If they are 300 grams now you will have to feed pretty heavy to get them to weight and that can cause them to go off feed thus negating what you are trying to do! Not sure what weight you are shooting for as your selected breeding wieght so it could be quite possible also! I like to see about 100 grams weight gain a month from 300 to 700 grams then maybe a little more than that say 120 to 140 grams a month till they make 1200. From that point if you are feeding them large enough prey, they pick up the pace pretty quick as long as they don't go off feed!