Editing Colt AR-15 (section)

===Lower receivers===
The lower receiver incorporates the magazine well, the pistol grip, the [[Stock (firearms)|buttstock]], the buffer and the buffer spring. The lower receiver also contains the trigger, disconnector, hammer and fire selector (collectively known as the [[fire control group]]). Full-sized rifles use a fixed buttstock, while carbines generally use an adjustable telescoping buttstock.

The early commercial SP-1 AR-15s used a pair of {{convert|0.250|in|adj=on}} diameter receiver push pins, identical to those found on the military rifles. In 1966 the company replaced the front pin with a paired nut and screw hinge using a {{convert|0.315|in|mm|adj=mid|-diameter}} pin to prevent shooters from being able to change receivers with military rifles or competitor rifles without the use of an adapter. They resumed production with the smaller and standardized 0.250-inch pin in the mid-1990s.<ref name="Sweeney1999">{{cite book|last=Sweeney|first=Patrick|title=Gunsmithing - Rifles|url=https://books.google.com/books?id=Eul8bA4KSCcC&pg=PA290|date=1 October 1999|publisher=Gun Digest Books|location=Iola, Wisconsin|isbn=0-87341-665-1|page=290}}{{Dead link|date=December 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>