One thing to take into account when planning to craft firearms, is that barrels have fixed mechanical stat requirements.
5mm requires 10 skill, 9mm 25 skill, 7.62mm and 12.7mm 35 skill and 0.44 45 skill in mechanic.
Since you can not make firearm without barrel, plan accordingly.
Anyway, I wold definitely put points on 1st level to crafting skills you want to focus, since later you will not have enough skills points to raise them quickly. But later, I would not raise them every level, only when I think that there is something worthy to get crafted.