Depends on your crafting level, hopefully you know what tincycalc is that is very helpful if you are new to crafting. Low level start making Pistols like Berretta M9, HK45 CT, Desert Eagle and also MP5K all while doing this having lots of pistol ammo to sell also. When you get higher in the levels craft SCAR, AK-101 I highly recommend you make them with Riffle compensator and also Red dot as people tend to buy full kitted guns more. Tinycalc will help you figure out profit margins like how much a gun costs to make, I tend to add roughly 15% - 20% on the crafting price.
Honestly i'd just craft pistol ammo and rifle ammo cause they sell the best, you can do crafting orders and possibly ask to become a crafter for vault corp down the line.