"A" Preventative Maintenance Program

[toddackerman]

You can't really fix a number to barrel life.

Assuming that the other parts of your shooting system are in order, when the barrel stops hitting what you are aiming at, it's probably time to replace it.

For highpower shooters, this might only be a few thousand rounds.
Threegun shooters will go longer on a barrel.
Slow-fire plinkers will go a long long time before wearing a barrel out.

If you don't shoot much beyond 100-150 yards, you probably won't even notice when your barrel IS getting worn out.

Other than a "No - Go" gauge, any thoughts on how to know if your chamber is getting too long, creating head space issues like case separation?

I thought I had such a problem, but in fact it was a bad lot of processed USGI brass from someone other than my usual source. This stuff must have been shot in a SAW or similar large chamber weapon. After loading only one round in these cases they exhibited extreme case length as measured in a Dillon Field Gauge, and maybe 1/10 % case separation in the middle of the case body. I haven't had this issue with any other lots than the one outlined. But it sure had me wondering! Now I have to check each case wall and trim after only one loading, Arghhhhh!!

Tack

[AR15barrels]

Other than a "No - Go" gauge, any thoughts on how to know if your chamber is getting too long, creating head space issues like case separation?

The proper gauge to use to check maximum chamber length is the "field" gauge, not the "no-go" gauge.
The "no-go" is only used on brand new barrels.

More in this thread

[toddackerman]

The proper gauge to use to check maximum chamber length is the "field" gauge, not the "no-go" gauge.
The "no-go" is only used on brand new barrels.

More in this thread

So is the No-Go Gauge a tighter tolerance (longer than the No-Go Gauge) as far as chamber length? Need a little more detail please.

Also, this question is in relationship to the throat. I can't see how throat erosion would be an issue as far as safety because the throat is ahead of the chamber, including the case neck. Is throat erosion something to be concerned about as far as excessive Head space, or because it is ahead of the chamber, it doesn't effect safety, brass stretching, head space etc.?

Thanks for your continued inputs.

Tack

[Diz]

I was reading through BR II tonight and noticed something interesting. The author states that Colt designed a better bolt and bbl extension for the carbine length models, designed to "cope with the higher rate of fire and impact" (BR II P.71). When they offered it to the gov't, they were turned down, because the gov felt it wasn't needed. No date is given so it could have been any time between say 1985 and 2003(?) I would be interested in seeing just what Colt came up with. Would there be any resemblance to the LMT BCG?

One other little nugget related to our discussion. The M-4/M-4A1 (initial acceptance) mil-spec calls for a min service life of 3K for: the ejector spring, FCG springs, extractor, and E. pin. But only 2K for the extractor spring. Hmmm. Four rifles from each lot of 1,000 are test-fired for 6,000 rds to see how things are going (PP 88-89).

So this pretty much backs up our 3K figure of merit for minor overhaul. With good lube, mags, and ammo we should be able to keep our "M-4's" up and running until the head space opens up past the safe limit. Which will probably be well over 10K for most of us, perhaps as high as 20K (or more, who knows).

Thanks to all for your comments.

OK, so here is "A" modified "M-4" maintenance Schedule:


Every 3K:
-Check head space. When out of spec, replace bbl and bolt. Re-check.
-Check extractor. Replace if chipped, worn, or broken.
-Replace HD extractor spring, black insert (optional?)
-Replace Crane O-ring or D-fender.
-Check ejector. Replace if worn or broken.
-Replace ejector spring.
-Replace gas rings.
-Replace cam pin.
-Check firing pin. Replace as req.
-Replace FP retaining pin.
-Check buffer spring length (10 1/16" - 11 1/4") Replace as req.
-Check key way for proper torque/stake
-Check reciever extension for proper torque/stake
-Function check fire control group.
-Check mags. Rebuild/replace as req.

At 10-12K:
All of the above plus:
-Replace fire control group and springs, as req.
-Replace mag catch/ spring, as req.
-Replace mag detent/ spring, as req.
-Replace pivot pins, springs and detents, as req.
-Re-finish if desired (KG GunKote, Lauer, etc.)


At bbl change:
Post at M4C how many rounds you got through your bbl.

[Heavy Metal]

If I felt it necessary to replace everything you list at the round counts you post Diz, I would have sold my AR's long ago and switched to AK's.

For example, D-Fender's are rated by MGI to be good for the life of the bolt. That is why you pay more for them.

I have also never seen an ejector spring go bad. Thought I did once but it was the extractor spring. Unless you are really cooking the bolt, they should easily run 10K rounds.

