how much will more weight on the buffer slow down a bcg to reduce blow back from a suppressed 16" barrel 6.5 creedmoor?   does the weight and spring really make a difference after the gas block is adjusted right?

MY BCG WEIGHS 17.75 OZ'S

MY WEIGHTS ON THE SCS ARE 2 TUNGSTEN AND 1 STEEL.  

ANOTHER TUNGSTEN IS LIKE 50 BUCKS,  WILL IT MAKE A DIFFERENCE?  

Start with buffer weight. Go as heavy as you can.

If you are severely over-gassed, you’ll need both mass and gas control. However, the more mass you have, the greater your performance envelope will be with gas adjustment.

I'm going to assume you have an adjustable gas block (if you don't, get one).  

As stated above, you want to go heavy on the scs, so add a 3rd tungsten weight.  As far as spring, that's all going to depend on your rifle.  The factory scs one is ok but I put in the heaviest one (black spring??? Can't remember. Maybe it's green. Idk).  But it was a noticeable difference.  But from my understanding, the spring kits were more ment for people wanting faster bolt speeds or people not wanting an agb.

The spring weight did not really do very much at all. Mass was the answer. Moving up in spring weight did not stop early opening and moving down did not let me cut the AGB setting lower and still run.

I started out trying to do an H1 weight, went H2, then eventually a third tungsten weight to get the rifle to cycle suppressed and unsuppressed AND not open the chamber early suppressed to make my brass and gun filthy when tuned to run on a single gas setting that runs unsuppressed. I did this with the LMOS carrier but it ALWAYS opened early suppressed when given enough gas to run unsuppressed, even with 3 tungsten weights.

Once I moved to the FMOS carrier, it was still doing the same with 1 tungsten weight, was almost good with 2 and only came back with perfectly clean brass with all 3 Tungsten weights and the full mass carrier.

It is for this reason, for anyone doing a suppressed large frame build in the future, I would reccomend an AR10/A5 receiver extension, the regular 5.4oz AR10 carbine buffer, and Armalite AR10 carbine EA1095 spring (or JP-OSC-308 polished spring). Because that is where I ended up anyway. And those parts (Armalite EA1095 spring or JP OSC-308 spring and the Armalite/H3 buffer) are WAY WAY less expensive vs the SCS kit with all tungsten weights.

The spring plus buffer is only $55-60.

The 308 SCS H2 with extra tungsten weight (which I bought off a friend) cost me over $200. (Retail is about $250 if you get 3 tungsten weights and also get the special JP SCS capture detent)

Unfortunately, you are tied into either a carbine receiver extension or an A5 receiver extension, depending on which way you go. For this build, I went with a V7 2055 receiver extension in carbine length and SCS was 7.4oz with 3rd tungsten weight.

This means the A5 length buffer tube, H3 buffer, and JP 308 spring weight is EXACTLY the same as the carbine buffer tube and 308 SCS with 3 tungsten weights. If you compare weights with the 5.4oz Armalite buffer, I could have been just a smidge UNDER the total weight of the carbine SCS combo. If my next barrel let's me cut 1 tungsten weight from the SCS, it cuts 0.8oz. Moving from the Armalite 308 carbine buffer (5.4oz) to H2 (4.6oz) is also a 0.8 oz cut.

In other words, the SCS does exactly NOTHING for the money other than have tunable springs, which I found to do basically nothing for my use case while requiring a special retention detent if you want that feature. I even use a the excellent  Magpul SL stock, which is free from wiggle and sizeded for the A5 tube length, so it was not like I am saving weight for a lighter stock or using a carbine PRS stock either.