Reaming 9mm Luger barrels to .357 Sig

Invalid · Jun 19, 2020

I have several pistol barrels in 9mm that I wish to ream to .357 Sig. (For those unfamiliar, .357 Sig is basically a 9mm projectile seated in a necked-down 40SW case.) I already have several factory conversion barrels from gun manufacturers (Glock, Springfield Armory) as barrel swaps into their production firearms, so I know this is do-able.

The question is - how do I get this done for pistols outside of Glock and Springfield (i.e. for CZ, Tanfoglio, etc. pistols)? I am an end-user, not a gunsmith or machinist.

From the research I have done, it appears there are two ways to accomplish this task: with a mill, or with a lathe.

For the 7-8 shorter barrels I want done (i.e. 4-5 inches) I presume a mill would work fine. Additionally, I am looking to ream an 8.5" or 10" AR pistol barrel, which I presume could practically be done only on a lathe. Am I right in these presumptions?

I have acquired an interchangable pilot reamer for the task from PT&G here: PTG Chamber Reamer: .357 SIG (SAAMI). Are there any issues with this choice?

Now the real question - how do I find someone to perform this work? It seems that reaming barrels is a very specialized business, and not many people are willing to do things they haven't done before. So, how do I find someone competent with confidence to perform this work?

Thank you.

GGaskill · Jun 23, 2020

The process you are describing is normally called "chambering" (or "rechambering," if done to a barrel that already has a chamber) even though it is done with a reamer. If the barrels are not mounted in a receiver or frame, a lathe would be the instrument of choice, even for short ones.

Any competent gunsmith should be able to do this kind of work.

steve-l · Jun 23, 2020

Invalid said:
I have several pistol barrels in 9mm that I wish to ream to .357 Sig. (For those unfamiliar, .357 Sig is basically a 9mm projectile seated in a necked-down 40SW case.) I already have several factory conversion barrels from gun manufacturers (Glock, Springfield Armory) as barrel swaps into their production firearms, so I know this is do-able.

The question is - how do I get this done for pistols outside of Glock and Springfield (i.e. for CZ, Tanfoglio, etc. pistols)? I am an end-user, not a gunsmith or machinist.

From the research I have done, it appears there are two ways to accomplish this task: with a mill, or with a lathe.

For the 7-8 shorter barrels I want done (i.e. 4-5 inches) I presume a mill would work fine. Additionally, I am looking to ream an 8.5" or 10" AR pistol barrel, which I presume could practically be done only on a lathe. Am I right in these presumptions?

I have acquired an interchangable pilot reamer for the task from PT&G here: PTG Chamber Reamer: .357 SIG (SAAMI). Are there any issues with this choice?

Now the real question - how do I find someone to perform this work? It seems that reaming barrels is a very specialized business, and not many people are willing to do things they haven't done before. So, how do I find someone competent with confidence to perform this work?

Thank you.

First, the process is called re-chambering not reaming. Second, not all 9mm barrels will have adequate wall thickness to be safe and thirdly, since the 9mm barrels have a must too fast rifling twist (1-10), it makes much more sense to simply buy a new barrel properly chambered and with the correct twist of 1-16. This would allow you to custom fit the barrel to the slide and give you the ability to shoot un-jacketed ammo without the danger of lead fouling. All this for very likely less cost of re-chambering.

72bwhite · Jun 23, 2020

given that 357 sig is using the same slide as a 40
you will find the barrel will have the same profile as a 40 barrel
just a smaller bore,
you will probably find the ramp and other things aren't the same as a 9mm
ramp on 9mm and 40 not the same
9mm is considerably shorter then a 40 or 357 sig.
just about any one with a lathe could re chamber the barrel.
getting it to actually work maybe not.

and the who tf wants a 357 sig sure wouldn't use one for competition
to big a pia to reload, too expensive to buy sw40 will get you major loaded right

Invalid · Jun 23, 2020

Thanks for those who corrected me, now I know this is a rechambering, but using a reamer.

The pistols I am doing this for started life as 40SW pistols but also have counterparts (factory chamberings) in 9mm. They all use the same OD barrels whether 40SW or 9mm (same slide, different extractors). Since I am using their 9mm replacement barrels to start with, I am confident wall thicknesses will be more than adequate.

