So far you can go from a crappy bow to an M4 with not much in-between in next to no time at all, this just not seem right. An M4 is a highly technical piece of kit, something that needs specialist tools and equipment to built not to mention a lot of technical training, it does not seem right that you can knock one out using just a furnace and random rocks you have found. I propose a change in which will see things a little more realistic and make the M4 much rarer.
Tier 1: Hatchet, Bow and Arrow. Real primitive weapons but one which anybody nowdays could make with a bit of incentive and next to no training.
Tier 2: Black powder weapons, not very accurate and slow to reload but they should hit much harder than Tier 1. You can have a Musket (Rifle) for long range, pistol and a Blunderbuss (shotgun).
Tier 3: Bolt action Rifle ALA Lee Enfield or Mosin Nagant, Double barrel shotgun and 6 shot revolver. With appropriate reload times and damage this will be the most common weapons for mid level players.
Tier 4: M4, Semi automatic pistol and Pump action shotgun. This is what should take a long time to make and should be quite rare unless you work in groups or are good at hiding your location, you should not have a naked man holding a rock running around with these weapons that he made with his own hands in only a few hours, you should need to manufacture various tools and presses first.