357 Sig conversion?

T

TrueBlue711

New member
So I'll start off by saying I've never given 357 Sig a thought until yesterday. Frankly, I didn't even know what it really was. When someone in conversation yesterday said it was a necked down 10mm to a 9mm bullet and will fit in a 40 magazine, it made me want to look more into it.

I have a S&W M&P in 40 (among many other pistols, but this is my only 40). Would I be able to do just a quick barrel swap to a 357 Sig barrel and I have new caliber? Or is it not that simple?
 
It's that simple. I have a KKM 357 SIG barrel for my M&P 40. Change the barrel and you're good to go.
 
That's awesome. I'm seeing the factory barrel is about $80 on Midwayusa or Brownells...if they're in stock. I'll jump on the wait list.

Are there any good online articles anybody can post about the 357 Sig? As mentioned, I'm fairly ignorant on the subject.
 
What do you want to know about 357 SIG?

One factor you might encounter is that 357 SIG often shoots to a slightly different point of impact if you are using the same sights as for .40 S&W and shooting to the same point of aim.

357 SIG being a higher velocity cartridge will shoot flatter at distance than than .40 S&W, so if you shoot at a great enough distance, 357 SIG might have a higher POI than .40 S&W because of less drop. But you would only see that if you were shooting at ranges well beyond 25 yards.

At the typical ranges most of us shoot handguns at, the 357 SIG often has a slightly lower POI with the same POA and sights as compared to .40 S&W. The reason is that .40 S&W has a longer "barrel time" than 357 SIG. The effect of recoil starts to result in some muzzle rise immediately or soon after ignition. Although there is not a lot of muzzle rise during the time it takes for the projectile to get out of the barrel, there is a little. The quicker the projectile clears the barrel, the less time there is for recoil to push the muzzle up. 357 SIG gets out of the barrel a little more quickly, so there is very slightly less muzzle rise.

The difference in POI between .40 S&W and 357 SIG at ranges of 25 yards or less is going to depend on the firearm and barrel length. It won't be very great but it will likely be enough to be noticeable. Most shooters simply compensate for this with a slight adjustment in sight picture (hold) but if you plan to shoot 357 SIG exclusively and the difference bothers you, you could switch the front sight to a slightly shorter one, the rear sight to a slightly taller one, or both.

There will also be a difference in recoil characteristics. Perceived recoil is highly subjective so different shooters will perceive this differently. I have heard many say they find the recoil of 357 SIG easier to manage when shot out of the same pistol as .40 S&W. I don't necessarily feel that way. I would be inclined to say that the recoil of the two cartridges is similar, but different. The .40 S&W tends to result in a little more muzzle flip than 357 SIG in my hands, whereas the 357 SIG seems to have a sharper jab straight back into the hand, sort of like 9 mm Luger on steroids. I have shot a lot more .40 S&W than I have 357 SIG so I do not feel that I can control 357 SIG more easily.

What I can pretty much guarantee you will notice with 357 SIG are the following (again, compared to .40 S&W):

1. 357 SIG is louder. This can really be a factor if shooting indoors and a person standing to the side will notice it more than the shooter.

2. 357 SIG has more muzzle flash. This might not be that apparent when shooting outdoors in bright daylight, but you will probably notice it when shooting indoors or in dim light.

3. 357 SIG ammunition is more expensive and the selection is more limited. This will be less a factor if you can buy ammunition on-line and shop around. But you will still likely find that FMJ practice ammo is still significantly more expensive than .40 S&W. If you have to buy ammunition locally, 357 SIG FMJ can be much more expensive, if you can find it at all. Curiously, the price difference between quality JHP self-defense ammo for 357 SIG and .40 S&W seems to be much less, but the choices will still be fewer for 357 SIG than for .40 S&W.

I won't get into the differences in terminal ballistic effects of 357 SIG versus .40 S&W as that would open a can of worms.
 
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You can do the same thing with a 45ACP. Get a 40 cal barrel and then buy you some 400 Corbon's ammo. Just like the .357 Sig is a hot rod, the .400 Corbon is even more impressive! it'll use same magazines and all. Enjoy...
 
Yeah, it is really that easy for owners of 40 S&W pistols. Just a drop in. Sometimes the mag's follower might be replaced. Usually a stiffer recoil spring is a good upgrade, too. I have 357 Sig barrels for all my Glocks in 40 S&W.
 
