I've heard both Yes and No on this. I've got lots of 40 S&W brass and 357 Sig size dies. Can this be done easy or is there a problem resizing 40 brass down to 357 SIg?

It won't be long enough.

OK

Thanks.  I actually thought it might be a little long.

10mm would work if you could find it with small primer pockets.

I've actually picked up some 40 S&W brass at range that was converted...might be possible sure made me do a double take.

It is one of those things that is possible, but not wise.

The neck comes out a bit short.  Since the neck on the 357 SIG is already so short, this is a problem.

In a universe where no proper brass was available, necking .40 down would be understandable.  If you can get 357 SIG brass, I wouldn't bother with trying to neck the .40 down.

I have loaded a few before without primer/powder just to see. It seems like it would work, but I am not sure how safe it would be to do a lot .

I did one piece one time.  Yes, its slightly shorter.  It was reloaded and is in the "mix" with the rest of the .357 SIG brass.

I read (somewhere) that .357 SIG brass will stretch (bottle necked case?? issue).  Maybe, one day, I'll notice that one piece of brass is the same length as the .357 SIG brass.  Then again, maybe not.

I've seen factory .357 SIG ammo get the bullets pushed back in them so the little short neck is not an asset.

Then again, a piece of .40 S&W brass necked down to .357 SIG should have a "thicker" neck wall than .357 SIG brass so, would that make it grip the bullet tighter??

As long as I have/can get plenty of .357 SIG brass I won't mess with necking any down.

I have tried it, it works. Probably not ideal. I reformed some multi-fired .40 brass and measured it against known once fired Speer and Federal brass, the difference was almost non-existent. None of them ( reformed or factory ) were close to as long as the specs. I was also concerned with neck tension/setback, I tested several and noticed no significant difference. As previously stated, it could be due to variations in neck thickness. I concluded that it works, I do have some in the rotation, but I won't do it for top end loads. YMMV.

My RCBS dies came with instructions to do just that. The RCBS dies are not a good choice for 357 Sig though. The shoulder isn't bumped back enough the crimp is the wrong type ofd a crimp. It's roll crimp and not the taper crimp requires for non-cannelured bullets. The early 40 S%W cases are thinner than 357 Sig cases. Federal had a run of weak 40 S&W cases failing and recalled the ammunition. Then Federal strengthened and thickened their 40 S&W cases. After Federal did that, all of the other US ammunition makers did the same thing. The current 40 S&W cases are similar in thickness and strength to 357 Sig cases now. The reformed 40 S&W cases end up only slightly shorter. The load data remains the same. I tried shorted 10 mm cases. They are tough to reform and the powder charge must be reduced for the thicker 10 mm case with now smaller capacity and the large primer compared to the small primer. with medium burning rate powder and mid-range loads, I got about the same velocity with a 1.0 grain reduction for 10 mm cases. These days there are plenty of Speer 357 Sig cases available though. The Speer 357 Sig flash hole is slightly smaller in diameter than the others and the early RCBS decapping pin will stick hard in them. Almost any other die brand is better than RCBS for 357 Sig. The necks are short in both 357 Sig cases and reformed 40 S&W cases. The Lee Factory Crimp Die now available for 357 Sig does a good job. Previously I made one by combining a 9 mm and a 40 S&W mm Factory Crimp Die but now Lee makes one for 357 Sig. Running the cases through a Lee carbide 40 S&W sizing die before the 357 Sig sizing die makes sizing easier with a minimum of sizing lube. The Lee carbide dies size the cases closer to the case head than most other dies. There is also a, "bulge buster" die now available too.