What is an upper and lower receiver?

An upper and lower receiver is the main housing for a firearm.

The receiver is where the firing mechanism, magazine well, and bolt handle are located. The upper receiver usually has Picatinny rail or KeyMod systems on its top surface for mounting optics and other ancillary devices.

For AR-15 rifles/M16s, the charging handle is also located on the upper receiver (though some designs now place it on the side by the trigger guard). For bolt action rifles, the cocking piece protrudes from the rear of the bolt carrier assembly, while pump action and lever action rifles have their respective charging handles attached to the forend or stock. The magazine release button and safety selector are generally found on either side of the lower receiver.

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An upper and lower receiver is a common set of parts used in assembling firearms.
The upper receiver Assembly includes the barrel, gas system, sights, charging handle, and bolt carrier group. The lower receiver assembly houses the trigger group and magazine well. In most cases, the upper and lower receiver are manufactured as separate pieces then joined together by the consumer.
 

 
It is important to note that on an AR-15 rifle platform, for example, the serialized number is located on the Lower Receiver (LR). The LR contains all of the firearm’s operating controls: pistol grip, trigger, and hammer specific to that firearm. The LR also serves as the mounting point for the buttstock, muzzle device, and forend/handguard.
In short, the upper receiver is where all of the action happens in a firearm. All of the moving parts that make a firearm function is contained in the upper receiver. The lower receiver is where the trigger and magazine are located. The lower receiver also serves as the mounting point for the buttstock, muzzle device, and forend/handguard.

An upper and lower receiver are the two main components of a firearm. The upper receiver houses the striker assembly, bolt carrier group, and barrel, while the lower receiver houses the trigger mechanism and magazine well. In a semi-automatic rifle, for example, the lower receiver also houses the buttstockand pistol grip.
 

 
The receivers are typically made from lightweight aluminum alloy or polymer. Some manufacturers produce complete firearms with both an upper and lower receiver made from a single piece of aluminum (known as a monolithic receiver). Monolithic receivers create a more rigid platform which can potentially improve accuracy.
The major modern sporting rifle calibers use AR-patterned Upper and Lower Receivers such as the AR-15, M4 Carbine, andM16 rifle.

An upper and lower receiver are the two halves of firearms that work together to fire a bullet. The upper receiver contains the barrel and bolt assembly, while the lower receiver houses the trigger mechanism and magazine. To fire a gun, the two receivers must be mated together to complete the circuit that ignites the powder inside the bullet.
 

An upper and lower receiver are the parts of a firearm that connect the buttstock, grip, and magazine well to the rest of the gun and house the trigger group. The upper receiver holds the bolt carrier group and expands or contracts to allow for different cartridges. Conversely, the lowers on AR-15s hold handgun ammunition. Each type of receiver has its benefits and drawbacks.