I have 2 M&P40c with 357SIG barrels in them as my EDC. I'm happy with my choice at the moment been carrying this way for 8 months. As for 357SIG round it's not for everyone try it and see what you think to each their own and I'm not knocking any other caliber here! Seems to be a lot of conflicting info out there as folks say don't you worry about over penetration and others say use your handgun to get to you rifle! For me and my environment the 357SIG offers what want. If I didn't carry this I'd go for 10MM but I keep that in my break glass in case of war collection right now!
 
Thanks for the info. Good to know the POI may differ. I'll just have to test it out at the range (whenever I get the barrel) and take note in the difference. This pistol is my designated truck gun and sits in my center consol. I'm not too concerned about how loud it is or muzzle flash.

I heard someone say that the 357 Sig is comparable to the 357 Mag in ballistics. That sound right?

[EDIT] I think I just answered my own question. Comparing apples to apples, I went to Hornady's website and compared their critical duty rounds for 357 Mag, 357 Sig, 40 S&W, and the 9mm +P. The 357s have almost the same velocity and energy using the same size bullet (135 gr). The mag is slightly higher on both, but I'm surprised how close they are. And they beat the 40 S&W and 9mm +P in both velocity and energy.
 
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It is as simple as a barrel swap, I had one for my G23 and loved it. It easily uses the same mags and is a fun round to shoot. To my understanding the round was designed to replicate the 357mag but in an auto format. For $80 I would do it again, ammo can be a little expensive but how often are you really going to be shooting it?
 
My experience shooting both 125 grain FMJ 357 SIG and 180 grain FMJ .40 S&W from the same pistol using the same sights and same hold (sight picture) is that the POI for the 357 SIG is approximately 1" lower in the 10-15 yard range than it is for .40 S&W. This is a difference you might not notice if you are shooting casually but might if you are trying to shoot groups in slow fire.

357 SIG in 125 grain comes close to 357 Magnum in 125 grain but there is still a significant gap in muzzle velocity, momentum, and kinetic energy. With 357 SIG you are pretty much limited to projectiles of 147 grain or less, as for 9 mm Luger. It is with the heavier projectiles that the gap between 357 Magnum and 357 SIG really opens up. 357 Magnum can propel a 140 grain projectile as fast or faster than 357 SIG propels a 125 grain projectile, and 357 Magnum can propel a 158 grain projectile faster than a 147 grain 357 SIG projectile and with 93% of the velocity of a 125 grain 357 SIG projectile, in both cases with much greater momentum and kinetic energy.

As for .40 S&W compared to 357 SIG, the momentum of the 180 grain .40 and the 125 grain 357 SIG is very close with the .40 S&W often having slightly greater momentum. The 357 SIG round does have about 24% more kinetic energy, but the .40 S&W makes holes that are larger in diameter.
 
My 357 Sig shoots to the same POI as it does with a 40 BBL or a 9mm conversion BBL.

The 357 Sig is somewhat of a one trick pony. Do not go to 147 grain bullets if you want a 357 Sig, stay with the 125's.
 
pblanc and I had a private conversation (which I initiated in an attempt to not further add chaff to the existing discussion). During that discussion he talked about his experiences when shooting the same rounds through the same gun. His observed results that seemed to run contrary what one might expect if all other things were equal. As I did some additional digging following that offline discussion, it became obvious that some other variables might be at play.

In searching through ballistic charts (showing how rounds arc and drop -- due to sight placement, gravity. bullet size, weight, shape and velocity) -- appears that the .40 bullet path follows a different (slightly higher?) arc, and drops more quickly after the zero point (25 yards) than does the .357SIG round. It's long been held that the .357SIG round flies a flatter trajectory.

The following chart was extracted from the following materials; a link is provided: http://www.gunnersden.com/index.htm.handgun-ballistics.html

bullet%20drop%202_zpsaeebbid9.jpg


The following information came from this site: https://www.realgunreviews.com/sig-sauer-sight-heights-sight-numbers/

It tells me that the standard SIGs use the following:

9mm & .357 SIG: #8 front / #8 rear
.40 S&W & .45 A​
CP: #6 front / #8 rear

For the SIG X-RAY sights (like the ones that come on the Legion series pistols), things seem to be a bit different, and the model does seem to matter:

P226 Legion 9mm (DA/SA & SAO): #8 front / #8 rear
P226 Legion .40 DA/SA & P229 Legion 9mm DA/SA: #6 front / #6 rear
​
But the author notes that there can be other variations.

