M16 interchangability

[us019255]

US Marine Corps Technical Manual TM 05538C-23&P/2 for rifle, 5.56-mm, M16A2, page 2-15 "Do not interchange bolt assemblies or components from one weapon to another. Doing so may result in injury to, or death of, personnel."


I am more familiar with the M1 Carbine and M1 Garand. During the service lives of these weapons, it was usual for soldiers to have cleaning parties where all the parts were mixed and reassembled without regard of which rifle they came from. My M1 Carbine Technical Manual shows a special bolt tool for checking head space to make sure that any M1 Carbine in specification bolt will properly head space.

Does anyone know the story behind the abandonment of the complete interchangeability policy and practice?

Information

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[Gun Surfer]

From Armalite AR 10 Technical Note 6, Feb 4, 2003:

Found this interesting tidbit on line from Armalite:

"INTERCHANGEABILITY: The AR-10 is dimensioned and toleranced for a high degree of interchangeability to allow easy repair or customizing. The AR-10 bolt, pivot and takedown pins, and receivers are produced to be completely interchangeable with few, early, exceptions. The SR-25 emhasizes tight selective fit more than interchangeability."

Bushmaster manual has the same warning for AR-15s as Marine and Army tech manuals for M16. What happens if a bolt cracks or gets lost? Has to be a way to fit a new bolt, right? I think they just dont want soldiers exchanging bolts uncontrolled in the field that may not fit or headspace in another upper, but one must be fitted at appropriate level of maintenance, armorer or above. I wonder if they originally intended interchangeability, but then found out the hard way that M-16 bolts are not fully interchangeable.

[browningautorifle]

Yes, they're head spaced but military demands that they be interchangable. When we chanced to the Colt family in the Canadian army they gave us the same warning. But it has no visible substance. No problems observed.

[jmoore]

Been swapping parts of all sorts on these weapons for over 25 years- no problems at all just as long as they're "mil spec" or otherwise known to be of good quality when procured. That applies to bolt assemblies that have been swapped so many times I have no idea for which upper they were originally intended!

That said, I've also seen large amounts of absolute junk sold! New or old, aftermarket or fringe parts suppliers have gotten a fair amount of bad stuff in the system. Fortunately, the military doesn't too often get saddled w a lot of this stuff, but it does happen.

The concern in military weapons mostly arises from those parts in heavily used M16/M4s not be introduced into lower round count assemblies, thus leading to unexpected breakages. Bolt carrier assemblies have part life expectancies of between 5-15,000 rounds, depending on the piece part, and the armorers take this into account. Not too big a deal if a weapon goes down during basic, but its a different matter on deployment!

[A square 10]

in our litigious society a company that manufactures things the public can potentialy take apart & reassemble , let alone interchange parts on is well advised to post a warning not to .........

that said , the rifles bolt/chamber are headspaced and the feed ramps ground to function as a unit , interchangeing parts without paying attention is ill advised , yet - the US military has incisted on interchangeability since the M1903s

[us019255]

"...from those parts in heavily used M16/M4s not be introduced into lower round count assemblies,..."

Makes sense, but except for general wear how does an armorer know the "round count"?

It would seem that setting up for x-ray or Magnafluxing would be a more productive way to weed out rifles that had ended their useful life.

Interesting that specs and field practice still assume general interchangeability. A good thing when things get rough.

[jmoore]

Educated eyeball to a large degree is used to judge parts. Bolts will usually fail through the cam pin hole long before lugs start coming off, but the ones nearest the extractor will go first as a rule.

Round count can also be estimated fairly well in rifles used for basic training, as there are known usages and overall round counts for the range, its an estimate, but they've been doing this for a while...

Another sign of high usage will show in extractor assembly failures, these usually occur before more critical parts let go.

Just inspected a M16E1 the other day that is still running, but its starting to malfunction in F/A because the trip pin hole is worn. No telling how many rounds that takes! (Sheriff's Dept. weapon)

X-ray is a bit of overkill as the failures work in from the surface, so dye penetrant, magnetic particle and some eddy current tests are allthat would ever be needed. The penetrant test is quick and cheap and is rather more portable.

If it was really important they would serial number the parts!

[Troublco]

Magnaflux or penetrant would be the way to go; XRay isn't nearly as sensitive for checking for cracks. Plus, magnaflux and penetrant are less expensive.

[RobSmith]

If I had to guess, I'd say it's mostly in the factory manual in order to limit liability or the ability of the end-user to file a lawsuit in the one-in-a-million chance that something goes wrong with the weapon.

[Wulf]

I was an armorer in a an Infantry company for a year. I was there when we change from A1 to A2. I'll guarantee these parts were interchangeable and they even taught it in armorer's school.