Regarding twist rate, that is an interesting question. I guess I will see what having a much faster twist rate will have on a standard 125gr .357 Sig projectile. I'm thinking not much.

steve-l · Jun 23, 2020

Invalid said:
Thanks for those who corrected me, now I know this is a rechambering, but using a reamer.

The pistols I am doing this for started life as 40SW pistols but also have counterparts (factory chamberings) in 9mm. They all use the same OD barrels whether 40SW or 9mm (same slide, different extractors). Since I am using their 9mm replacement barrels to start with, I am confident wall thicknesses will be more than adequate.

Regarding twist rate, that is an interesting question. I guess I will see what having a much faster twist rate will have on a standard 125gr .357 Sig projectile. I'm thinking not much.

The difference between 1 in 10 and 1 in 16 is huge. The 9mm Parabellum was designed by the Germans so the officers behind the battle line could provide harassing fire over long distances. That twist rate is way too fast for the cross sectional density of the bullet, even in the highest weight possible. That twist rate also mandates a bullet jacket unless you are using hard linotype cast bullets. The correct twist rate is 1 in 16 as is used in all the 38 and 357 weapons. If you should check, you will find that most aftermarket barrels will be offered in 1 in 16.

Invalid · Jun 23, 2020

steve-l said:
The difference between 1 in 10 and 1 in 16 is huge. The 9mm Parabellum was designed by the Germans so the officers behind the battle line could provide harassing fire over long distances. That twist rate is way too fast for the cross sectional density of the bullet, even in the highest weight possible. That twist rate also mandates a bullet jacket unless you are using hard linotype cast bullets. The correct twist rate is 1 in 16 as is used in all the 38 and 357 weapons. If you should check, you will find that most aftermarket barrels will be offered in 1 in 16.

One of the mfrs offers 38sup which I believe has 1:16, so I can go with that instead of the 9mm barrel.

72bwhite · Jun 23, 2020

well if you are wanting to do it to a cz talk to cz customs.
I know with like the CZ TS you can just switch the barrel and mag to go from 9 to 40
all though they recommend a new slid be fitted for max reliability.

wyop · Jun 24, 2020

Barrel twists in 9mm barrels:

1 in 10 is fast for the 115 grain ball ammo - much faster than needed in 9x19 applications. I don't think it would hurt anything IF you were launching your 9mm bullets at 9x19 velocities.

But that's not what is going to go on here. You're packing more powder behind a 9mm bullet because you want it to go faster. Everyone wants "MOAR velocity!"

Well, now we can get to a problem: When bullets are spun "too fast," the forces involved can actually cause a copper jacket to come off the bullet. .38 Super shooters have seen this happen even in 1-in-16 twist rates.

Here, I'll give a link to some background from a 1911 barrel maker, making barrels for .38 Supers as well as .45 ACP:

http://blog.westernpowders.com/wp-content/uploads/2015/04/Webfile_Barrel-Twist-Rate.pdf
As you see, they're even talking of backing down the twist rate from 1-in-16 on a .38 Super barrel.

If you're loading a .357 Sig to .357 Sig velocities, you're apt to see some issues in a 1-in-10 barrel.

smaug · Feb 6, 2022

Invalid said:
Thanks for those who corrected me, now I know this is a rechambering, but using a reamer.

The pistols I am doing this for started life as 40SW pistols but also have counterparts (factory chamberings) in 9mm. They all use the same OD barrels whether 40SW or 9mm (same slide, different extractors). Since I am using their 9mm replacement barrels to start with, I am confident wall thicknesses will be more than adequate.

Regarding twist rate, that is an interesting question. I guess I will see what having a much faster twist rate will have on a standard 125gr .357 Sig projectile. I'm thinking not much.

The issue with the twist rate makes some sense, but the twist is imparted by the rifling no matter what length of barrel or speed the bullets are traveling. The projectile spins at the rate of the barrel rifling twist, no matter what length of barrel. Higher pressure means higher velocity, but going faster means it travels the 1:10 or 1:16 distance that more quickly. Too high a twist can possibly cause a bullet to come apart mid-flight, or in some other way de-stabilize the projectile. In High Velocity rifles, twist rates are optimized to bullet weight and caliber: to different bullet weights within a specific caliber. Or am I wrong?

GGaskill · Feb 6, 2022

A higher linear velocity will impart a higher rotational velocity to the bullet since they are directly related. This will magnify the stress on the jacket and any out of balance condition of the bullet.

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