This table, from https://www.realgunreviews.com/sig-sauer-sight-heights-sight-numbers/ shows the actual heights of each SIG front and rear sight number. The front heights shown are the actual heights of the sight — including the dovetail base, not just the blade. Front and rear sights with the same numbers are NOT the same height.

SIG%20SIGHT%20CHART_zpsfnzuvfhj.jpg


According to various folks each step down or up in the number on a front sight (given that the rear sights are the same) should result in a 1" rise or drop at 25 yards, respectively, if the round stays the same.

That would make me think that if a .357 factory barrel is used in a .40 slide, it might NATURALLY shoot 1"-2" lower than a .40 round fired from the same slide. The reverse might also be true -- if a .357 slide using a .40 barrel might cause the .40 round to hit 1"-2" higher. No matter which slide is used, the rounds should give different points of impact when mounted in the slide for the other caliber based on the performance specs above.

The sight differences might account for the many variances and difference we see discussed when this topic comes up when barrels are switched. Or, it may simply be that this continues to confuse me and others.

Conversion barrels may be different than factory barrels -- might -- and if there is a difference it could conceivably vary by barrel-maker. I don't know how or where CONVERSION barrels fit in this discussion. About the only difference between a .40 SIG slide and a .357 Sig slide is the sights, and the barrels are supposedly interchangeable. When I"ve owned factory barrels for the .40 and .357 SIG rounds, they seemed identical (except for bore size).

The only part of this exercise in data collection that doesn't make sense to me is the fact that the front sight on the .40 slide is .28 mm higher than the front sight on the .357 SIG (which would seemingly cause a slightly lower point of impact than if the front and rear sights used the same #, like the .357 SIG model does. That difference is a quirk whose existence isn't intuitively obvious to me. (As shown above, front and rear sights with the same #s are not the same height.)
 
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the sig would be more like 40 S&W necked down to 9mm.
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Yea, sorta, but not quite.

If you neck a .40 down to 357SIG, and its easily done by mistake, you end up with a case thats to short.

Its also my understanding that the 357SIG case is also structurally different in other places from the .40S&W.

Ive unintentionally reloaded .40 S&W into 357SIG on a number of occasions, and shot them without issue. I wasnt even aware Id done it. I dont even know how many times I reloaded those rounds either, just that I caught the mistake after the fact, when I noticed the headstamp while priming the cases.

I didnt use lube when sizing, instead, used a carbide .40 S&W sizing die as the first step, then a 357SIG sizing die. Once sized, the .40 cases go right up into the 357SIG die with no effort, and come out looking like a 357SIG. If youre not paying attention, and even if you are, its very easy to miss the .40's in the mix.

The main problem (other than the structure of the case) is the necks come up
a few thousanths short in sizing, and just aggravates the neck tension problem the round is noted for. The load I used to use, was a compressed load (13 grains of AA #9 under a 357SIG 125 grain bullet), and that kept the bullet from setting back, and helped nullify the tension issue.
 
When someone in conversation yesterday said it was a necked down 10mm to a 9mm bullet
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That would be a 9 x 25 Dillon--the sig would be more like 40 S&W necked down to 9mm.
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Well, since the 40 S&W is really just a "10mm Short", yeah, the 357 Sig is basically a 10mm Short necked down to a 9mm bullet.
 
Not at all. 10mm uses a large pistol primer.

I reload and shoot my own necked down 10mm cases converted to 9x25 dillon.

The 357sig case was designed from the ground up, not necking down and shortening a 10mm case.

As long as you aren’t hot roding necked down .40 cases into 357sig, the slight difference in case length isn’t an issue.
 
I’m now using .357 Sig as my carry round. It’s a flat shooting, hard hitting cartridge with slightly more recoil than a 9MM. The bottleneck case makes for good reliability too. I have not been able to make my Sigs jam with this round.

You have to be a reloader to fully enjoy this cartridge because factory ammo is expensive and bullet variety is limited. Despite some negative comments from a few amateur reloaders, .357 Sig is really no harder to reload than any other bottleneck cartridge.
My woods carry rounds use Lehigh Defense Xtreme Defense and Xtreme Penetrator bullets. These are the solid copper fluid transfer technology bullets. Underwood has been selling loads with these bullets for some time and Black Hills has also started loading these bullets now. They call it their Honey Badger line.

Give the Sig round a try. I think you’ll like it.
 
Give the Sig round a try. I think you’ll like it.
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I plan on it. I just ordered the barrel for $80 on Brownells. They're not in stock at the moment, but they said they expect a shipment in early Oct.
 
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