The first modern combat helmet was the French Casque Adrian, which was designed to address the threats soldiers faced in the Great War. The ballistic helmet was never designed to offer protection from small arms fire. The steel helmets of WWI and WWII were developed to protect against indirect fire, such as mortar and shell fragments. These helmets were not officially rated to stop any handgun or rifle projectile. Military planners knew well that indirect fire from mortars and artillery can inflict terrible casualties, and the first helmets were designed and issued to counter those particular threats.
In WWI, explosive or fragmenting munitions were responsible for roughly 60 to 70 percent of all combat casualties. At the battle of Verdun, fragmentation and shrapnel from artillery bombardment caused at least 70 percent of the approximately 800,000 casualties that both sides suffered. The remainder were, for the most part, inflicted by relatively heavy rifle and machine gun rounds which even the best helmets of today would not be able to stop.
The French Adrian was made of mild steel, 0.7 to 0.8mm thick, with a tensile strength of at least 415 MPa and moderate ductility (18 percent tensile elongation). This helmet was capable of resisting a 230-grain, .45 caliber ball round at 400 to 450 feet per second, which is roughly half the .45 ACP’s muzzle velocity. Notwithstanding this poor performance against bullets, it is estimated to have defeated 75 percent of all shrapnel impacts from airburst munitions, and it had, therefore, an immediate positive impact on troop casualty rates and morale. In the Adrian’s wake, every other participant in WWI—except for Russia—hastened to develop and issue steel helmets of their own. Like the Adrian, these helmets had very poor resistance to small arms impacts, but were highly effective at protecting their wearers from shrapnel and fragmentation.
These same steel helmets, with minor modifications in some instances, were employed by all American and European forces through WWII. Here they proved even more vital, for whereas fragments and shrapnel accounted for approximately 65 percent of all WWI casualties, they accounted for 73 percent of WWII’s wartime wounds.
The Adrian helmet specifically holds up well in some respects when measured against 21st-century combat helmets. A 2020 NPR report claimed:
A recent study done by a team of Duke University researchers finds that the 105-year-old "Adrian" helmet used by the French army in World War I can provide better blast protection than the Advanced Combat Helmet (ACH) widely used by the U.S. military.
It was only when those helmets were exposed to overhead blast waves that the 1915-era Adrian helmet outperformed the others. The Duke researchers point to the raised metal crest running from the front to the back of the Adrian helmet—a design feature also found on helmets used in those times by French firefighters—as a likely explanation for its superior protection from overhead blasts."
Sources: Adept Armor; NPR, 21 February 2020