.375 H&H Brass to .340 Weatherby Brass Conversion Question

J E Custom

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Not trying to argue, but This is the Weatherby Pressure (Actual) for the 340. There is no SAMMI standard for the Weatherby's but the factory loads fall within SAMMI guidelines.


I hold all Of my wildcats to 62,000 to 63,000 psi and find all the performance I want so the pressure you want to use should be fine unless you want factory performance.

Just Saying :)

J E CUSTOM
 

longestrange

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Not trying to argue, but This is the Weatherby Pressure (Actual) for the 340. There is no SAMMI standard for the Weatherby's but the factory loads fall within SAMMI guidelines.


I hold all Of my wildcats to 62,000 to 63,000 psi and find all the performance I want so the pressure you want to use should be fine unless you want factory performance.

Just Saying :)

J E CUSTOM
JE, I am sorry but you are becoming a serious problem on this thread. I am trying to get conversion data, and you have countless times repeated yourself on an issue that is beat to death and distracting from the thread. Below are the 340 and 375 specs from SAAMI that you could have found merely by clicking on the link I provided above. As stated above, they are all 'voluntary' but nobody in their right mind would ignore them.
Please play nice in the sandbox or I will just delete every one of your posts.
375HH.jpg


340Press.jpg
 

yorke-1

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Longestranger, I think you're overthinking the conversion. It's really as simple as running a 375 H&H case into a 340 Weatherby die until the bolt will close on a piece of brass in your chamber, trimming to length and either COW forming (like MagnumManiac described) or firing a moderate load in the converted case.

I also wouldn't be so quick to dismiss what J E Custom is saying. The internal web design of 375 H&H brass is likely different than the specs on 340 Weatherby brass. It may not make a big difference, but the difference is still there. As an experiment, once you've successfully formed the 340 brass from 375 H&H brass, measure the internal case capacities of Weatherby headstamped 340 brass and your converted 375 H&H brass. I bet you'll end up with different capacities and those are caused by differences in the internal case dimensions. I believe that's all J E was trying to explain.
 

longestrange

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I understand you are converting 375 to 338, it’s a simple process.
Neck down to 338 and fireform. The case will fill the chamber and produce a nearly fully fire formed piece of brass.
Can you explain to me what difficulty you are having chambering a 375/338 necked piece of brass in your rifle?
I just necked down a 375 piece to 338, chambered it and it needs to be trimmed, this is the only issue I can see, once trimmed, it chambers perfectly.
Cheers.
OK, hopefully we've buried the pressure thing that's been beaten to death, let's move on to this as it is more closely related to my question.
I agree that you can neck down a 375 to 340, fireform and trim to fit nicely in the chamber. I have some serious safety issues with actually loading magnum charges into such a case.

For starters, necking down 'work hardens' the brass, thus it has a higher yield strength. It also thickens the material in the case neck. A thicker, higher strength neck is going to resist releasing the bullet far more than a soft, annealed thin neck. That will result in higher pressures.
Second, the neck expanding is what releases the bullet and establishes a seal between the chamber and the case. If the neck is too thick, it may expand on the outside, but on the inside it may not fully release the bullet. This also will result in higher pressures and possible gas leaks.

Thus it is standard practice to:
1. Anneal a case neck after it has been necked down back to factory specs.
2. Turn or ream the neck to thin it back to factory specs.

So my original question in my original post that has remained unanswered after more than a page of posts was about the neck annealing and reaming.
 

MagnumManiac

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Again, I will mention case design.
As most know on here, I own a pressure trace and I DO actual destructive testing such as sectioning cases to SEE web thickness, case wall thickness and even hardness tests.
I also anneal cases and carry out the same tests to see what DIFFERENCE it makes, not only to performance but case longevity.
Weatherby case dimensions are metric and slightly different to H&H imperial dimensions...but it matters not as the difference is minuscule.
Now, here’s the kicker and, my final word on this matter. Norma manufacturers Weatherby head stamped brass, it is excellent brass if used within it’s limitations, which are two fold:
1) It is soft and does NOT take to max loads for very many firings, 3 is tops for it.
2) It is the ONLY brand made with webs as thin as .025” on the market. This fact makes it even more prone to early failure (read as primer pocket expansion) than nearly every other brand out there.
After sectioning multiple brands of brass, two stand out as the toughest on the market; Lapua and older Winchester. Both are HARD in the head and have thick webs, Winchester having the thickest web I have seen so far, above .060” thick in most cartridges of .473” and above head size. New Winchester brass is no longer the same quality unfortunately.
So, take this as you will, but I DO NOT use Norma/Weatherby brass to make or convert to other cartridges that require large fire forming steps.
I was given 20 boxes of Norma factory 22-250 rounds from a deceased estate, even that stuff is soft in the head...go figure it’s around 30 years old, so Norma have never changed their internal structure on their brass.

Cheers.
 
Last edited:

Lefty7mmstw

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I've been making 7stw from 375 h&h brass for years whenever I need to. It's just fine... Don't overthink this stuff... Neck, fireform, trim, and shoot...
An alternative is for the o.p. to neck up 300 roy brass; it's easier to find and will fill the bill just as well...
 

