The “Nazi army helmet” referenced in the question wasn’t a NAZI helmet.
The word “helmet” derives from the diminutive of “helm”, which is traditional head armor going back centuries. The distinctively shaped German Stahlhelm (translation: steel helm) was created in 1916 during WWI to replace the spiked Pickelhaube, and its body made of stiffened leather.
Kaiser Wilhelm with staff in 1915 wearing Pickelhauben with cloth covers.
The Pickelhaube’s leather may have protected against saber blows, but did very little to protect against the shell fragments and shrapnel that had increased by orders of magnitude in the battlefield’s trench warfare as compared to prior wars, even as late as 1905. While high ranking officers, especially generals and field marshals continued to wear their Pickelhaube, the rank and file soldiers in the trenches were issued the new Stahlhelm. Its shape evolved some through the years as its manufacturing was simplified and improvements were made to facilitate better hearing while wearing it. Shown is an M35 Stahlhelm (the 1935 version). It continued to have revisions until 1944 while maintaining the same basic shape of the original 1916 helmet.
M84 PASGT Helmet (without camouflage cover or night vision device mount).
The initial M84 PASGT introduced in 1984 didn’t resemble the M42 Stahlhelm as closely as people generally believe, even though some soldiers referred to it as the “Fritz Helmet”. The iconic M1 helmet from WWII, aka “Steel Pot” had evolved some in minor details since WWII (mostly in the removable shell liner), but not its general shape, and it only stopped low velocity shell fragments and shrapnel. Its steel did not stop 9mm or .45 ACP pistol rounds. Major General Maurice Rose’s helmet with its three bullet holes (and possibly a fourth) is on display at the U.S. Army Museum. He was shot in 1945 by a German tank commander using his 9mm Parabellum machine pistol (fired pistol rounds). MG Rose was the commander of the 3rd Armored Division when he was killed. A PASGT would have stopped them, although he would have died from the additional rounds that hit him in the thorax.
For comparison, the iconic M1 helmet without net or camouflage cover. It has Less coverage around ears and back of neck.
MG Rose’s helmet showing its 9mm Parabellum bullet holes (without liner). Large white stripe on back was used on officers’ helmets during WWII to identify them as such to other soldiers.
The Kevlar PASGT (aka “K-Pot”) had significantly better ballistic performance (NIJ Level IIIA) with ability to stop 9mm and .44 Magnum rounds. Ability to stop rifle rounds (notably 7.62x39 Soviet/Chinese AK rounds) depended greatly on distance and how it hit the helmet. Direct hits at 90 degrees to the helmet surface from close range would easily penetrate. At an angle from a longer range of 100 or more meters might or might not. Numerous anecdotal trophies exist of a PASGT stopping an AK-47 or AK-74 round, but those are the exception, not the norm. It had better coverage over the ears and back of neck, but did not flare around the lower edge as the Stahlhelm did. It bulged some over the ears and had a much smaller well-defined brim. The only similarity was lower coverage around the ears and back of neck, providing better protection of the lower head on the sides and back. Iterations since the M84 PASGT with the U.S. Army’s MICH, ACH and ECH have adjusted the shape to provide for easier use of communications and hearing protection along with modestly improved ballistic performance. The USMC went to the LWH (lightweight helmet) which looks almost identical but with reduced weight using improved manufacturing methods without sacrificing ballistic performance.
ECH helmet, the shape of which didn’t change much from the ACH. Gone is the brim and the cut isn’t quite as low around the ears and back of neck. No night vision device mount or camouflage cover to show shape better.
The ECH and LWH are being replace by the IHPS (Integrated Head Protection System) made using a polyethylene material versus the Kevlar previously used. Note the cut around the back of neck that’s lower than around the ears. Includes mounting in front for night vision devices and rails on sides for other accessories.
The shape of the 1984 PASGT and its successors have provided better protection coverage and substantially better ballistic performance compared to the M1.
Source: John Lind Former Major at U.S. Army (1976–1997)