To clarify this further:
There are two different .40 S&W Sub-2k's that use Glock magazines. If you have the Glock 22 version then you can only use Glock 22 magazines or longer (15 rnds or more). If you have the Glock 23 version then you can use Glock 23 magazines and longer (13 rnds or more), which includes Glock 22 magazines.
I just checked my gun and it doesn't say which one I have anywhere on it. The factory magazine doesn't help either since it comes with a 10rnd to make it California legal. Hopefully it says on your original box. If it doesn't, then the simplest thing to do is buy a Glock 22 magazine (15 rnds), which is 100% guaranteed to work. Then see if it fits exactly right or hangs out the bottom a little. If it hangs out then you have a Glock 23 version and can use all mags. If it doesn't hang out then you have the Glock 22 version and can only use Glock 22 magazines or longer.