In a PCC build you are supposed to use their buffer in a regular carbine extension, not the A5 extension due to the difference in bolts/carriers. It may work in the A5 extension but you will want to make sure the overall length is legally compliant.
In terms of weight, I’m running the KAK extended heavy buffer which is 10 oz., the same as the A5H4 buffer. I would suggest starting with the A5H4 and swapping out weights to go lighter if needed.
Not sure about the green spring, a .308 carbine spring is the normal spring used. I tried a Sprinco red spring but switched to a Tubbs 42 coil flat spring - mostly because I am tuning for 147 grain loads and needed a spring with a little less force.
I used a Vltor A5 pistol tube with a A5H2 buffer and Sprinco Green Spring with a LAW folder and Tailhook Mod 1 under a BCM KINO. My thought was I didn’t need a heavier buffer due the additional weight of the LAW folder carrier extention. Also, you will need an open end style wrench to tighten the castle nut.
I ended up calling Vltor tech support to ask for their opinion and to make clear of some of the questions I had.
He educated me and advised me to use a carbine extension with a rifle spring since I plan to use QC10 bolt. Said he would recommend an A5 extension if I were to use a CMMG bolt since their bolts are longer compared to the QC10.
He also told me that he believes a sh1 should be where I should start so that’s what I bought. I low key feel that I should of started with a sh2, though. He stated that he himself has a 10mm pcc and is using the sh3 but thinks he’ll be stepping down to a sh2. him having a 10mm and feeling he should step down to sh2, is why he feels I should start with a sh1 because I’m doing a 9mm
FWIW, Pistols can be any length. (I might have misunderstood you) The main “rules” are anything shorter than a 16" barrel, and any rifle shorter than 26" (regardless of barrel length) is an SBR and needs to be registered. Unless it has a pistol brace, and no verticle forgrip. Then it is a pistol. An AR pistol can be any length under 26".
For what it’s worth, my 9mm has a full heavy Odinworks 9mm adjustable buffer in a carbine length tube and it runs nice. It also has a LAW tactical folder so the carrier extension adds a little weight but I had to cut my carrier weight by about 5/8" to fit the extension so it’s probably not far off a non-folding setup.
I haven’t bothered trying to tune it with weight because it runs fine. I think the spring is just standard 5.56 carbine spec.
You don’t need to complicate things if simple works…