My experience reloading 357 Sig

 The following is my experience with reloading 357 sig. A little background, the 357 sig was designed to be a law enforcement round that replicated a 357 mag ballistics; specifically 124gr bullet traveling at 1450 fps. To do this the designers at Sig Sauer necked down a slightly shortened 10mm case to 9mm (the 40 cal S&W is also based on a shortened 10mm case, and while close in length, if you neck down a 40 cal shell case it will be about half a mm too short). A 357 sig round is not that difficult to reload, but there is a learning curve. First, because it is a necked down round, it is best reloaded like a rifle cartridge (I'll explain this in more detail later). One issue that needs to be dealt with is the neck of the 357 sig case is very short, so bullet design is very important; as an example the bullet should be short (IOW "squatty" for lack of a better term) with parallel sidewalls, often referred to as "Truncated cone flat point bullets", 124gr.* That way the bullet uses the maximum inner surface of the neck of the case without being too long. Along with this, it's best not to bell or flair the case mouth prior to pressing in the bullets similar to reloading some rifle rounds with boat tail bullets. This allows you to press in the bullet without the need to crimp the shell, as crimping the shell mitigates how well the neck of the shell holds the bullet.   

 

Dies for the 357 Sig come in 2 flavors, because the 357 sig is a necked down cartridge, like rifle cartridges, most the dies only come in steel (not carbide; carbide allows for sizing shells without case lube).  Now there is a common work a round for not using case lube. Since the bottom half of the 357sig shell is 10 mm, you can use a 10mm/40 S&W carbide die to size the bottom and decap and then follow up with the 357 sig steel sizing die for the neck; because the neck is so short I've found it's not necessary to use lube with sizing the neck with 357 sig steel die. Now Dillion has a die that combines these two features, a carbide ring for the bottom and a steel portion to form the top the neck of course its twice the price of an RCBS die set and they still recommend using case lube, though it's probably not necessary.

 

One issue I ran into is the RCBS 357 sig forming die does not work well. Here we are back to sizing the case like a rifle cartridge. Most necked rifle cartridges use the shoulder of the cartridge for proper head spacing, that is how the bullet fits into the chamber, based on the length from the shoulder to the rim. This is opposed to rimless pistol cartridges that headspace on the case mouth.  RCBS die forms the 357 sig case to headspace on the case mouth, (not the neck) which after a few reloads will increase the length from the base to the shoulder so the base of the round end up ending out of the chamber so the slide wont completely close (IOW, don't use the RCBS forming die). The reason for this is every time you resize a necked shell casing it gets a little longer. If the die is designed to head space on the shoulder, it will keep the shoulder position relative to the shell base the same. However if the die is designed to headspace on the case mouth, the shoulder position relative to the shell base will lengthen farther every time it is formed, until the slide will not be able to close on the round. Yes, like a rifle case you will need to trim the the 357 sig case to length, but if the distance between the shoulder and rim is too long, trimming the mouth of the shell will not solve the fitment problem

 

 Powder wise, 357sig is not real picky as long you can load enough in the relatively small shell. I have used Unique (not recommended but will work), Accurate #7/#9, Power Pistol, Silhouette and Blue Dot; a big positive about 357sig is the increased pressures lead to most powders burning very clean and due to the limited case capacity it is rare you will find a load that will allow a double charge.

 

When I first decided to reload for 357 sig, I bought some RCBS 357 sig dies. The RCBS dies have a ball mouth expander in the forming decapper die, but not a separate expander die, which it tunns out is for food reason. When I first started loading 357 sig I used the 357 mag expander die, but I found using a flaring/ bell style expander die is problematic because it mitigates the how securely the necked of the shell holds the bullet. This is because a 357 mag flare die is designed not to just bell the top of the shell, but to open a channel, which reduces the short necks ability to hold onto the bullet. What I eventually ended up with is a hybrid Lee/ RCBS forming die, using a Lee carbide 10mm forming die, but replacing the Lee decapper pin with the RCBS expander ball decapper pin. What this does is forms the die with the headspace at the shoulder, and opens the case mouth enough for a 124gr truncated cone flat point bullet to be pressed in without and further expansion of the case mouth. This is important due to the issues explained in the following paragraph.

 

Another issue that plays into the forming and loading of 357sig rounds is something called rebound deformation. As I mentioned, it better to not use a belling or expansion die when pressing in the bullets of the 357 sig due to the short neck, this is less of an issue with a longer neck  or straight wall shells because there is more surface to hold the bullet. The reason is, expanding a shell and then crimping the shell around the bullet compromises the friction that holds the bullet in the case, due to rebound deformation. When brass is deformed or sized with a die, it rebounds, meaning it expands or contracts larger or smaller than the die, depending on whether you are expanding or constricting the shell. Because of this when you bell or flare open the end of the shell to accept the bullet, the walls of inner surface of the shell that holds the bullet will no longer be parallel, which comprises the holding strength of the inner sidewalls. Usually with other calibers this only affects a small portion of the interface between the bullet on shell, but because the 357 sig neck is so short, anything that mitigates this interface is problematic.This issue can cause what is called "bullet setback" or extraction, where recoil changes how the depth of bullets in shells of the rounds in the pistol magazine. This is why I replaced the decapper in the Lee 357 sig die with the expander ball from an RCBS 357 sig die. This ball expander is actually less than .9mm/ .355 (I believe it is .353"), which makes it easier to press in the bullets without using a more extreme expander die. I have found that using the RCBS expander ball holds the bullet better than expanding the mouth of the case and then crimping it closed. Bullet setback/extraction in a 357sig round is enough of a problem that most manufacturers fill the entirety of the case with powder and/or use a bullet adhesive/sealant, but when using the expander ball without crimping I have not had any problem with setback or extraction.

*There are some other bullets out there including a JHP/ notched 147 gr 9mm bullets that were fairly easily to load without belling the case mouth.