Sucks to hear about your cylinder.
As you said, it was assembled from the factory and were able to fire off 5 mags so obviously this one would have passed QC. Challenge kit or not, that just as easily could have happened with a pre-built gun.
I really don't want to sound so evangelistic for the challenge kit, mostly because I know I'm going to jinx my good experience. The second I finish tying this and go home, I know something will go wrong
The other thing to note is that there is definitely going to be a greater volume of people on the Systema boards having problems or asking questions about their challenge kits just due to the fact that there is assembly required. Call it a squeaky wheel, chalk it up to the fact that people don't generally post about things that just work or sometimes it's easier to post than it is to read the instructions a couple of times......whatever.
Sell somebody a toaster that requires you to insert a pin before it will work and there will be a 4000% increase in support requests
Anyway, hope things work out with your cylinder.