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

longestrange

Active Member
Joined
Dec 26, 2013
Messages
29
I would like to convert .375 H&H (parent case) to 340 Weatherby Magnum Brass. RCBS formerly made a die(s) to do this, but has long since discontinued doing so. 375 brass is readily available and relatively cheap. 340 brass is costly and hard to find. I have attached the SAAMI specs for each case/chamber. Notable differences are that:
1. The 340 case has less taper, thus a fireform at some point is required
2. The 340 case uses Weatherby's double radius neck vs. the conical frustrum neck on the .375
3. The 340 case length and distance to neck are just a smidge less than the .375, a nice feature
4. LOA can be identical and be in spec
5. Both headspace off the stupid belt, but Weatherby's has tighter tolerances both for bolt and chamber

So what I have in my possession is a full length .375H&H (taper crimp) and a full length 340 Wby (taper crimp) two die set, along with a 340 Wby finish chamber reamer and a small lathe if needed.

So my first inclination would be to oil the cases and size them with the 340 die set sans plug/decapper. The die would be run down until the neck was all the way resized to the 340 dim's. on the outside. That would be .366 OD at 2.485 from the base. Not sure what the shoulder would look like at this point.
NOTE: If the cases were un-fired I would probably size them with the .375 die first to make the necks round.
Then I would 'somehow' ream out the inside of the neck to possibly .3353 or .3383 not sure, they are both in spec. I want to make sure there is enough case mouth thickness to grab the bullet when seating but not too much that it would push the case shoulder back instead of seating the bullet. Don't know what the standard practice is. Don't know whether the case mouth would have to be supported externally whilst reaming either.
Then I would trim to length.
Then I would anneal the cases and fire-form them. Do I need a real bullet for that? Then I would reload as normal except headspace off the shoulder if possible, maybe a full length resize would be needed on the first reload?
Anyway have never done a case conversion before so please add lots of comments even though it may seem obvious to you.


375Dim.jpg


340WbyDim.jpg
 

Doublezranch

Well-Known Member
Joined
Mar 24, 2011
Messages
1,277
Location
North Dakota
I really like your enthusiasm.

I apologize I do not have any experience converting 375 H&H to 340 Weatherby. However, I really like your enthusiasm.

IMHO, The reality is that there are so many factors that could possibly go wrong during the process, it’s not worth the time nor effort to make something that is already available.

I’m sure there are others out there that feel differently (which I’m very okay with). That’s the beauty of this forum.

Good luck on your project!!
 

HARPERC

Well-Known Member
Joined
Jan 28, 2011
Messages
6,424
Location
Spokane, WA
.....I really like your enthusiasm.
I apologize I do not have any experience converting 375 H&H to 340 Weatherby. However, I really like your enthusiasm.IMHO, The reality is that there are so many factors that could possibly go wrong during the process, it’s not worth the time nor effort to make something that is already available.
I’m sure there are others out there that feel differently (which I’m very okay with). That’s the beauty of this forum.

....Good luck on your project!!.....

Have you tried chambering .300 H&H or .375 H&H cases in the rifle?

Mostly keeping the conversation going, until someone with specific experience kicks in.
 

longestrange

Active Member
Joined
Dec 26, 2013
Messages
29
Please, everybody, I have some very good reasons for doing this conversion as in the original post. I have 375 brass and 340 dies, and 340 brass is very expensive and rare. Please post suggestions on the conversion process rather than why it won't work (RCBS used to make a conversion die) or how I can just get 340 brass for pennies.
Harperc: Thank you very much for your suggestion. I don't have any .300 H&H. The .375H&H would not fit in a 340 chamber at all.
 

MagnumManiac

Well-Known Member
Joined
Feb 25, 2008
Messages
2,404
I convert 375H&H to 375 Weatherby one of two ways.
I either fire form with COW (Cream Of Wheat/Semolina) and a fast pistol powder like Universal, about 1/3 of a max load in the PARENT case gets a good fire form. I actually use a load of 35gr of Universal under COW and a good tightly packed wad of TP.
The other way is to neck the round up/down, as I did when making all my Weatherby cases from 300H&H, load a middle charge for the CONVERTED case you wish to make and shoot to get your brass.
The beauty of belted cases is the fact you don’t need other steps, like false shoulders, the belt takes care of headspace just right.
I have even turned 300WM into 270Bee when brass was hard to come by, never had an issue doing this.
I even fired 300H&H rounds in my 340 Bee to get brass once...it worked perfectly.

Cheers.
 

MagnumManiac

Well-Known Member
Joined
Feb 25, 2008
Messages
2,404
I don’t understand why you say H&H brass doesn’t fit in the 340 chamber, you need to trim it to 2.825” to fit prior to fire forming. I have it fit without trimming.
It will shrink in the neck, somewhere around .020” in my experience. This is NOT an issue, as it will grow after each sizing anyway.
Please ask if you’re missing something, it’s quite a simple process.

Cheers.
 

longestrange

Active Member
Joined
Dec 26, 2013
Messages
29
I convert 375H&H to 375 Weatherby one of two ways.
I either fire form with COW (Cream Of Wheat/Semolina) and a fast pistol powder like Universal, about 1/3 of a max load in the PARENT case gets a good fire form. I actually use a load of 35gr of Universal under COW and a good tightly packed wad of TP.
The other way is to neck the round up/down, as I did when making all my Weatherby cases from 300H&H, load a middle charge for the CONVERTED case you wish to make and shoot to get your brass.
The beauty of belted cases is the fact you don’t need other steps, like false shoulders, the belt takes care of headspace just right.
I have even turned 300WM into 270Bee when brass was hard to come by, never had an issue doing this.
I even fired 300H&H rounds in my 340 Bee to get brass once...it worked perfectly.

