I also had Army MEU, their gun is pretty good for the money. I suggest you use .28 g BB Bastard, They are pretty good and very consistent for the gun that yo have.
I tested the .20 g and .25 g with 10 meters distance, they are not consistent even I aim right on the target then I switch to .28 g and test it again at the same range, oh ! I love .28 g BBs. It almost punch on the same hole on each shot.
You might want to get .30 g to test out as well but I prefer .28g with 2000 rds just only 15 $.