini-holographic scopes is they can be mounted on telescopic rifle scopes. You can find numerous scope mounts that allow you to mount Insight's MRDS or Trijicon's RMR on your scope, rail, or to the scope rings. This allows a precision marksman to use his rifle for CQB should he suddenly find himself in that situation, or it gives you a backup sight should your primary sighting system fail.
Another advantage of the miniature reflex sights is their toughness. Both of these sights are waterproof to beyond 60 feet and will run nearly forever on one battery. Should the battery die on the Trijicon RMR, fear not; the dot will glow thanks to its dual power fiber optic.
Best of Both Worlds
As much as I like the mini red dots, I have grown fond of the low magnification variable power scopes for use on an M4. Apparently I am not the only one, because more and more military troops and numerous police are using these scopes. The new generation of tactical scopes I have used feature both low variable power magnification and a lighted reticle with a dot or crosshair. This gives you the best of both worlds: fast target acquisition and precise shot placement.
When it comes to these new tactical scopes I think the first and oldest is Leupold's Mk IV CQT. This is a 1-3X 14 scope with an illuminated circle dot reticle. It is one of the toughest scopes I have seen and mine rests on a SIG 556. I say tough because a buddy dropped one mounted on his M4 while sitting in his UTV in Iraq. He hit a bump and the scope slid about 100 yards down the road and all it needed was a new cap for the horizontal adjustment, which had cracked.