Enlarged gas port with adjustable gas block, and can play with buffer mass as well in needed.

Also, before going too far, make sure you don't have gas leaks, or blockages of the gas system. Hence gas block port aligned with block passage port, no leaks between block to barrel, tube to block, carrier key to top of carrier, and gas tube is correctly aligned with carrier key as well.

Also, leave recoil spring alone, so rig does have enough tension on the forward stroke to correctly strip a round out of the mag, and lock the bolt up correctly.

Since rig is not full automatic, and don't have to worry about bolt bounce on lock up, so if you need a lighter buffer, can either remove the weights from the buffer, or just mill up a solid buffer out of Delrin.  

Here is a system that use shorter ones  on each side of the recoil spring, but single longer one the length of the standard buffer works just as well.
https://www.tacticallink.com/taccom-ultra-lightweight-carbine-buffer-for-ar15.html

Hence buffer first order of business it to keep the carrier key from crashing into the lower receiver threads where the receiver extension tube threads on to it, and the very first buffers for the rig where just made of plastic.  Later on, military wanted to slow down the fire rates of the rigs in full auto, and that is where the dead blow effect type buffers where introduced as upgrades (which also changed the ejection path of the spent cases from forward, to backwards out of the port, and the upper receiver deflector came into effect so south paw shooters would not get the spent case into the face).

So on full charging handle pull, face of bolt should stop 1/8"~1/4" in front of the back edge of the ejection port window, and last inch of pull should be the same tension at the middle of the pull (spring not too long and coil binding up on itself before the back of buffer touches the back wall of the void of the receiver extension.

Also, keep in mind that you can produce sub sonic loads with slower burning powder and heaver bullets,  will produce more barrel gas pressure to being with.  Hence at sub sonic speeds, no bullet is going to preform well with any kind of expansion, so your really just lobbing a chuck at lead at the target, and the fatter the nose is on the bullet, the harder is going to hit and transfer it power into the game faster.

Lost me for a second, since "touching the void" sounds like something "Six"  would say from "Tripping the riff".

As for what I meant, corrected it above, and back of buffer should touch the back wall of the receiver extension, before the recoil spring binds up on itself.  To check for such, just compress the spring all the way down on the buffer,and spring should spring bind out on itself, with the top of spring below the back of receiver buffer end.  If bring this up, since if you use a full lenght spring in a carbine receiver extension and  shorter carbine buffer, will be the spring that coils bind out, before buffer bumper can touch the back wall of the receiver extension itself.

Also, the carbine spring is made from thicker gauge wire than the longer rifle spring, so bad idea to chop down a rifle spring, to turn it into a carbine length/coil amount wind spring.

Hence different type receiver extensions have different inner void depths, and since amount that bolt should come back is a constant so back of key does not crash into lower receiver threaded ring face, different lenght buffers and springs, to go with each type of receiver extensions void depth.