Wear in an M4?

[WS6]

What determines when an M4 is "worn out"?

I understand less wear is always better, but does the relationship between the BCG and the upper matter, or only to the extent that excess friction to the point of failure is not created during cycling, and that the bolt is guided into the locking lucks of the barrel extension?

What exactly are the "critical" wear areas (if any, besides the bolt/barrel extension/throat) in the M4/AR-15?

[Iraqgunz]

In the military world "worn out" means that the weapon is not serviceable. The way that thisis determined is by inspections and gaging. Any part that fails is replaced until everything is corrected and serviceable.

I really can't make sense of the remainder of your paragraph.

What determines when an M4 is "worn out"?

I understand less wear is always better, but does the relationship between the BCG and the upper matter, or only to the extent that excess friction to the point of failure is not created during cycling, and that the bolt is guided into the locking lucks of the barrel extension?

What exactly are the "critical" wear areas (if any, besides the bolt/barrel extension/throat) in the M4/AR-15?

[mkmckinley]

I think he means besides barrels wearing out and bolts breaking, what wears out on an AR? Does the BCG moving inside the upper receiver ever wear it out to the point that either the carrier or receiver is unserviceable?

[WS6]

I think he means besides barrels wearing out and bolts breaking, what wears out on an AR? Does the BCG moving inside the upper receiver ever wear it out to the point that either the carrier or receiver is unserviceable?

Correct, how "Critical" is this area? It's the only other thing I can think of wearing out besides the bolt breaking, gas-key coming loose, barrel shooting out, etc.

[mkmckinley]

I've never seen one worn out but someone like IG would have a lot more experience than most in this subject. I've seen ARs from the 60's that were still going strong even with the anno starting to fade. The weapons in the Philippines are all Vietnam era and they're still using them on a large scale with mostly original parts. In an arms room in the US I've seen a Mk 12 SPR built on a well worn lower stamped XM-16. I don't know how old that must be but if it still works then I'm not worried about one of my many ARs wearing out significantly.

[Iraqgunz]

Things that wear are springs. Even parts like the trigger hammer and disconnector can wear. If you examine your upper closely and the movement of the BCG it can only wear so much.

I have seen M16's, A1's and A2's that have had years of service put on them and continue to function. People need to get around the concept that stuff wears out. The basis of the weapon may stay the same- lower receiver, upper, extension, but anything else will at some point need replacing.

[TurretGunner]

What determines when an M4 is "worn out"?

I understand less wear is always better, but does the relationship between the BCG and the upper matter, or only to the extent that excess friction to the point of failure is not created during cycling, and that the bolt is guided into the locking lucks of the barrel extension?

What exactly are the "critical" wear areas (if any, besides the bolt/barrel extension/throat) in the M4/AR-15?

Unless you are shooting over 1000rnds a week, for YEARS, you are not going to wear out a reciver/BCG. Most shooters will never even wear out a barrel in their lifetimes.

[Sgt_Gold]

What exactly are the "critical" wear areas (if any, besides the bolt/barrel extension/throat) in the M4/AR-15?

The critical measurement for an M4 is head space. As others have said the Army doesn't measure gas port erosion, and when springs fail they get replaced. Every once in a while I saw gas rings fail, and there is the very rare trigger\sear pin rounding out issue, but head space is critical above all else.

Head space gauges come in three flavors, but unless you build rifles under the TDP, or are really into QA\QC, you really only need a Go and a Field gauge. The go gauge tells you you have enough head space to safely chamber and fire a round. The field gauge tells you when you have too much head space and it's time for a new barrel. Some manufacturing standards require a new rifle chamber to have a head space reading the falls within a small range of measurements. What a no-go gauge does is measure to see if the new chamber is too long. If a rifle fails (i.e. closes on) a no-go gauge, it doesn't mean the gun is unsafe. What it means is there is more head space then the spec calls for. I've seen plenty of rifles fail a no-go and come no where near failing a field gauge test.

[Cameron]

As IG said the reality is that a worn part really doesn't cost much to replace. That said I really hope, even more than hope, I am actively working towards having the funds and time to wear out an AR.

Cameron