Chamber throat questions

I recently had a customer bring in a near new (<50rds) Remington 700 SP in 338 LM. Since the get go, he's been splitting cases using factory Lapua rounds as well as Remington. There is definite case head separation. Headspace is spot on. I have borescoped the rife and found a burn mark right at the throat. SAAMI spec's for this only call for a .152 throat dimension, which this probably has or even less. I intend to do a chamber cast to get exact dimensions. I did the bullet slipped in fired retensioned case and found that I have about 0.50" to touch the leade. I am thinking that having a short throat, that it is not getting enough jump or is hanging up on a rough throat causing the overpressure. I currently plan on using a throat reamer and deepening the throat to one caliber diameter. Can anyone offer any experience on a situation like this? This guy has better than 5k in this rifle and I don't want to ruin it more than it already is. By the way, he emphatically does not want to send it back to Remington. Suggestions?

Factory loads splitting? Where does the cases split? and how?
High pressure alone should not cause this, and not before hard extraction and leaky primers. I'd triple-check headspace and chamber dimensions before final judgment. Something must be waaay of...

Near new, splits factory ammo, 5K in the rifle and doesn't want to send it back to Remington......is there something he is not telling you ????
A picture or two of the split cases would help. Like you said I'd do the chamber cast and compare to SAAMI specs before planning a fix.

He said that he doesn't want to wait on Remington and not get it fixed in the long run. I agree there. The splitting is where you see the average head separation. Chamber cast is definitely my first step. One of the reamer manufactures tells me that the SAAMi specs for the throat are much shorter than normal for this caliber, especially if you consider Weatherby's with their 3xcaliber freebore. I am just wondering if it's got a real rough throat that is hanging the bullet there long enough to build pressure up. 0.050" off the lands is pretty good jump too. I will post more after the chamber cast.

So you are saying he is getting head separations? How does the shoulder length on the case compare with the shoulder length of the chamber? Could the brass be too short in the shoulder dimension or the chamber too long?

I was going to post a link to the SAAMI drawings for .338 Lapua Mag but their table of contents says "PENDING" and there is no link.

The .338 Lapua Mag. has been listed by SAAMI as pending for at least 5 years now. To the best of my knowledge SAAMI has not moved on this cartridge. It is listed on the CIP web site and a link to the chamber and cartridge dims can be found at the following: 338 Lapua Mag. en.pdf

CIP updated this drawing this year and is the most current version available. It would be nice if SAAMI adopted this cartridge. However, until that happens CIP is the official last word on the .338 Lapua Mag. specs.

I would look first at chamber near where case splits..

Are the fired cases oversize in OD at separation???

A hung up reamer holder may have cut rear of chamber oversize..

You are checking throat with what bullet? This is only a good throat measurement for that bullet..

Ogives vary wildly, and are hard to measure visually...

Case head separation, is not a usual sign of overpressure, in a properly cut chamber..

How is headspace checked?? Clean chamber, no ejector or extractor interference.?

Bolt stripped of FP and spring??

False bearing of bolt handle, only one bolt lug bearing, scope mount screw interfering with bolt lug...???

Sounds to me like he is sizing his cases down too far causing his cases to stretch to far.

Jim Kobe said:

Sounds to me like he is sizing his cases down too far causing his cases to stretch to far.

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That would be hard to do with a factory load, don't you think...

Thanks for the responses! There are some things there for me to continue to checkout and will probably be more evident when I do the chamber cast.
When I say "cases splitting", it would have been more accurate to say that he had "case head separations". The OD of the brass was a couple of thousandths larger...so it did expand at the head area. The separation was nearly all the way around. The fact that this happened with factory loads really does make me doubt real over pressure. The comment on the shoulder dimension of the chamber being long, or the head area of the chamber being over spec are some real possibilities to check. This is happening both with light reloads and factory loads, so the brass stretching and separating makes sense to me.
Head space checked with the extractor removed, along with the FP and spring, so no tension on the bolt. Closes on GO, doesn't on NO GO. The bullet seating was check with a Sierra Match King, which I though would be more typical than a VLD type.
The specs I used were from the CIP site. I loosely called them SAAMI specs. Thanks for that point though. I actually received that info via one of the reamer manufactures and that is where they get them from.
Thanks for all the ideas. I will let you guys know what I find after I do the chamber cast.

