What is port size?
20" barrel so I'm going to guess that this is a rifle gas system, correct?
Rifle or CAR buffer? If rifle, then 5.9ish oz buffer whereas if you are running a CAR buffer it can be anywhere from 2.9 oz for a standard CAR to 5.6 oz for an H3.
What chamber is in barrel? Original 6.5 Grendel, 6.5 Grendel II, 264 LBC or ??? Differing amounts of freebore and different lead angles, will have an effect what you can load.
What brand barrel?
What load you'll be able to run is going to depend on a lot of variables, only a small portion of which I asked about above.
Load that runs like a raped ape in my 21", rifle gas system, rifle buffer setup is not very good in my 18", mid gas, H2 buffer rifle. More data needed from you on rifle configuration to be able to give you any useful information.