MagnumManiac

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So my original question in my original post that has remained unanswered after more than a page of posts was about the neck annealing and reaming.
I have never had to turn necks after necking down from 375 to 338.
I have to turn down my necks on my 338-416 Rigby Improved due to the huge difference from 416 to 338, but I also have a tight neck chamber.
Yes, I anneal my brass prior to fire forming AND necking down, but a word of warning, in some dies that do not have a tapered expander, an annealed case can crush the neck, so depending on this, it may be better annealing after the neck is reduced.
The issue with the neck increasing pressure is only partly correct. The neck releases the bullet early in the ignition phase of the combustion sequence, it only takes a little pressure to do this. If you don’t believe this, then a simple test will show that even the pressure developed from a magnum primer is enough to expand the neck and release the bullet. If you prime a case and seat a bullet that is one cal smaller and fire it, the distance that bullet will travel is rather amazing and measuring the neck before and after will show just how much the neck expands just from that little test.
So, the pressure increase you are concerned with really is moot. I doubt you will need to ream or turn your necks, unless you have a tight neck chamber, which I doubt as it sounds like a SAAMI chamber and most of these have a .006”-.008” clearance in the neck anyway.
My only SAAMI chamber in my Weatherby rounds is my 270Bee, it has the SAAMI .376” long throat, all my other chamberings have the
.500” throat that Roy originally used and they are .0003”-.0005” over bullet diameter, which is tighter than most modern throat designs.
In fact, Roy’s own original designs ran 1/2”, 3/4” and 1” throats when they were still proprietary and had no SAAMI influence. In fact it was CIP in Europe who first recognised them as a factory offering.

Cheers.
 

longestrange

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Joined
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Messages
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Again, I will mention case design.
As most know on here, I own a pressure trace and I DO actual destructive testing such as sectioning cases to SEE web thickness, case wall thickness and even hardness tests.
I also anneal cases and carry out the same tests to see what DIFFERENCE it makes, not only to performance but case longevity.
Weatherby case dimensions are metric and slightly different to H&H imperial dimensions...but it matters not as the difference is minuscule.
Now, here’s the kicker and, my final word on this matter. Norma manufacturers Weatherby head stamped brass, it is excellent brass if used within it’s limitations, which are two fold:
1) It is soft and does NOT take to max loads for very many firings, 3 is tops for it.
2) It is the ONLY brand made with webs as thin as .025” on the market. This fact makes it even more prone to early failure (read as primer pocket expansion) than nearly every other brand out there.
After sectioning multiple brands of brass, two stand out as the toughest on the market; Lapua and older Winchester. Both are HARD in the head and have thick webs, Winchester having the thickest web I have seen so far, above .060” thick in most cartridges of .473” and above head size. New Winchester brass is no longer the same quality unfortunately.
So, take this as you will, but I DO NOT use Norms/Weatherby brass to make or convert to other cartridges that require large fire forming steps.
I was given 20 boxes of Norma factory 22-250 rounds from a deceased estate, even that stuff is soft in the head...go figure it’s around 30 years old, so Norma have never changed their internal structure on their brass.

Cheers.
Very well stated, thank you. What you are saying is in direct contradiction to what JE was saying - that the 375 brass is a lot weaker than the 340 brass due to web thickness. If what you say is true, necked down 375 brass is far superior to original Norma/Wby brass in terms of strength.
I also have sectioned a belted magnum case to reveal a fatal flaw. Belted cases headspace off the belt. If the neck does it's job and seals to the chamber, it also clamps the mouth of the cartridge. There is always slop in the belt to boltface distance by design, and cases stretch just ahead of the belt where it is thin until the case head touches the bolt. They definitely will thin down there until eventual failure IF the case is fully resized. Weatherby chamber/cartridge specs are tighter in this respect, so it may not be as rapid or pronounced. That being said, I never full length resize belted magnum cases after firing because they will go back into the same rifle. This results in headspacing off the neck instead of the stupid belt, eliminating the problem.
This brings up another question I had in the first post, not answered after over a page of replies, as to whether the case should be full length resized (once) after the initial fireforming or not. There may be some case taper issues down where the brass is hard, that is my worry.
 
Last edited:

yorke-1

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OK, hopefully we've buried the pressure thing that's been beaten to death, let's move on to this as it is more closely related to my question.
I agree that you can neck down a 375 to 340, fireform and trim to fit nicely in the chamber. I have some serious safety issues with actually loading magnum charges into such a case.

For starters, necking down 'work hardens' the brass, thus it has a higher yield strength. It also thickens the material in the case neck. A thicker, higher strength neck is going to resist releasing the bullet far more than a soft, annealed thin neck. That will result in higher pressures.
Second, the neck expanding is what releases the bullet and establishes a seal between the chamber and the case. If the neck is too thick, it may expand on the outside, but on the inside it may not fully release the bullet. This also will result in higher pressures and possible gas leaks.

