Compared to some historical American small arms, that were made by many contractors, only a few fims made the M-16 and M-16 A1 rifles. Most of them were made by Colt, but nearly a quarter of a million each were made by Hydramatic Division of General Motors, and Harrington and Richardson.
Later, M16A2s were made by Colt and by FNMI. In addition to these guns made of American troops, some third parties made military-specification firearms that were purchased by US DOD and supplied to US allies under the Military Assistance Program.
The service life of a rifle is limited. The US Armed Forces rebuild small arms when their age or condition calls for it. Every unit’s weapons are supposed to be subject to a thorough third party Technical Inspection at periodic intervals, and prior to combat deployments. Weapons which fail some aspect of that TI are transferred out for depot maintenance and replaced with new or freshly overhauled weapons. This ensures that no one goes to combat with a clapped-out rifle or carbine.
At the depot, every rifle is disassembled and each individual part is cleaned and inspected. If it meets specifications for reuse, it goes into a parts bin and is reused in assembling an overhauled rifle. (In some cases it is refinished first).
Specifications for a single measure might comprise an array of values: one, the narrowest, for acceptance of a new part; one, the loosest, for persistence of a part in service on a serviceable firearm; and an intermediate one for reuse of a part during an overhaul. (Alternatively, the part may be required to meet new specs during an overhaul, depending on the part’s nature and criticality). Parts that fail may be scrapped, or overhauled themselves, either in-depot or sent out to a contractor.
During the overhaul process, improvements are made. This is how, for example, three-prong flash suppressors were gradually weeded out of the Army’s stockpiles. A complete rifle exits the depot resembling a new weapon in most respects, and in significant measures it is up to the latest version of the technical data package (some older parts can be retained in non-critical uses).
An overhauled rifle is made up of a mixed bag of new and recycled parts, but is guaranteed to match the minimum key performance parameters that a new rifle would meet. By now, all original M16 series rifles are likely to have been depot-overhauled several times, and their original parts scattered to the four winds. There is no such thing as a “matching numbers” American military rifle.
Therefore, it should be obvious that a barrel from any manufacturer may turn up on an M16, A1 or A2 rifle. You might find a Colt barrel on a H&R or Hydramatic upper, for instance (in fact, it’s extremely common, as Colt produced surplus barrels which it then supplied to the other sources), and any combination of these on a Colt lower.
In addition to the three 1960s manufacturers of the M16A1, and the two of the M16A2 (Colt and FNMI), there were additional contracts let for replacement barrels and other wear parts. Barrels were replaced at depot as specifications changed, or as old barrels flunked inspections (from erosion or corrosion).
For example the barrel twist rate specification changed from 1:14 to 1:12 even before the main M16/M16A1 contract got underway, and in the late 1960s chrome chambers and later chrome bores were mandated. Any barrel that came through rebuild and was not chrome bore, 1:12, was discarded. Later, a new run of barrels were made that were M16A1 profile, chrome bore, and 1:7 to allow free use of M855 as well as M193 ammunition. (The heavier M855 bullet is not stabilized by the slower original twist). We have observed these barrels from FNMI and Colt manufacture.
These replacement barrels were not only made by original contractors. We’ve observed FNMI-marked M16A1 profile barrels in 1:12 and 1:7 configurations, and we’ve observed SAK-marked (Saco Defense, Saco Maine) barrels in 1:12. According to one reference, at least 385,000 of the SAK barrels were made by 19971. FN barrels seem to be less common; in a protest that FN filed to a replacement-barrel award to Colt, FNMI cited only three barrel contacts with a total of 29,500 barrels, mentions other contracts with the Defense Logistics Agency without citing numbers, and mentioned another contract for 26,275 complete rifles2. It’s impossible from this reference to understand how many spare or replacement barrels FNMI has made, but it is likely to be many fewer than Saco’s considerable output, and fewer than Colt’s.
As technology has improved, newer barrels tend to be more consistently rifled than earlier ones, and the newest barrels by all GI makers are superior to the original 1960s barrels. There may be a limit in the degree to which they improve: for example, the technical data packages for the M16A1, A2 and M4 all specify that barrels must be button-rifled, while FNMI knows they could produce superior barrels and greater accuracy by using the hammer-forging process over a mandrel. (They do use hammer-forged barrels for the SCAR-H, but the paperwork requires them to use a button broaching process on their M16 and M4 barrels). In our experience, SAK and FNMI barrels have excellent accuracy when correctly installed.
1. Demeritt, Dwight B. Jr. Maine Made Guns & Their Makers. Augusta, ME: Friends of the Maine State Museum,1997. pp 369-373.
2. US Government Accountability Office. Decision, Matter of FN Manufacturing LLC. File: B-407936; B-407936.2; B-407936.3. Washington: GAO, 19 April 2013. Retrieved from: http://www.gao.gov/assets/660/654276.pdf