My P226 is chambered for .40 S&W but could shoot .357 SIG by swapping the barrel.
.40 S&W seems perfect in the P226.
What are the advantages of .357 SIG?
The .357 SIG has its supporters, but with all due respect, I will make the opposite case and say that it has no advantages and some disadvantages.
This cartridge is essentially a 9mm (.355, not .357) bullet in a necked down .40 S&W case going at 9mm +P+ velocities. One advantage, therefore, is that it allows a private citizen who does not have access to 9mm +P+ (arguably the best stopper in the 9mm field, although that could even be argued these days - it certainly was true at the time this SIG cartridge was introduced) in an over-the-counter, consumer-available-without-restriction cartridge. The round is also very accurate. That, unfortunately, is the end of the good news.
SIG, which has a relatively narrow line of guns aimed mostly at agency and officer sales, with the attendant commercial hangers-on that always come with LE contracts, wanted a cartridge bearing its name. Essentially, they wanted a "coup" like S&W had achieved with the .40 S&W. The .40 S&W recreated the FBI 10mm Auto ballistics developed after their search for the "ideal" cartridge following the infamous Miami shootout, but in a shorter case. Thus, the .40 S&W achieved the desired ballistics in a case length that could fit into the envelope of a 9mm sized pistol, which was needed because of the increasing numbers of officers with smaller hands.
At the time, following the Miami shootout, however, the term "9mm" had an ineffective connotation, and agencies were leaving that round in droves. On the other hand, the new cartridge which SIG hoped would take over the LE market had to take advantage of existing technology and fit into existing platforms to keep development costs at a minimum, hence the use of the .40 S&W case and the 9mm bullet.
The name selected for the cartridge is a piece of marketing genius, if not a jab at weapons committees and police administrators, who were duped into thinking they were getting real .357 Magnum revolver ballistics in a .40 caliber pistol. Recall that the last revolver round regarded as devastatingly effective (as opposed to merely adequate) used by LE prior to the en masse switch to semi-auto pistols was the .357 Magnum 125 grain JHP at 1,400 to 1,450 fps. The .357 SIG falls short of this goal and instead performs more like a .357 Magnum in a snub or a hotly loaded 38 Special in a 6 inch barrel.
In fact, with a more effective bullet, the .38/44 of 1929/1930 will do the same job as the .357 SIG, albeit with less rounds on tap and in a larger weapon. For that matter, a properly set up 38 Super is also capable of the same ballistics, as is the .356 TSW (there's that S&W name on a cartridge again) and 9 x 23 Winchester. There are perhaps others as well. SIG had to have a cartridge that would fit into a short action like a 9mm length of cycle or envelope, as opposed to a long action like a .45 ACP/38 Super envelope in order to fit a wider variety of hands. Plus, S&W and Winchester had already done the homework on the .40 S&W case.
The fact that the .40 S&W already did the same job ballistically was not the point. Marketing for SIG was the point. That is fine, I suppose, as long as everyone understands it for what it is.
In actual fact, the .357 SIG did not really catch on, and only a handful of agencies actually use the cartridge. It is frightfully expensive, much more so than the 9mm +P+, making consumers turn up their noses. Those who have to have one because it is used by the Texas Rangers, Secret Service, etc., get one, but must also have a Texas sized bank account or oil rig to feed the weapon. Agencies contract for ammo and compared to hobbyists, most officers shoot surprisingly little.
So, in its favor, the SIG cartridge has accuracy and 9mm +P+ ballistics, for those who cannot find 9mm +P+ ammo.
Against it is prohibitively high cost for the consumer (agencies take note as you are spending OUR money). This may not be important as it seems many gun owners seem not to shoot much. Or, a box or two a year is deemed "shooting alot" by these folks. Also against the SIG cartridge is high chamber pressures, which makes it punishing for the pistols which chamber the round. It is so hard on the weapons that when Homeland Security announced winners in its huge weapons contract a few years ago, SIG's own pistol could not stand up to the pounding over the length of the test and was not approved in that caliber. HK, which got approved in all calibers, was the only pistol approved in the .357 SIG cartridge.
If your goal is to just try it out, by all means, have at it. If, on the other hand you believe it will give you something not available in .40 S&W, then pass. What you will get is 9mm +P+ ballistics but at a greatly enhanced cost, less rounds in the magazine than 9mm in the same size magazine and your pistol will sustain a pounding that most cannot take over the long haul.
Compare:
125 grain .357 SIG at 1,350 fps
115 grain 9mm +P+ at 1,300 fps
127 grain 9mm +P+ at 1,250 fps
These are, for all practical purposes, the same and I doubt any criminal hit with any of these could tell the difference. Thus, for less cost for practice ammo and greater magazine capacity including less wear and tear to the weapon, I would choose a 9mm over the .357 SIG. I would still choose a .45 or .40 S&W before either the 9mm or .357 SIG, however.
I have also posted a picture comparing bullet performance which I saw in another thread and which pretty well tells the story of the .357 SIG versus 9mm:
