The Colt SMG has only been available in the 9mm configuration. The standard AR15 magazine well is too narrow to accomidate double stack .45acp magazines (such as the M3 Grease Gun). This has been successfully negotiated by Cavalry Arms out of Gilbert, AZ with their MK II lower reciever, that does accept the Grease Gun magazines.

Regarding the penetration issues, a 5.56mm round is safer than a 9mm round. IF the shooter does his part, the 5.56mm round will stop in the target. If you miss, a 9mm will go through more walls (interior and exterior) than a 5.56mm will.

re: .357 magnum vs. .357 Sig, if you look up the loadings on any commercial ammunition manufacturer's web site, you will see that their performance is identical. The only difference, is that you can use the Sig round in a standard autoloader, while the traditional .357 S&W Magnum will only work in a Desert Eagle, a tad much for daily carry.

The secret service uses either the HK MP5 Submachine Gun or the FN P90 Submachine Gun the light weight round of the FN P90 is a 5.7x28mm is a light weight round specifically designed to penetrate body armor and is highly accurate.. The pistols they use is either the FN five-seven chambered with the same 5.7x28mm round or the Sig Sauer P229 used a .357 Sig round. They also use Remington 870 Shotguns. Some of their snipers use the HK PSG1, some use custome built 7mm magnums, and some use completely custom built guns... Hope all that helps

I don't think you can really say a 5.56 is less effective than a 9mm at any range. Some argument could be made if the 5.56 is restricted to fmj ammo,perhaps,but I doubt if the Secret Service is restricted to those rounds. With proper ammo,the 5.56 is better at pretty much everything than the 9mm. The 9mm,with good HP rounds, is probably less likely to over penetrate and cause collateral damage and is also available in smaller,lighter and more managable packages than the 5.56. You can obviously get some pretty short 5.56 carbines but when you start using that short of a barrel you really take away the 5.56's advantage which is speed.
As to why the SS uses the .357 Sig, I am a little puzzled at that choice. Many test over the years have shown the .357 Magnun and the .45 ACP to posses very similar stopping power, and although the Sig has a snappier recoil,over all teh .45 with full house loads,in a comparable weight pistol,would probably posses overall greater recoil. I'm sure there are numbers published somewhere that bear this out.
I do recall that the FBI originally adopted the 10mm round but has since switched to the .40 because the recoil was too much for many of the agents. The 10mm is indeed a bit of a handful in a light pistol,but in pure "power" stand well above the .357 Sig.

I've seen a .357 bring down a deer. It truly hits hard. Honestly, I love the .357, always have. I wanted to buy a revolver soon and it'll be a .357 if I do get one.

I think the .357 is probably a happy medium, which is why they probably use it.

The .357 magnum has taken everything from rabbits to moose and grizzly bear. This was done during the time the round was being developed, mainly as an attemt to show people what it was capable of. Of course the .357 is by no means ideal for hunting large game, but it proves that with proper bullet choice and perfect shot placement it can indeed take very large game. I'd say it is quite adequate for taking medium size thin skin game(like deer) at ranges up to 50 yards provided a proper bullet is chosen. Personally I'd probably take a 180gr SP or a good hard cast for those applications.
The .357 built it's reputation as a man-stopper with 125gr hollowpoints and that's probably what I'd put in it for self defence. If you want a really interesting and versatile combo,pick yourself up a Marlin or Rossi Puma leveraction in .357. The extra 10-12 inches of barrel length on the rifle turns the .357 into a whole 'nother animal, and actually makes a very viable self defense weapon. Being short cartridges,you can fit about 10 rounds in it and the lever throw is short enough that you can fire it quite rapidly. With practice you should be able to reach accurately to 150yds or so,far greater than your revolver. Plus, you don't have to worry about carrying two different types of ammo,and both will take .38 Special rounds as well,making for cheap and recoil free plinking!

As an LEO, I carry the Sig 2022 in .357 Sig as my duty weapon. It is a man stopper but is also highly preferred for it's accuracy. It is a very flat shooting and gives pin point accuracy out to 100 yds. I can't do it, my eyesight is not what it use to be, but our instructors can.
As far as the a handgun round being a low powered round, I have to disagree. A gun is a tool and just as you woud not use a .45 acp to snipe with you would not use a rifle as a belly gun. A handgun, any handgun, (within reason, don't go crazy here) is an effective weapon under 25 yds and that is the distance most gun fights occur. I had just as soon have a high capacity .22 pistol at my disposal when needed, as a .44 mag. There is no kenetic energy in the .22, but it is like shooting a surgical scapel. It enters the body and bounces around a while. Seriously, it may enter the shoulder and come out of the thiegh while slicing through everything in between. The last murder in my town was by .22. The guy died in place where he was hit.
Simply put, a gun is for killing, and all of them work.
Seeashrink

All of them work,yes,but some much better than others. The .357 Sig is decently fast and therefore fairly flat shooting for a pistol. Still, to me it seems that a flat trajectory would be about the last criteria I;d be looking at when choosing a self defence pistol round. The average gunfight takes place at somewhere around 7 yards,I believe. Very few take place beyond 25 yards,and trajectory on any of the major calibers will not be an issue.
Accuracy is determined first by the firearm,not the cartridge. Very few people can shoot a pistol as accurately as the weapon is capable of,even under the most ideal conditions. Some pistols are earier to shoot accurately because they produce less severe recoil,thus reducing the chances of developing a flinch. Thats why people often shoot most accurately with a .22, not because it is inherently more accurate but because they are not anticipating recoil and muzzle blast.
Plenty of people have been killed with a .22,probably more than with most other calibers. This doesn't make it more deadly than the others, just far more prevalent and avialable. About the only way to guarantee a one shot stop with a .22 is to shoot someone in the head. Even then some have survived. In a gunfight, I want something that will most likley stop the person right now, not let them continue on and maybe expire hours later.

Im only Eleven years old but im a hardcore conspiracy theorist. im positive that the US NAVY SEALS also use the sig saure p229. Im shocked that the US and every other nation and military superpowers are not using the polymer AK74? If i was creating a gun and survival cache i would have a AK74 and many loaded magazines. If your shocked of my knowledge of military superpowers and conspiracies its because i spend all day on my computer learning about politics,superpowers,conspiracies and weaponry. if you have any military questions or such you can email me at Sk8truntd@gmail.com