Thus it is standard practice to:
1. Anneal a case neck after it has been necked down back to factory specs.
2. Turn or ream the neck to thin it back to factory specs.

So my original question in my original post that has remained unanswered after more than a page of posts was about the neck annealing and reaming.

I personally don't worry about annealing after going from 375 down to 338. I would neck them down then fireform the brass however you see fit. I would anneal after the first firing though. If you were necking down beyond 338 or doing it in multiple passes it might be worth annealing after every step I suppose. I've necked down literally thousands of 375 cases (a mix of H&H, RUM, and Ruger) to various neck diameters and the only time I anneal them before fireforming is when I have to size the neck down in multiple steps or going beyond 338.

Instead of inside neck reaming, why don't you turn the outside of the case necks? Just run an expanding mandrel like the K&M Expand-Iron through the necks of your formed cases and it will push the donut to the outside of the case neck where it can be turned down to the appropriate diameter. I find that to be much easier than inside neck reaming.
 

MagnumManiac

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Feb 25, 2008
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I do not FL size any of my brass, I partial FL size so that the shoulder is bumped .002”-.003” and the case head spaces off the shoulder and not the belt.
You need to look at the belt as a fire forming tool only, once that is done the shoulder takes over from there.
This interweb notion about expansion above the belt and thinning there on the first firing is a myth. The case will only thin there if the shoulder is bumped back too much, say in the order of .005”-.008” each time it is fired.
Once you fire form the brass, I would adjust a FL die so that only a partial neck reduction takes place, say 3/4 of the necks length, Weatherby’s have long necks so there will be ample bullet grip.
After that second firing I would be bumping the shoulder .002”-.003” max and be done.
The cases will need that second full power load to fully form to the chamber.

Cheers.
 

Wild_Bill

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794
Ok i will answer
Size the 375 cases in your 340 FLS die as you do this dont size so the shell holder touches the die slowly size 1 case at a time until a case will just close with slight feel on the shoulder. This will help the cases form without creating a stretched case. If you have an annealer then anneal if you want to it is not critical then trim to max 340 cas length if required. Next seat a projectile in a case and measure the loaded neck diameter. Now measure your reamer and make sure you have at least .003" clearence if it is tighter than this neck turn dont ream, reaming is really concentric and can give irratic release of the projectile. Now you have sized cases that fit in your rifle look for a starting 375 load with a slow powder that matches your 338 projecile weight and compare that to a 340 starting load, use the one that is the lightest.
Fire form the cases.
After fire forming full length size your cases and trim to trim length however forget that they have a belt. Size them like a 270 etc by only bumping the shoulder back .002" use hornady shoulder comparator to measure that. You can anneal between firings if you wish to it should extend your brass life. Now you have to work slowly up from your forming load to a max load in your rifle. You might find it matches the max data in the loading manual but it might also be less depending on the fireformed brass volume and your match chamber dimensions.

Full length size everything but use compartors to know when they are sized to the minimum required that will help you obtain maximum brass life.
 

Wild_Bill

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794
I do not FL size any of my brass, I partial FL size so that the shoulder is bumped .002”-.003” and the case head spaces off the shoulder and not the belt.
You need to look at the belt as a fire forming tool only, once that is done the shoulder takes over from there.
This interweb notion about expansion above the belt and thinning there on the first firing is a myth. The case will only thin there if the shoulder is bumped back too much, say in the order of .005”-.008” each time it is fired.
Once you fire form the brass, I would adjust a FL die so that only a partial neck reduction takes place, say 3/4 of the necks length, Weatherby’s have long necks so there will be ample bullet grip.
After that second firing I would be bumping the shoulder .002”-.003” max and be done.
The cases will need that second full power load to fully form to the chamber.

Cheers.
Mate you do full length size you are doing it correctly
 

MagnumManiac

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A belted case does not STRETCH on the first firing at the web, this is a myth.
The belt holds the case, the rest is blown forward and out. I have cases here in partial fire formed condition with the first third of the case from the neck down blown out while the rest of the case is not. A second COW load is needed. I can deliberately add powder in small amounts and blow the cases out further down until one is fully formed. No stretching occurs and the necks shorten as the brass is pulled from here, not from the web or lower case walls.
I have never stretched a case fire forming it, not a single belted case or an AI case.

Cheers.
 

Wild_Bill

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Messages
794
A belted case does not STRETCH on the first firing at the web, this is a myth.
The belt holds the case, the rest is blown forward and out. I have cases here in partial fire formed condition with the first third of the case from the neck down blown out while the rest of the case is not. A second COW load is needed. I can deliberately add powder in small amounts and blow the cases out further down until one is fully formed. No stretching occurs and the necks shorten as the brass is pulled from here, not from the web or lower case walls.
I have never stretched a case fire forming it, not a single belted case or an AI case.

Cheers.
Measure the heaspace guage then measure the belt of the case. Come back to me after that. Most belts are a lot smaller than the go guage we usually fit belted mag barrels to the brass not go guages because of the differences Belted mags can and will stretch if loaded without regard to headspace off a shoulder when resizing.
If the shoulder is pushed back to far it can stretch.
 

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