I have one that takes S&W59 9mm mags. Changing the magazine catch can convert it to a few others, like Baretta, Kel-Tec, etc.
Some people have modified Springfield XD9 mags by cutting a notch in the side as well. (Glock versions are not convertible.)
It is fun to shoot, and gets a lot of attention at the range. It fits inside a standard laptop bag, along with a few extra mags, so I find it the ideal back-up to a CCW Kel-Tec P11 handgun.
It gets dirty fast due to the blowback design, and after about 150 rounds at a time, the bolt will sometimes get sluggish enough to cause FTEs (depends on ammo). Pulling out the bolt, wiping the crud off, and liberally oiling it will put the gun back into action in under 2 minutes. (This gun likes to run WET, far wetter than you might normally consider prudent.)
The 1 inch stock extension, and a tube cover are must-haves in my book. The grip is annoyingly rough and I have a piece of bicycle innertube on it, which is a nice addition as well.
It is as accurate as I am and I've had no problems with it. It is a very light, super compact rifle that fits a very unique niche by folding and taking common handgun mags.
I would not ever consider turning a .40 cal S2K into a 10mm as the bolt does not lock, (blowback design) and the weight of the bolt and the spring is what keeps the breech closed until the bullet exits the barrel. The engineering required would be prohibitive, and even 9mm hits .357mag ballistics from the longer barrel, so I'm not sure it needs any more power.
It is not a $1000+ AR or a $750 AK, so it isn't really fair to compare it to those platforms.
It is kind of like a Mazda Miata: Fun, cheap, kind of impractical because of it's inherent limits, and isn't a performance champion. But it still turns heads, and does what it is designed to do very well. Few people would have a Miata as their only car, or for certain types of travel, but tooling around with the top down is still a blast despite that.
If you can get one at a reasonable price, I would say get it. I know people that will pay a real premium for Glock-capable versions on Gunbroker, but $350 or less is a fair price IMO.
