Only difference between .357 and .38 special cases is the length not the thickness. Trimming .357 cases back will give no more neck tension then an untrimmed case.kalifornia, they will work but you'll get best results by trimming .357 Magnum cases to .38 Special length. That way you'll get better case neck tension on the 9mm bullet. When I have done this in the past, I also used my REDDING 9mm taper crimp die to apply crimp on the shortened cases. I used a number of powders that are slightly faster than the typical Magnum choices starting with Blue Dot but had better luck with V-V 3N37, AA#7 and the now unavailable Vectan SP-2. My reason for doing this was also an abundance of Remington 124 gr. 9mm JHPs, but also with 125 gr. 357" bullets in the "Short Magnum" cases to prevent extraction problems with the 7 shot Taurus Tracker and short barreled .357 Magnum S&W revolvers with short ejector rods and to make speed loading a bit quicker. Another good reason for trimming down .357 Magnum cases is for identification to help insure these loads never found their way into a .38 Special revolver.
Doesn't matter, your still shooting an undersized bullet through the gun. If its lead then you will get leading no matter what. If its jacketed you will get excessive copper fouling. Either way its a loose loose load.I see that nobody suggested using a 9mm sizing die. Since 9mm has a tapered case you should only attempt to size the 38Spl case as far down as you seat your bullets. I've used a 9mm taper crimp to taper crimp 38s and it works just fine. Although I've shot .357 out a 9mm, I've never tried it the other way.
Shooting undersized cast lead bullets in an oversized bore is a bad idea, but the OP stated his bullets were FMJ. Concerning jacketed, if that were the case, Ruger would have stopped selling the 9mm/.357 Convertable Blackhawk a long time agoDoesn't matter, your still shooting an undersized bullet through the gun. If its lead then you will get leading no matter what. If its jacketed you will get excessive copper fouling. Either way its a loose loose load.
I don't see an advantage to trimming .357 cases back. I used the standard sizing die for a .357, then switched out the .357 expander ball for the expander from my 9mm dies.
Maybe that's part of your problem. I was never satisfied with caseneck tension using full length .357 Brass even when using a 9mm expander. As I said before, my main motivation was to decrease extraction issues with thin cylinder walled 7 shooters and revolvers with short ejector rods. Most of my shooting was done in my wife's model 65 Ladysmith with a 3" barrel and accuracy was decent enough to hit ten cans out to 40 yards shooting DA. Powder selection was important in my experiments, but you didn't list the powder you were using. The powders I chose were for ballistic efficiency in 3" barrels as well as low flash characteristics important to me with defense loads.It worked yet I had less than stellar results. As a matter of fact accuracy was quite poor...
Yeah, it has a barrel length that is 1" longer, thicker cylinder walls, heavier frame, coiled mainspring and it's a Ruger. I mentioned the Ladysmith because I had good results using it, but I've never had any extraction issues with full length .357 Magnum cases and it has a full length ejector rod. The extraction issues were with a 4" 7 shot Taurus Tracker that belonged to my shooting partner so I solved that problem while making a round that would also eject better with S&Ws that have barrel lengths of 2 1/2" or shorter and short ejector rods.A GP100 is quite a bit different than a model 65 lady smith.
Using 9mm bullets in a .357 was for me, simply an excercise in curiosity, one I undertook years ago...and while I did...I can't say that I recommended practice. Just because you can doesn't mean you should...lol
Absolutely true.Only difference between .357 and .38 special cases is the length not the thickness. Trimming .357 cases back will give no more neck tension then an untrimmed case.