I am building a 9mm upper to go one one of my existing lowers.  It is a rifle length buffer tube.  I will be running mostly subsonic 155 gr cast bullets for use with my suppressor.  I have read all I can on buffers and springs and spacers and broken bolt catches.  I know heavier spring and buffer will slow the cycling, which is good because the bolt will stay closed longer and the system will be quieter, and it will decrease felt recoil.  But it may not function properly if I go too heavy.  Also a heavier spring may increase the chances of bolt catch breakage.  I am hoping for some input and maybe some experiences with what I am trying to accomplish.

Thanks!

A2 stock assembly,  std 16oz colt pattern bolt, A2 rifle buffer with brass dowel that weight 7oz, for total of 23oz of moving weight.. I shoot what ever with no issues

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I've never used a rifle stock, but in my carbine tubes I have used the JP Silent spring (It comes with a spacer for a rifle stock) and one of Slash's heavy buffer and spring. Both work well and I've never broken a bolt catch in many thousands of rds fired. If you can afford it the JP is nice, especially if you plan to add a silencer.