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

[Swamplord]

look what I just found at a local gs

guy just put them out & these are the last 5 bags of Rem 375 H&H brass we will ever see here, was tempted to take all 5 but the 375 H&h is kind of popular out here and some poor bahstad will come looking for some, so I walked away with two bags

gonna pop 1 bag into 340 Wby and the other bag into 350 Rem Mag

.....

....

 

[longestrange]

look what I just found at a local gs

guy just put them out & these are the last 5 bags of Rem 375 H&H brass we will ever see here, was tempted to take all 5 but the 375 H&h is kind of popular out here and some poor bahstad will come looking for some, so I walked away with two bags

gonna pop 1 bag into 340 Wby and the other bag into 350 Rem Mag

.....View attachment 227333

....

Couldn't have happened to a more deserving guy. Thanks for all the help!

 

[hunter0528]

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.


View attachment 226569

Huntingtons.com/dies_caseform.html offers case forming dies for the 340 Weatherby

58260

.340 Weatherby Mag from .375 H&H Mag

G

91.98

 

[longestrange]

58260

.340 Weatherby Mag from .375 H&H Mag

G

91.98

Thank you very much for looking into this hunter0528, instead of just dismissing it 'out of hand'.
First, the bad news:
Before filling out the order form and sending in your $92, please click the 'shop online' link where you will find this statement (Their upper case, not mine):
'AFTER 70 YEARS OF MAKING SPECIAL AND CUSTOM DIES, THE BEAN COUNTERS AT VISTA (RCBS) HAVE DECIDED TO DISCONTINUE MOST (95%) OF THE ITEMS THE SPECIAL ORDERS/CUSTOM DEPARTMENT MADE. Including the 58260, unfortunately.
Now for the good news:
swamplord, over the last 10 posts or so, has detailed a method using a standard .340 weatherby sizing die as well as some kind of .375 die to do the same thing, along with actual examples. Others have chimed in as well. Assuming you have a 340 Wby die, which you will need to reload 340 anyway, you will need some kind of .375 die to enable the cases to feed into it. Twenty bucks or so should do it. Be warned, however, that SAAMI pressure specs for the 375 are about .8% less than for the 340 Wby. You can read the discussion above and judge for yourself.
That is a great elk, BTW. Am assuming you didn't get it in the peoples republik of MA

 

[longestrange]

Got some Cutting Edge 200gr MTH for this but thinking probably going with something else, although actually 'getting' anything else is a bit problematic. These have probably the best BC available bu for the shorter ranges probably not ideal. Anybody have any ideas?