How much for and aft play do you get on a factory rd with a stripped bolt? Or better, if you have some way to compare the 'headspace' dimension of the cartridge to your headspace gauge. Either could be off spec, or at opposite ends of the tolerance range. Although this is why the spec's for these dimensions are the way they are, to 'prevent' this.

A too short throat would be more likely to result in flattened or pierced primers I'd think.

The primers are very flattened with brass on the bolt face.
This is my Christmas vacation project. I just want to nail the problem down well. Once I get all of the dimensions from the cast, I will post it for further comment.

Have him chamber a loaded round and then eject carefully. I think the reamer/chamber may be short for the case neck. It may be easier to see if the neck/bullet junction is colored with a sharpie.


good luck
R

Case head separation is usually caused by too much headspace (and is not necessarily to overpressure). Too much headspace would make the round easy to chamber. A tight neck or short chamber/throat would make a round stiff to chamber. Given the situation with less than certain industry standards, it makes me wonder what specs factory ammo is made to. Brass on the bolt face can be caused simply by rubbing on the case when the bolt is closed and openned.
How much longer is a fired case when compared to an unfired case?

Very flattened primers are also an indication of a headspace problem.

Ditto

Headspace is THE most likely cause of case separation.

A partial list of things, that can throw headspace measurement off, is in my last post.

A rifle headspace problem, and possibly the gage is faulty, indicating headspace is ok..

Unless I do a headspace check myself.. I cannot trust anyone else's results.. Too many things to screw up/not take into account.

At severe excess headspace level, a simple comparison of shoulder to base measurement (new brass vs separated case with base held together) will show up with just dial calipers.

I am going to recheck headspace, do some of the brass comparisons tomorrow. I down loaded the CIP spec's to mark up as I check it. I did a very quick check of the base to shoulder measurement on one of the popped pieces of brass and it looks to be about .5 to 1.0mm longer on the bad one. base to shoulder spec is 54.9mm. I measure about 55.6mm.
With a rounded shoulder, how to get a real accurate measurement?
My headspace are new just out of the box. Hopefully, they are accurate.
The new rounds that separated were Lapua factory ammo.
This time when I check headspace, with the bolt stripped, I will put some dye check on the locking lugs to see how they mate up too. I didn't do that last time.
The brass on the bolt face looks more like "sprayed" on rather than "rubbed" on. Primer is flattened with cratering.

If you have approx .5mm or more excess headspace, you will have primer backing out upon firing. As case expands.. it stretches rearward, jamming slightly expanded primer back into case... Cratering will also occur..

Since the brass is being slammed into bolt face, not much rubbing action..

Section a separated case, and you should see additional thinning right where internal wall starts to thin. When case is fired, the entire thinner section of case expands to grip chamber walls.

Rear of case is thick, and does not expand/grip chamber. With no grip, internal pressure pushes head to rear, stretching cartridge brass until failure..

I became very familar with case head separations, and excess headspace over 30 years ago.. My very first centerfire rifle, was a #4 Mk1 .303 with excess headspace, and the longest bolt head installed.

Combined with the oversize/chamber (to allow for wartime dirt and grime) short case life was to be expected (even with careful, neck only sizing)...

Years of servicing belt fed machine guns (that had to have headspace adjusted, everytime barrel was changed in the field) in the military.. Have provided many chances to see separated case heads, due to improper headspace adjustment...

Tozguy said:

Case head separation is usually caused by too much headspace (and is not necessarily to overpressure).

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The key word is "usually". However, not ALWAYS. Further head separation can be caused by an improper chamber that has nothing to do with headspace.

Tozguy said:

Too much headspace would make the round easy to chamber. A tight neck or short chamber/throat would make a round stiff to chamber.

