Histone methyltransferase that dimethylates histone H3 at 'Arg-17', forming asymmetric dimethylarginine (H3R17me2a), leading to activate transcription via chromatin remodeling. L-arginyl-[protein] + 2 S-adenosyl-L-methionine = 2 H(+) + N(omega),N(omega)-dimethyl-L-arginyl-[protein] + 2 S-adenosyl-L-homocysteine Belongs to the methyltransferase superfamily. METTL23 family.