Avoid all the spacers, different buffers and broken bolt catches with the correct parts.
JP bolt, SCS and a carbine buffer tube is the answer. Functions flawlessly and eats everything you feed it with out broken bolt catches.
Expensive up front but in the long run saves coin and headache.
Can't say that this works for for every receiver on the market but does with the QC10 Colt pattern.
This has been what's keeping me from building a 9mm AR...so I'm glad you posted this!
But I don't know what SCS is?