Simply put, the receiver holds the mechanical parts of a gun. These parts include the trigger housing and bolt carrier group. Most firearms have a single receiver although a few--most notably the AR-15 and the AR-10--have both a lower and an upper receiver.

In most cases, the receiver is the only ATF-controlled part of the gun as it is marked with the gun's serial number. From a legal standpoint, therefore, the receiver is the gun. In the case of a gun with two receivers, the lower receiver is typically the serialized part.
http://weapons.about.com/od/glossary/g/Receiver.htm

Пистолетът се състои от следните главни части и механизми:
- ложа с цев и спускова скоба;
- затвор с ударник, изхвъргач и предпазител;
- възвратна пружина;
- ударно-спускателен механизъм;
- ръкохватка с винт;
- затворна задръжка;
- пълнител.
http://www.centre-point.eu/index.php/курсове-за-боравене-с-о... (На същия линк има инструкции за сглобяване и разглобяване.)
http://bg.wikipedia.org/wiki/Пушка
http://www.guns.bg/ICS_M16_RAS-cat143-scat144-p23743.html?se...