Wear in an M4?

[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

[skyugo]

I would think that the contact points between the bolt and the upper receiver would start to show excessive wear. We are talking steel against aluminum moving back and forth rapidly and frequently.

Or is there enough contact all around the bolt akin to a piston in an engine cylinder instead of just a couple of rails?

i seem to recall colt or someone stating that an upper receiver was good for either 40k or 70k rounds. proper lubrication would probably play a big part in how long this would last. regardless, it's a very long time.
springs bolts and barrels are what i'd watch.

[Cagemonkey]

When a weapon is gaged it gets recorded. Chances are before your port has eroded enough to cause issues, the headspace or bore erosion will already be an issue and stuff will start getting replaced.

No one dicks around with buffers or spring unless defective.

Thanks for the info IG.

[WS6]

i seem to recall colt or someone stating that an upper receiver was good for either 40k or 70k rounds. proper lubrication would probably play a big part in how long this would last. regardless, it's a very long time.
springs bolts and barrels are what i'd watch.

This is mainly what I was curious about. I wonder how long it would last run dry, etc. and how critical the tolerances in it are/what tolerances are critical.

[Iraqgunz]

Some people need to do more shooting and less thinking about the theoretical stuff.

[Tactical Joke]

If I can afford to shoot it enough to wear it out, then I can probably afford to replace it.

[TurretGunner]

If I can afford to shoot it enough to wear it out, then I can probably afford to replace it.

Yea man, You will spend approx 50x in ammo as you will in weapon parts last time I crunched the numbers. One of the reasons I don't buy into the BFH or CHF hype of the day that most of the vendors are pushing now. Its the SPR MOD 1 or Reece of the month.