I’ve got a .375 wildcat that’s very similar to the.375 Weatherby and I’d like to form cases from magnum basic brass. Yes, it’s easier to just fireform H&H or Weatherby brass, but that leaves me with improperly headstamped cases. What’s the best way to neck down the basic brass to something I can chamber for fireforming?

I can’t tell from picture but how much are you taking it down? If it’s .010 or less I would just lube it up good with Imperial and run one in my die and see what it looks like.
If it’s more than that I use an intermediate die “if you have one” with the expander removed.
Rule of thumb I use is go down about .010 increments although I have exceeded that.

Either way I would anneal once they are fire formed.

I would use a 44 pistol die pull the stem out size to the kneck or a little more then go to a 41 if you have one doe the same the use your 375 die set it a little high to create a false sholder to form from, used to do this with 375 h&h brass making stw brass before you could buy it

A while back I was considering building or buying an upper for an AR-15 in .375 SOCOM. I wondered to myself how case forming might go. I may not be remembering this correctly, so don't quote me or hold me liable...but I think I ran a .45 Colt case up into a .375 Winchester sizing die just too see what I could see, and this was the result:







I don't know if that will help to inspire a solution, but I guess it won't hurt a thing.

The basic brass is slightly larger than 458 Lott in diameter. I’ll try to step down dies with the expanders removed and see what happens.

Super Blackhawk with a 270g bullet please

Basic to 458 Lott to 416 Remington to 375 H&H with plenty of imperial sizing wax.

Confused. Just a little (or maybe a little more than that).

So, will .375 H&H fireform to your wildcat chamber?

Think of the wildcat as a 375 AI or 375 Weatherby minus the radius shoulder. I can fireform H&H or Weatherby brass, but that leaves me with a head stamp that doesn’t match the rifle. The wildcat has an aggressive shoulder and very little taper. Going from 416 Remington to the wildcat die destroys the case, but the gentle H&H shoulder seem to work. I’ll trim the case length then fireform and we’ll see what we see.

I see (I think). You gradually necked down basic to .375 H&H dimensions for fireforming, but do not want to fireform .brass that is headstamped as such. I think I'm up to speed.

That's the kind of thinking that I arrived at shortly after photographing it. As soon as I caught myself wondering what, where and how, I threw that case away.

I probably should trash the image file as well.

Yep, the whole soap opera is to end up with cases that don’t conflict with the chambering marked on the rifle for international travel.

Nope, it’s mine now. I will remind you periodically. Someone just needs to ream a cylinder then slap a .375 barrel on it!!

You can have it, but I'd not neck it down so far. About 70% of what you see there should be about right. More powder capacity and less silly neck length!

Well you’re lucky I’m not smart enough to add a photo to my signature. I tried though. MODS!! Help me out!!

Here. Try saving and working with this file:

I yield for the night. 03:30 comes early.

RR your necked down case reminds me of the 40-454 JDJ

Soooo…. RiverRider, the sierra # 2900 is a 200g flat nose flat base prohunter designed for the 375 Winchester functioning well around 2000 fps.



https://www.sierrabullets.com/product/375-caliber-200-gr-fn/

Look at Hawk bullets for it as well. The more I shoot them in the 375 the more I like them for hunting. The rifle bullets in .375 have different jacket thicknesses .025 is the same as on the handgun bullets.

https://hawkbullets.com/bullet-selection.html