The easiest thing to do is replace all small springs and the buffer springs with Chrome-Sillicon springs, they are far more resistant to losing temper thru heating and can withstand tens to hundreds of times the cycles of conventional springs.

[LukeMacGillie]

So is the No-Go Gauge a tighter tolerance (longer than the No-Go Gauge) as far as chamber length? Need a little more detail please.

Also, this question is in relationship to the throat. I can't see how throat erosion would be an issue as far as safety because the throat is ahead of the chamber, including the case neck. Is throat erosion something to be concerned about as far as excessive Head space, or because it is ahead of the chamber, it doesn't effect safety, brass stretching, head space etc.?

Thanks for your continued inputs.

Tack

Tack,

Its not so much a safety issue as its a reliability issue. If the throat errodes, the cartridge seats deeper and you get extraction problems.

[Heavy Metal]

I thought the shoulder set the forward limit on headspace and not the leade of the rifling?

[AR15barrels]

So is the No-Go Gauge a tighter tolerance (longer than the No-Go Gauge) as far as chamber length? Need a little more detail please.

Did you carefully look through my headspace info PDF which was linked in that thread?
Your headspace questions point out (to me) that you did not.
I will defer to your reading/understanding the provided documents and gaining a further knowledge from them and then being able to ask more finely honed questions.

Also, this question is in relationship to the throat. I can't see how throat erosion would be an issue as far as safety because the throat is ahead of the chamber, including the case neck. Is throat erosion something to be concerned about as far as excessive Head space, or because it is ahead of the chamber, it doesn't effect safety, brass stretching, head space etc.?

Throat erosion is completely different thing than headspace.
It does not effect safety at all.
It does effect accuracy and (ever so slightly) velocity.

[AR15barrels]

If the throat errodes, the cartridge seats deeper and you get extraction problems.

Don't make me slap you over the confusing wording of your reply.

The shoulder in the chamber stops forward movement of the cartridge.
Shoulders don't (measureably) wear with use like throats do.
Bolt and extension lugs do wear and these effect net (total) headspace.

The bullet does NOT seat in the throat until it starts leaving the case in the course of firing.
The length of the throat does not effect the depth to which a cartridge seats in the chamber.

[AR15barrels]

I think it's time to establish some basic terminology to ensure that we are all on the same page.
Holding a cartridge in your hands, look closely at it and follow along.

The base (head) of the cartridge is where the headstamp and primer are.
The extraction groove is the area between the front face of the rim and the rear of the chamfer at the beginning of the case body.
The case body ends at the shoulder.
The shoulder ends into the neck.
Headspace is established as a standard length between case head and shoulder to a standardized datum diameter on the shoulder.
The case neck ends at the front of the case.
The bullet is seated into the end of the case.
There will often be a short section of the bullet shank at full bullet diameter before the bullet starts to taper to a point.
The chamber has a corresponding cut at or above bullet major diameter called the throat or freebore.
The throat terminates into the rifling via the leade which is basically just a long angle between throat diamter and bore diameter.

Here is an extreme close-up drawing of a 6.8 SPC neck and throat showing the different areas of interest:



Of particular note is the case neck in yellow and the bullet drawn in blue.
Note that the straight shank of the bullet terminates into the bullet's ogive (curve towards the point) roughly over the "o" in "throat" on the drawing.
Also note that the actual throat in the chamber is further forward than the straight shank part of the bullet.
Also note how the Ogive of the bullet mor than clears the Leade angle of the throat.

Zooming out to a wider view, look at this drawing of 223 and 5.56 throats:



This drawing is NOT a scale drawing.
It shows the neck drastically shorter than it really is, but it does show the actual case body ending into the shoulder and then into the neck.

Now look at this picture of a 300x221 chamber reamer:



The 0.336" area cuts the neck portion of the chamber.
The 0.308" area cuts the throat/freebore of the chamber.
The 0.306" point is a mid-point of the leade angle between throat and bore diameter.
The brass bushing at the top is sized to the bore and would typically be around 0.300" diameter.

Is this starting to make sense yet?

Throat erosion is basically when the throat gets larger in diameter AND the leade angle wears forward, creating a longer (than when new) cylindrical section forward of the case neck.
The longer throat then presents a longer gap for the bullet to jump when it's pushed out of the case by the rapidly expanding gasses created by the burning powder.
It is this longer jump that allows the bullet to mis-align with the bore/groove and let the bullet engrave into the leade ever-so-slightly crooked that causes accuracy to suffer.
If you have looked closely at my 223 & 5.56 chamber reamers comparison, you would note how much larger a 5.56 chamber's throat is over bullet diameter.
You might even have a light come-on in your head and start to understand why the 223 Wylde is such a great chamber after seeing the differences between the reamers.