H3 would be the better choice. It is close to the optimum weight (5.36 oz.)for the Grendel. I have a H3G buffer in mine with a Sprinco blue spring. I remember when I installed these parts I had to open the gas valve on my gas block. But you definitely want to back down on your charges. Just because 28.3 gr is safe in one rifle is no guaranty it is safe in yours. You would have seen signs of trouble before this if you started at least 10% below.