Editing M16 rifle (section)

===XM16E1 and M16A1 (Colt Model 603)===
{{Redirect|M16A1|the mine|M16 mine#Variants}}
[[File:NARA 111-CCV-349-CC43120 101st Airborne soldiers firing XM148 Operation Cook 1967.jpg|thumb|XM16E1 with [[XM148 grenade launcher]] ]]
[[File:M16A1 brimob.jpg|thumb|M16A1 rifle with 30-round magazine]]

The U.S. Army XM16E1 was essentially the same weapon as the M16 with the addition of a forward assist and corresponding notches in the chrome bolt carrier. A rib was built into the side of the receiver to help prevent accidentally pressing the magazine release button while closing the ejection port cover.

The M16A1 was the finalized production model and was produced from February 1967 until 1982. To address issues with the XM16E1, for the M16A1 a closed, birdcage symmetric flash suppressor with open side slots to the top, bottom, left and right replaced the XM16E1's three-pronged flash suppressor, which caught on twigs and leaves, from 1967 onwards.{{Sfnp|''Retro Rifles''|2020}} Various other changes were made after numerous problems in the field. Cleaning kits were developed and issued, while barrels with chrome-plated chambers and later fully lined bores were introduced. A small storage compartment inside the stock was introduced.{{Sfnp|Meehan|1985|pp=145-147}} Covered by a trapdoor with checkered anti-slip texturing, it is often used for storing a basic cleaning kit. To promote reliability and durability, the mechanical behavior of the operating system was revised to make it compatible for using US military issued ammunition loaded with WC846 ball powder (which reaches peak pressure significantly quicker than the extruded IMR8208M powder and increases the cyclic rate of fire for which the operating system was originally designed). Revisions like reducing the diameter of the gas port to mitigate the higher port pressure caused by the ball powder to properly gas the operating system again, updating the buffer assembly, changing the bolt carrier surface finish to manganese phosphate and the gas tube material to stainless steel contributed to improved mechanical behavior.

The rib was extended on production M16A1s to help in preventing the magazine release from inadvertently being pressed. The hole in the bolt that accepts the cam pin was crimped inward on one side, in such a way that the cam pin may not be inserted with the bolt installed backwards, which would cause failures to eject until corrected. With these and other changes, the malfunction rate slowly declined, and new soldiers were generally unfamiliar with early problems.

The M16A1 saw limited use in training capacities until the early 2000s,{{Sfnp|''DOD News, 1 March 2010''}}{{Sfnp|''DOD News, 1 July 2000''}}{{Sfnp|''Navy News, 29 December 2017''}} but is no longer in active service with the U.S., although is still standard issue in many world armies.