After the Ichord report of 1967 identified the early failures of the M16 rifle in combat, particularly failures to extract, solutions to the problem of fouling were investigated by Colt, as well as Rock Island Arsenal (a government entity), and Olin/Winchester. Colt’s effort was the 703, which they somewhat presumptuously offered to the Army as the “M16A2” in 1969. The 703 was not only one of the first – if not the first op-rod AR-15s, it was also the first “hybrid” AK/AR-15 design, utilizing a very AK-esque gas block and fixed flexible piston:

The disassembled Colt 703/”M16A2″. Note the gas block and piston, both of which greatly resemble that of an AK. Also of note is the different bolt stem design, and the enlarged cam pin, charging handle, and cam pin bulge on the upper receiver. This design added almost half a pound of weight versus the M16A1. Image source: The Black Rifle, Edward Ezell, R. Blake Stevens.

The 703 did not just modify the gas system, it also incorporated changes to the fire control (adding a fourth position for burst fire), bolt, handguard retention system (eliminating the frustrating delta ring design), and adding a cleaning kit trap in the buttstock – a feature quickly incorporated into the production M16A1.

The Colt 702/”M16A2″. To add to the confusion, the export variants of the later M16A2 second-generation US Army AR-15 were also referred to as the “Colt 703”. Image source: The Black Rifle, Edward Ezell, R. Blake Stevens.

As the propellant issues of the M16 family shook out, the need for the 703 evaporated, and the M16A1 continued in service with the Army until its replacement by the M16A2. Two Colt 703s still exist today, in Reed Knight’s collection.

Aftermath Gun Club has some more information about the Colt 703 on their website, including a full spec sheet.