Reflex sights: Open style red dot sight & tube style red dot sight. 

Tube style red dot by 2nd Amen

This can get quite confusing so I have tried to simplify it as much as possible. 

Reflex sights are a popular type of optic that are used to improve accuracy and speed when aiming at a target. These sights use a reticle, such as a red dot or crosshair, to help the shooter aim their firearm. There are two main types of reflex sights: tube style and open.

Tube style reflex sights are characterized by their cylindrical shape, which is similar to a traditional telescopic sight. They are designed to be mounted on a firearm, such as a rifle or shotgun, and they use a reticle to help the shooter aim. Tube style reflex sights are known for their durability and reliability, and they are often favored by shooters who want a simple and effective aiming solution for their firearm.

Open reflex sights, on the other hand, are characterized by their open design, which allows the shooter to see through the sight and keep their peripheral vision. These sights are typically smaller and lighter than tube style sights, and they are often used on handguns or other firearms where size and weight are a concern. Open reflex sights are known for their fast target acquisition and ease of use, and they are a popular choice among shooters who need to aim quickly and accurately.

When choosing a reflex sight for your rifle, it is important to consider your specific needs and preferences. Both tube style and open reflex sights have their own unique features and benefits, and the best option for you will depend on your shooting style and the type of firearm you are using.