Overview & Obtaining
Military helmets offer cheap (albeit inferior) protection for one's head, but impose a move speed penalty of 0.04 c/s. The main pro of Military Helmets is the cheapness to manufacture and purchase, requiring little more than half of the materials that Kevlar Helmets require, and also less than half as expensive when purchased, and Military Helmets can also be made out of various metallic materials which will affect their durability and therefore their lifespan. They can be purchased via trading, produced at a machining table, or looted from Outlanders and Pirates.
Military helmets must be manufactured at a machining table, which in turn requires machining to be researched in order to be constructed. A military helmet requires 40 building materials (i.e. steel, plasteel, uranium, silver, gold), 2 components, and 100 seconds of work (6,000 ticks) to be created, assuming baseline global work speed and no enhancements or hindrances to consciousness, sight, or manipulation.
Usage of different materials does not affect the protection offered.
Conclusion and Comparison
Military Helmets are one of the three main headgear protections available in RimWorld (the others being the kevlar helmet and power armor helmet), but also the least protective out of the three. While being the least protective, Military Helmets do have a remarkable lifespan due to the inferior damage absorption, especially if made out of Plasteel or Uranium. They also impose the greatest movement speed penalty along with power armor helmets, with a 0.04 c/s penalty, whereas kevlar helmets only impose a 0.02 c/s penalty. Military helmets are intermediate in terms of mass with a mass of 2kg, whereas kevlar helmets have a mass of 1.2kg and power armor helmets have a mass of 2.7kg.
The following table is an hierarchy of the combinations which a military helmet may offer with varying quality levels (and full durability):