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Given that the bolt will close on a go gauge and not close on a no-go gauge. It is highly unlikely that headspace is the issue. This could be confirmed by shimming the bolt face a few thousandths and chambering go gauge. Typically there is less than 0.010" difference or less between a go gauge and a no-go gauge. I would shim the bolt face 0.010" - 0.012" and see if I could still close the bolt on a go gauge. If so then I would start to suspect a defective gauge. However, I have never seen a defective headspace gauge... Not saying it can't or hasn't happened. just saying it would be a rare occurrence.

Tozguy said:

Given the situation with less than certain industry standards, it makes me wonder what specs factory ammo is made to.

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There is NO uncertainty AT ALL regarding the specs to which either Lapua or Remington load the .338 Lapua Mag. Please see C.I.P. web site under headings TDCC (Tables of Dimensions of Cartridges and Chambers) linked above.

Tozguy said:

Brass on the bolt face can be caused simply by rubbing on the case when the bolt is closed and openned.

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Agreed... Brass on the bolt face will tell you nothing unless the base of the cartridge is engraved. Since OP made no mention of the ejector pin hole being transferred to the base of the cartridge...

Tozguy said:

How much longer is a fired case when compared to an unfired case?

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In the case of a head separation I doubt any meaningful data could be gained from such a measurement. Once the web is weakened to the point that a head separation can occur it is no longer possible to determine the amount of displacement within the chamber vs. the amount of displacement as a result of extraction / removal. Casting the chamber would be the only reliable measurement.

wesg said:

Very flattened primers are also an indication of a headspace problem.

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But not necessarily... It can be a sign of over-sized primer pockets... Further once the web has been compromised and head sep. occurs, nothing meaningful can be gleaned from a flattened or cratered primer.

abarnsley said:

Headspace is THE most likely cause of case separation.

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Most common to be sure.

abarnsley said:

A partial list of things, that can throw headspace measurement off, is in my last post.

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Yes!!! Post #7 Line 2... Given the information available at this time... That would be the direction I would be looking!

abarnsley said:

A rifle headspace problem, and possibly the gage is faulty, indicating headspace is ok..

Click to expand...

Possible, yes. Not as likely as a miss cut chamber... A mis-cut chamber would pass QC MUCH easier than improper headspace. Remington checks headspace on EVERY firearm. The chance of passing QC with a headspace issue then getting a bad gauge that also indicates proper headspace... Again, COULD happen. But not very likely.

Your description and understanding of head seperation, as stated, is spot on... I'm just not buying that this rifle has had 2 bad no-go gauges chambered, which allow it to pass a headspace check incorrectly TWICE.

OP, Nothing posted here is of any real value until you cast the chamber. At this point the best ANY of us can do is speculate. Once you cast the chamber take multiple measurements of the casting to insure the chamber is NOT out of round. Measure, record, rotate, repeat... I'm interested in hearing what the casting uncovers.

Lost longer post due to database error..

OP stated only .002 or expansion.. It takes MUCH more to cause case failure near base.. Shoot a .40 Glock lately

Oblong/miscut chambers, that are bad enough to cause case failure, should clearly visually show uneven expansion on fired case..IE bulge at case base.

Chamber cast is not going to provide much more info, than fired brass vs new brass in this case.. More precise, but we are talking .012 or more, instead of .002..

And will not show anything about headspace.. Fired case (both parts) has more than enough shoulder/headspace info...

Civilian/Non ISO 9000 certified mfg.. gages may not be calibrated like in the military, TB 43-180 Annually or biannually??... been 11 years, since I had to schedule that in..

Gages could have been mismarked. It happens. Have they been sent in for cal??

Bolt lugs might have cracks. Bolt handle bearing instead of lugs, plenty more.

Shoulder angle of chamber could be different than proper, but still headspace right. This still should not stretch base of brass to separation on one firing.

Pictures are worth a thousand posts

I want to see clear closeup pics of the failed brass, side by side with unfired...