Forming .458 win mag from .300 win mag

I did this very thing back when I owned a 458.
Anneal case.
IIRC 8-9 grains of fast pistol powder.
Small piece of cotton pressed on top of powder.
Fill to neck with corn meal/dry used coffee grounds.
Plug of wax or old bar soap in the neck.
Point straight up in the air, pull the trigger, and magic. Most of the time.
May have to trim case, I don't recall.

I recall I had WW, Federal, and Hornady brass, and even annealed, all the Hornady brass split. But when life gives you lemons, make lemonade. I trimmed the cases down to 2.10" and you basically have a 45-70 or 450 Marlin twin with a big bore jump. Considering the amount of freebore, they shot not bad at all.
 
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I fire formed them with the cream iof wheat method. This opens the case except for the end because the 300 Case is longer.. run the case over progressively larger expander balls until you can get the 458 expander in, then trim to length. It would probably be easier to trim them first, but I never did.
It would be easier to start with 338 Win brass, COW them and you’re done.
 
Dogleg said:
I fire formed them with the cream iof wheat method. This opens the case except for the end because the 300 Case is longer.. run the case over progressively larger expander balls until you can get the 458 expander in, then trim to length. It would probably be easier to trim them first, but I never did.
It would be easier to start with 338 Win brass, COW them and you’re done.
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I was given a bunch of .300 win mag brass a while ago. I have thought about fire forming but the rust of cracking the cases is a bit high. I have annealed them. I have some powders on hand like hs6 and 2400 might have others theirs a flake type powder that was in the lee load I I was given said win571 on the hopper but who knows and theirs a MEC bottle with another Winchester powder in it.
 
I suspect a variety of shotgun or pistol powders could be used for fire forming. Hate to use primers, given availability and cost.
I've heard that cornmeal has abrasive qualities. Would CoW be better?
Annealing and progressively sizing the necks up, then trimming might be the way to go.
I'm setting up to go the other way - shortening .458WinMag down to make .458x2" cases.
 
I used Unique (14 grains) because I had lots. I doubt it matters too much, any fast powder would work, just start low and creep it up until they stop going poof and start going bang, then play with it until your’re getting mostly filled out cases.you might be surprised how hot the barrel getting a hurry. You can shoot grasshoppers with the fireforming loads at a few yards. :)


I use the same method for making my 458 Lott brass out of 375 H&H, and what I call 458-300 with untrimmed 300 cases in a Lott chamber.
 
tiriaq said:
I suspect a variety of shotgun or pistol powders could be used for fire forming. Hate to use primers, given availability and cost.
I've heard that cornmeal has abrasive qualities. Would CoW be better?
Annealing and progressively sizing the necks up, then trimming might be the way to go.
I'm setting up to go the other way - shortening .458WinMag down to make .458x2" cases.
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It’s said that you can make .458 2” from any belted magnum I don’t know if the chamber cut allows for thickness of cases or if they need to be reamed/turned. I have cream of wheat on hand for blowing .303 out to .410.
 
I have tried making cases from bottlenecked magnums, .264 and 7mm, and case wall thickness was a problem. That is why I bought the .458s.
 
antiqueguy said:
It’s said that you can make .458 2” from any belted magnum I don’t know if the chamber cut allows for thickness of cases or if they need to be reamed/turned. I have cream of wheat on hand for blowing .303 out to .410.
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I found that keeping c.o.l. to 2.75" with 400/405 grain bullets and 2.85" with 500's created no issues. Shorter than 2.65" and you could tell it was starting to tighten up. YMMV

Edit to add, that I'm talking about a shorter cartridge case in the 458 WM, and not the 458 x 2" or 458 American.
 
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antiqueguy said:
What tiriaq is talking about is the .458 2 inch American. Basically a .450 Marlin with a standard belt.
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Yes, I understood that. I was just pointing out that if you used a stubby case in a 458 WM and tried seating to 2.50-2.55? you may run into the problem of needing to ream the inside of the case, but as long as the cartridge col was kept to 2.75", no problems, at least in the rifle I had.

Other post edited to clear it up
 
saskgunowner101 said:
Yes, I understood that. I was just pointing out that if you used a stubby case in a 458 WM and tried seating to 2.50-2.55? you may run into the problem of needing to ream the inside of the case, but as long as the cartridge col was kept to 2.75", no problems, at least in the rifle I had.

Other post edited to clear it up
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Too much of a jump to the rifling with the tapered throat and all.
 
I did sort of what you are thinking - started with 8x57 case - run in an expander for the 9.3x57 - all in one go - Hornady Custom Grade dies seem to have an expander ball made for that - none of the case shoulders collapsed or split - was unfired, new 8x57 brass though.

I have a number of previously fired 7mm Rem Mag that I have intended to COW out to 458 Win Mag - never tried that yet - no doubt could be done with expanders - 7mm to .30; .30 to .338; .338 to .366 - then use the .41 expander - might be able to get the .45 expander from the 458 Win Mag in there. I also have an RCBS .416 Rem Die Set - P/N 22701 - that expander looks tapered - maybe can go from .30 cal directly to .416 cal??

Thinking about it - that would expand the neck area enough to get a .458 bullet in there - I suspect it still needs to be fire formed to blow out the taper wall from the parent cartridge?

If you can tell me which brand of brass cases that you have - I might have some here - I could try to size them up from 300 Win Mag to 458 Win Mag without using a primer - I have never done so - thinking end result might be "waisted" or look like snake swallowing a golf ball.
 
Potashminer said:
I did sort of what you are thinking - started with 8x57 case - run in an expander for the 9.3x57 - all in one go - Hornady Custom Grade dies seem to have an expander ball made for that - none of the case shoulders collapsed or split - was unfired, new 8x57 brass though.

I have a number of previously fired 7mm Rem Mag that I have intended to COW out to 458 Win Mag - never tried that yet - no doubt could be done with expanders - 7mm to .30; .30 to .338; .338 to .366 - then use the .41 expander - might be able to get the .45 expander from the 458 Win Mag in there. I also have an RCBS .416 Rem Die Set - P/N 22701 - that expander looks tapered - maybe can go from .30 cal directly to .416 cal??

Thinking about it - that would expand the neck area enough to get a .458 bullet in there - I suspect it still needs to be fire formed to blow out the taper wall from the parent cartridge?

If you can tell me which brand of brass cases that you have - I might have some here - I could try to size them up from 300 Win Mag to 458 Win Mag without using a primer - I have never done so - thinking end result might be "waisted" or look like snake swallowing a golf ball.
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It’s in the tote I can dig it out in the morning. I think I have some mixed. Lightly annealed it shouldn’t be a problem going in steps for 7mm I’d start at .30 and work up. And if a bullet fits you can fire form with a normal load.

I’d like to try some of the things I been reading about like round ball loads I read at one point it was common in .45-70 to use a light charge of black powder(5-10gr or so) with a ball seated directly on top. Theirs also stuff like tri ball and obviously cast bullets ect basically down loading the .458 to plink and hunt everything from squirrel to elephant.

For my .410 from .303 brass I have of all things a bicycle pedal shaft that has 3 sections first smallest is .280 next step is .332-.335 and the next is around .415-.425
 
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