For AR 9mm PCC guns that are going to be run suppressed, the buffer and spring needed is basically like adding a few inches to the barrel length.  So if you have a 5.5" barrel, it is like having a 7"-8" barrel so you could really use several buffer and spring combos and have them work, but optimal would be our RB5007 standard 9mm buffer and the JP 308 Carbine Buffer spring (part no. JPS-OSC-308).  If you have a barrel longer than that running suppressed, then just move up to the RB5015HD buffer with the JP 308 carbine spring and you'll be good to go.  These are simple, drop-in combos that will run great.  If you want absolute optimal performance with a suppressor, you could add our PCC buffer weight to the set-up and get an AR10 carbine (or A5) buffer tube to put it in.  This will give you more weight and smooth it out a bit more and if you are concerned about gas blowing back, this will help with that.  It is not required and a lot of our customers run suppressed with just the buffer and spring and it works great, but this option is there if you want a bit more weight in the package.  But we recommend that you only add the weight with an AR10/A5 carbine buffer tube so that it remains a full-stroke system.  Otherwise, you will increase the recoil impulse.  

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