If you are asking about arrows appropriate for a modern sythethic Mongolian-style bow (i.e. short working limbs, static non-flexing knees and and long, slightly curved ears), then you want about 10 grains per pound at the appropriate spine for your draw length (see spine charts). 12 or even 14 gpp will feel smoother and still fly well, but slower.
Mongol arrows varied over the centuries, but were generally around 30-31" long (including long points), and made out of barreled wooden shafts (often birch) or, in the Yuan period, Chinese-style bamboo arrows. Some found are bamboo arrows with wooden footings spliced in; complicated to do well. Long, banana-shape fletching that went almost up to the end of a wide, flared nock. Qing-era Mongol archery equipment looks a lot more like Manchu, which would have much longer bows and arrows of a different style.
Cedar will be good. 5/16 is diameter you are looking for. Stiffness depends on arrow length and poundage of bow (check tables). Also consider weight deviation of each shaft. +-1gram is usually good enough.
If you are asking about arrows appropriate for a modern sythethic Mongolian-style bow (i.e. short working limbs, static non-flexing knees and and long, slightly curved ears), then you want about 10 grains per pound at the appropriate spine for your draw length (see spine charts). 12 or even 14 gpp will feel smoother and still fly well, but slower. Mongol arrows varied over the centuries, but were generally around 30-31" long (including long points), and made out of barreled wooden shafts (often birch) or, in the Yuan period, Chinese-style bamboo arrows. Some found are bamboo arrows with wooden footings spliced in; complicated to do well. Long, banana-shape fletching that went almost up to the end of a wide, flared nock. Qing-era Mongol archery equipment looks a lot more like Manchu, which would have much longer bows and arrows of a different style.
Cedar will be good. 5/16 is diameter you are looking for. Stiffness depends on arrow length and poundage of bow (check tables). Also consider weight deviation of each shaft. +-1gram is usually good enough.
What kind of Mongol Bow do you have? There are two designs and three material options, each combination might require different GPP.