Go here for an overview and some great links including manuals: http://www.operation-helmet.org/helmets.html
US troops in ACH's talking with Republic of Korea soldier in an M-1 steel pot pattern helmet.
The M-1 steel pot standard US Army issue from 1942 to the 1980s. Instructions on this helmet can be found in FM 21-15.
Troops of the 101st Airborne Division wearing PASGT's, aka "Fritzes" aka "K-Pots."
Personal Armor System for Ground Troops (PASGT) helmet (also called the Kevlar, K-Pot and Fritz) was first fielded to U.S. military units in the early 1980s. The helmet, available in five sizes, provides ballistic protection for the head from fragmenting munitions. It is a one piece structure composed of multiple layers of Kevlar ballistic fiber and phenolic PVB resin. For a complete story, go to olive-drab.com. This helmet is now being removed from inventory in ALL services, replaced in the Marines by the LWH (see below) and the Air Force by the Army-style ACH helmet.
Link for the K-Pot operator's manual:
Manual for Advanced Combat Helmet:
Marine Light Weight Helmet (LWH)
Lightweight Helmet (LWH) was developed for the US Marines to replace the PASGT. Looks very similar to the PASGT, but has a four-point chinstrap built-in and is about 6 ounces lighter. Originally it had a web suspension system and being upgraded to padded suspension. Primary users: Marines and Navy FMF.
LATER: Cost of helmets around Alabama/Southeast:
ACH or MICH $100-$150 each at gunshows and surplus stores. Currently I can get them for about half that.
PASGT/K-Pot $50-$75 each regular price (depending on condition and size). I can get them for $40.
M-1 Steel Pots. $20-$30 each. Sometimes I can find them for less, sometimes not.