I think Im going to to look into this.
Couple of things Im noticing looking at this.
1. For .357, cylinder-to-frame gap would need to be, at a minimum, 1.52mm(0.06") based on rim thickness. For 9mm it would be a minimum of 2.17mm(0.085") to include both the 'rim' as well as the width of the 'extractor groove' formed in the case just above the 'rim' where the 'moon clips' would hold the rounds. Do most S&W .357s have enough cylinder-to-frame gap too allow for that 0.65mm(0.026") difference?
My Taurus 905(9mm J-frame revolver) measures 2.14mm(0.084") cylinder-to-frame gap in full lockup but the moon clip that the rounds headspace off of is only 0.5mm(0.02") which is 0.4mm(0.016") less than the 'extractor groove' of a 9mm case which C.I.P. specs as 0.9mm(0.035").
If most .357s dont provide enough gap to make up the difference between the cartridges then I would have to shim the cylinder forward but then I would be concerned about cylinder-to-barrel gap.
The cylinder needs to be machined for moon clips at the same time it is converted from 357 Magnum to 9MM Parrabellum
The moon clips sit BELOW the rear edge of the cylinder. The cylinder spacing in the frame window does not change for this conversion
2. Case body diameter. .357 is straight walled at 9.63mm while 9mm is tapered from 9.93mm to 9.65mm. Does 9mm drop into S&W .357 chambers without modification?
No a 9mm Parabellum should not be able o drop into a 357 Magnum chamber, but a 38 Super will drop in most of the time.
The 357 Magnum cylinder needs to have a 9MM Parabellum finishing reamer run through it.
While 357 Magnum ammunition can still be chambered and fired in the converted cylinder, if you are a hand loader you will not like the brass that comes out.
3. Bullet diameter. .357 is 9.12mm, 9mm is 9.03mm. Ive read that 9mm bullets are inaccurate out of barrels cut large enough of .357 rounds. Granted this was for one of those Taurus cylinder-swappable revolvers so that could just be Taurus being Taurus but Im wondering if the same could be said of ALL .357 barrels.
This is mostly untrue
Accuracy is mostly a function of proper bullet spin.
Bullet spin is caused by the rifling engaging the projectile
Most people forget that there is both a land and a groove diameter involved when they talk about bore diameter. What is actually measured when folks slug a barrel is the groove diameter.
The difference between the land and the groove is the depth of the rifling and what engages the projectile and causes spin. This difference is somewhere around .008" - .013" depending on the type of rifling and who did it
Image courtesy of Hand Gun Safety Course
Jacketed, plated and coated 9MM projectiles measure anywhere from .355" - "3565" depending on the manufacturer. There is still plenty of projectile for the lands to grab onto and get spin even in a .357" barrel.
I have been shooting 9MM projectiles through many different .357" barrels for several decades now.
The accuracy exception comes when we are discussing lead projectiles. For least amount of leading and therefore best accuracy the lead projectile needs to properly fit the barrel it is being fired from