Cheers.
Ah, I see the confusion here. I am converting 375 H&H to 340 Weatherby, not 375 Weatherby.
Thanks for the reply.
 

HARPERC

Well-Known Member
Joined
Jan 28, 2011
Messages
6,424
Location
Spokane, WA
.....Anyway have never done a case conversion before so please add lots of comments even though it may seem obvious to you.........

MagnumManiac covered it really well, better than I would. It's where I was leading to with my question before deletion.

I've not done specifically the .340 is why I hesitated a bit, but that silly belt makes some conversions fairly straightforward.
 

MagnumManiac

Well-Known Member
Joined
Feb 25, 2008
Messages
2,404
Ah, I see the confusion here. I am converting 375 H&H to 340 Weatherby, not 375 Weatherby.
Thanks for the reply.
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.
 

J E Custom

Well-Known Member
Joined
Jul 29, 2004
Messages
10,688
Location
Texas
Most all belted cartridges are based off the 375 H&H because it was the first. (Developed in 1912) so it is possible to form most belted cases from it. The problem I would have with doing this is the design requirements of the old case. In the beginning it was loaded with Cordite sticks (The reason for the long angle shoulder and loadings normally ran from 44,000 CUP's to a max of 50,000 CUP's. The 340 Weatherby can be loaded to 66,000 Psi and if the 375 cases were loaded to those levels, the results could be catastrophic.

Normally Manufactures hold all dimensions to factory spec. so the chances of getting an improved/heavier design would be slim. The 375 H&H is one of the greatest designs but like every old case design it has its limits.

It is a great idea, but cost should not dictate using something that could be problematic.

Just My opinion

J E CUSTOM
 

MagnumManiac

Well-Known Member
Joined
Feb 25, 2008
Messages
2,404
J E,
I run my 375Bee converted from regular everyday Remington made 375 H&H brass at full Weatherby pressure, never had an issue with excessive pressures, pockets stay tight even when a 300gr pill is pushed to 2830fps from my 24” barrel.
The cases are not weak and they weren’t made at the turn of the last century.
The OP’s issue is still not clear.

I have used 300Bee to form many other Weatherby cases and had no drama, yet I find Weatherby stamped brass soft, so when Remington were still making 300H&H brass I would buy it up. Same goes for their 375H&H brass, it’s good stuff. I also use Winchester brass, yet it’s harder to obtain in the last decade.

Cheers.
 

J E Custom

Well-Known Member
Joined
Jul 29, 2004
Messages
10,688
Location
Texas
J E,
I run my 375Bee converted from regular everyday Remington made 375 H&H brass at full Weatherby pressure, never had an issue with excessive pressures, pockets stay tight even when a 300gr pill is pushed to 2830fps from my 24” barrel.
The cases are not weak and they weren’t made at the turn of the last century.
The OP’s issue is still not clear.

I have used 300Bee to form many other Weatherby cases and had no drama, yet I find Weatherby stamped brass soft, so when Remington were still making 300H&H brass I would buy it up. Same goes for their 375H&H brass, it’s good stuff. I also use Winchester brass, yet it’s harder to obtain in the last decade.

Cheers.


I hear you :) but was not worried about when they were made, just that the design was for much lower pressures. It is good to hear that they will take the higher pressure and survive. I have found many older designed cartridges that would not take modern pressures that were newer brass. Maybe some companies recognized this weakness and beefed up their cases.

Anyway I just wanted to point out to the OP that there could be problems.

J E CUSTOM
 

memtb

Well-Known Member
Joined
Dec 30, 2013
Messages
1,365
Location
Winchester, Wy.
JE, I use “fire-formed”, .375 H&H brass for my .375AI....which as you know, is almost a twin to the .375 Weatherby. I have run .250 TTSX’s to well over 3100 fps, with no apparent issues. In fact, I settled on a load giving me 3130 fps. I have no idea what the pressures are running, but can speculate that they are pretty high. I’m not going to suggest that it is completely safe nor that others follow my lead, but, I haven’t blown-up my rifle.....yet! :eek: Oh....with a similar load was getting 10 firings per case before discarding the case! memtb
 

longestrange

Active Member
Joined
Dec 26, 2013
Messages
29
Most all belted cartridges are based off the 375 H&H because it was the first. (Developed in 1912) so it is possible to form most belted cases from it. The problem I would have with doing this is the design requirements of the old case. In the beginning it was loaded with Cordite sticks (The reason for the long angle shoulder and loadings normally ran from 44,000 CUP's to a max of 50,000 CUP's. The 340 Weatherby can be loaded to 66,000 Psi and if the 375 cases were loaded to those levels, the results could be catastrophic.

Normally Manufactures hold all dimensions to factory spec. so the chances of getting an improved/heavier design would be slim. The 375 H&H is one of the greatest designs but like every old case design it has its limits.

It is a great idea, but cost should not dictate using something that could be problematic.

Just My opinion

J E CUSTOM
Thank you for your input. Prior to undertaking this project, I had a look at the SAAMI 'voluntary' pressure specifications for both the 375H&H and the 340Wby Mag. Max average pressure for the .375H&H is listed as 62 ksi. Max average pressure for the 340Wby mag is listed as 62.5 ksi. That is about .8% difference. I doubt the lawyers for the ammo mfg's let them use antique brass and corderite anymore, but I'd love to have a few for my collection if you find any. Meanwhile, I'm willing to accept the risk and move on to converting some modern brass.

Here is a link to the specs, pages 33 and 32 for the 375 and 340:
SAAMI Specs
 
Last edited:

Primary

LRH Assistant
Here are some related products that LRH members are talking about. Clicking on a product will take you to LRH’s partner, Primary, where you can find links to LRH discussions about these products.

 
 
Top