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Thread: Forming 357 Sig from 40 SW

  1. #1
    Boolit Buddy
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    Forming 357 Sig from 40 SW

    Is anyone doing it . does it work well or a pain in the neck?

  2. #2
    Boolit Bub

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    They worked fine for me but do run a few thousands short in length , other than that they seem fine .

  3. #3
    Boolit Buddy
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    Does being short affect feeding ?

    I have heard that the 357 Sig is a very good feeding cartridge. What say you.

  4. #4
    Boolit Master dkf's Avatar
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    I made some already out of .40 already, they were .020" short. Easy enough to make. The neck is small enough so as it is and it is not 1999 anymore so 357sig brass is available. So if I need brass I'll buy once fired .357sig brass an done. When you got around 1/8" of neck gripping the bullet .020" can make a difference.

    It does not affect feeding from my experience.

  5. #5
    Boolit Buddy
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    Thanks for the feedback.

  6. #6
    I'm A Honcho!

    DukeInFlorida's Avatar
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    CAUTION

    One of the known issues with reloading the 357 Sig (my current carry chambering, in a Sig P229) is that of bullet set back. That is to say, that the grip length on a proper 357 Sig case is already so short that just installing and reinstalling cartridges into a magazine can cause the bullets to push enough into the case to create an over pressure (dangerous over pressure) situation.

    When trying to reform .40 S&W into the 357 Sig, as has been previously pointed out, the grip length on the neck/bullet is even worse!

    The body diameter will be fine. The shoulder will be in the right place. But, there won't be enough remaining material to form a full length neck. And, so the grip length will be too short, and bullet set back an even worse problem for anyone who tries this.

    The only brass you should try to reform into 357 Sig is the 10mm auto. But, that presents an additional step, trimming to length. Add to that the factor that 10mm brass is currently difficult to find, and you are better off, in my mind, to just seek out and find proper 357 Sig brass.

    I deal with the bullet setback on proper 357 Sig brass by using a Lee Factory Crimp Die in the 357 Sig size, and give the bullets a tighter squeeze.

    I caution you to heed my warnings about reforming .40 S&W brass into 357 Sig. But, it's your gun, your fingers, your eyes.


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  7. #7
    Boolit Master
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    My BIL loaded 357 Sig. He used a load that slightly compresses the powder to prevent bullet setback. I did not get any details on the load.
    Spell check doesn't work in Chrome, so if something is spelled wrong, it's just a typo that I missed.

  8. #8
    I'm A Honcho!

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    The "Go-To" powder for 357 Sig is Blue Dot. However, that powder will NOT stop bullet setback. And, if you over pressure it, it goes off with a BRIGHT flash Bang.


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  9. #9
    Boolit Bub
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    I reload 357. sig and I agree , get the proper brass for reloading . Don't try to " make " something else work . Bullet set back can be a problem with such a short case neck . I use the LEE factory crimp die , problem solved . My two favorite powders is Accurate #9 or Blue Dot . Accurate #9 is my most favorite . I would like to add that I have found that using a case gauge has eliminated that problem of having a case that does not load . It doesn't happen often , but the case gauge sorts out those that otherwise wouldn't feed / load . Regards Paul

  10. #10
    Boolit Buddy
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    Very good info guys. Extra caution is the word when reloading.

    My body parts are priceless.

  11. #11
    Boolit Master dkf's Avatar
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    Quote Originally Posted by ulav8r View Post
    My BIL loaded 357 Sig. He used a load that slightly compresses the powder to prevent bullet setback. I did not get any details on the load.
    AA#9 will allow a compressed charge with most bullets in the sig and help prevent the bullet from going further into the case.

  12. #12
    I'm A Honcho!

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    AA#9 might "help" but it does NOT stop bullet setback. And, that's with a proper 357 Sig case. Reform one from a .40 S&W, and end up with an even shorter neck, and you are barking for trouble.


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  13. #13
    Boolit Buddy
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    Ive shot about 100,000 357 sig loads with 40 cases so far.

    Works fine. Take care to ensure your crimp is solid and bullets don't setback as mentioned above.

    It's not a good feeding cartridge imo.

    We have to use 357 sig for standard major in IPSC here in Australia. 357 sig cases cost a lot, 40 cal is often free.

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check