I have two KA sigs 556: a "shorty" and a DMR - they are generally very well made, but... poorly designed. Don't be fooled by their sturdy looks and feel, hefty weight and not-so-bad reviews on the internet, the devil is in the detail:
1. marriage of a GB V3 with an "M4" type hop up chamber required an addition of a plastic extension tube to the latter, which joins it with the GB. That extension is rather flimsy, resulting in a really poor air seal (read: erratic FPS, dropping to 150s with M100 spring). You can try to remedy it by using teflon tape, silicon mastick, epoxy glue, but these are all half measures and sooner or later will fail because of the vibrations coming from the GB.
2. RIS/front grip is a bit wobbly. This can be remedied with a few drops of Loctite.
3. the charging handle is made of a rather cr*ppy metal and sooner or later will fall off unless you disable the blowback feature and will take special care when transporting and usign the gun.
4. barrel develops a bit of a wobble over time - you will have to remember to tighten a couple of screws every now and then, again, remembering that Loctite is your friend
5. unless you change the stock - you will have to use a battery designed specifically for the "shorty".