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Thread: Loading 357 magnum pistol: is flaring and crimping necessary?

  1. #11
    Member jtcarm's Avatar
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    Quote Originally Posted by okie john View Post
    This.

    Revolvers can be extremely accurate, even at long range, but there are some tricks.

    The first is to use flat-based cast bullets sized to .002-3" over bore diameter. From there you want to ensure that bullet pull, which is the tension that the case exerts on the bullet, is uniform. Most people loading for accuracy size normally, then use a slightly undersized expander, such as a .410" for the 44 Special and Magnum. This results in a loaded cartridge with a little waist on it like a Coke bottle just below the base of the bullet. This, combined with crimp, helps make ignition more consistent.
    A couple points of order:
    Cast revolver boolits (actually most cast boolits) need to be sized to throat, not bore diameter. Throats need to be slightly larger than bore. If throats are too tight, the boolits hits the bore undersize and doesn’t form a gas seal. If the boolit is larger than the throat, it gets swaged down on firing, a scenario that can increase chamber pressure.

    On the other hand, throats can be too big, which is not uncommon with older .45 revolvers, where throats may run up to .458 with a .451-.452 bore. The only solution I know of here is to limit loads to mild pressures. A .456 bullet crashing a .451 bore - you get the picture.

    Since a revolver has multiple chambers, the throats not only need to be sized correctly, they need to be consistent.

    I’ve DIYed opening chamber throats, but nowdays ship the cylinder off to DougGuy over on the cast boolits forum.

    Though not undersized, all the S&W .357 throats I’ve measured were very tight. I have mine opened to .358. That minimizes or eliminates sizing of my home-cast boolits and helps ensure a good seal in the bore.

    I assume your reference to a .410 expander ball for a .44 bullet is a typo, as that simply isn’t possible.

    Given sufficient neck tension, a super-tight crimp usually isn’t necessary. For best accuracy, I use just enough taper crimp to allow chambering.

    A slight flare bearing against the chamber wall helps align the bullet with the throat.

    The exception is for rounds that that need to feed fast and 100% reliably, like competition or SD. Those get run through a Lee carbide factory crimp die.

  2. #12
    The R in F.A.R.T RevolverRob's Avatar
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    Quote Originally Posted by Duelist View Post
    Yes, and yes.

    Excess belling/flaring may overwork the case mouth and cause it to fail prematurely, and if you really overdo it, you may have trouble getting it to pass through the seating die. Too little, and you’ll shave bullets, ruin concentricity, smash cases trying to seat in some situations.

    Crimp is necessary to get proper ignition from some loads and to prevent bullet pull in adjacent chambers.

    I use a separate crimp die, and just use the seating/crimping die for seating.
    This ^^^^ has been my experience 100%.

    My first few times reloading .38, I tried not to flare the case mouth, but immediately started shaving bullets and ruining concentricity of the bullet and then when I crushed a case I thought, "Yea...maybe I should do that."

    I adjusting my flaring down to flare the case mouth enough to fit my .358 sized poly coated bullets without shaving polymer.

    And like Duelist here, I use a separate crimp die and just seat with the seating/crimping die. I found I was getting an inconsistent crimp with the seating/crimping die for what I wanted. Bearing in mind most of my .38s are destined for airweight guns, I definitely wanted a crimp that allowed me to not suffer bullet pull in a light gun.

    Similarly, when I'm loading hot .357s I do the same. The example @okie john posts of a .45 LC is what I aim for when loading SWCs in any revolver caliber.

    If I were loading powder puff (think Trail Boss-esque), I would flare and use the light taper crimp I get when I seated the bullet with a seating/crimping die. But why I would load powder-puff .357s, except to avoid a powder ring forming in the chamber, I couldn't say. I would want a real crimp on them if going through a lever gun.

  3. #13
    Site Supporter OlongJohnson's Avatar
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    Mar 2015
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    Quote Originally Posted by jtcarm View Post
    A couple points of order:
    Cast revolver boolits (actually most cast boolits) need to be sized to throat, not bore diameter. Throats need to be slightly larger than bore. If throats are too tight, the boolits hits the bore undersize and doesn’t form a gas seal. If the boolit is larger than the throat, it gets swaged down on firing, a scenario that can increase chamber pressure.

    On the other hand, throats can be too big, which is not uncommon with older .45 revolvers, where throats may run up to .458 with a .451-.452 bore. The only solution I know of here is to limit loads to mild pressures. A .456 bullet crashing a .451 bore - you get the picture.

    Since a revolver has multiple chambers, the throats not only need to be sized correctly, they need to be consistent.

    I’ve DIYed opening chamber throats, but nowdays ship the cylinder off to DougGuy over on the cast boolits forum.

    Though not undersized, all the S&W .357 throats I’ve measured were very tight. I have mine opened to .358. That minimizes or eliminates sizing of my home-cast boolits and helps ensure a good seal in the bore.

    I assume your reference to a .410 expander ball for a .44 bullet is a typo, as that simply isn’t possible.

    Given sufficient neck tension, a super-tight crimp usually isn’t necessary. For best accuracy, I use just enough taper crimp to allow chambering.

    A slight flare bearing against the chamber wall helps align the bullet with the throat.

    The exception is for rounds that that need to feed fast and 100% reliably, like competition or SD. Those get run through a Lee carbide factory crimp die.
    My understanding is:
    You want no thread constriction where the ID of the barrel gets tighter where the threads interface with the frame. That will swage the bullet down, and then you can have accuracy and leading issues farther along the barrel. I've read that Colt Pythons actually have a slight taper the length of the barrel, so the bullet is very gradually swaged and achieves a super-fine fit, which delivers excellent accuracy but tends to lower velocity.
    For lead, you want the bullet diameter to be about 0.001" larger than the groove diameter of the barrel so it is swaged just a tiny bit by the forcing cone to fit the barrel and seal more or less perfectly.
    You want the chamber throats to be about 0.0005" larger than the bullet. The bullet should slip through with little to no drag, but just barely. If the throats are smaller than the bullet, as mentioned, they will have to be swaged down. The force to swage them is reacted by all the thrust surfaces of the cylinder, yoke and frame, so you're liable to end up with severe endshake in short order.
    If either the throat or bore is significantly larger than the bullet, the gases driving the bullet will blast past the sides of the bullet, melt them, and spray the molten lead on the surfaces ahead of the bullet. You can get really severely nasty leading in short order.

    I have measured several pre-lock K frames, and the throats were all close to .357. Assuming the groove diameters were .357, that is excellent for jacketed bullets and for inexpensive swaged lead wadcutters sized .357, which most are.

    I have measured one pre- and one no-lock J frame, and the throats were all a no-drag .359. The one I shot some inexpensive swaged lead wadcutters sized .357 in had really severely nasty leading in short order.

    In addition to bullet pull with recoil, if the cartridges may be used in a lever gun with a tube magazine, bullet setback can be an issue if they are not well crimped. This will increase pressures significantly, possibly dangerously.

    I have seen a bunch of Geco branded ammo with inadequate crimp in .380, .357 and .38 SPL. Enough that I don't buy it in .357 anymore. Seems to be a thing with that brand.

    The last time I considered Freedom Munitions ammo was when I looked at some .357 in their Houston retail store and the case mouths were still flared. I brought it to the attention of an employee and was told they were supposed to be like that.
    .
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    Not another dime.

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