1000 / 500 gram per bullet would be good, if I do say so myself. Since the actual bullets costs 10$, which means that the grams have to be a lot in order to get the bullets.
Since we are able to sell irons in F7, players are selling 1 gram of iron for 10$ (sometimes 12 or 11), and MG bullet is already for 10. Crafting 4 grams (which if you sold, you will get 40$) to get 1 bullet (which is sold for 10$) of MG is kinda unfair because it will be like getting 40$ then crafting it for 10$(one bullet). Crafting 2 grams of iron to get 1 bullet of MG will be good imo.
Well, I like your idea but don't you think that if we added it to crafting iron GUI it will be useless ? because if you tried to use the MG you will note that it takes much ammo in the sec. so you wouldn't get anything by working for many hours and craft your iron to ammos that you will shoot it in few seconds ..