UK aircrew issue helmets (Alpha Mk3/4/10) offer better impact protection but are heavier to wear.
US aircrew issue helmets (Gentex HGU-5/P etc) etc are less impact protection and more of a system to keep everything attached to your head. They are far lighter and more comfortable though.
I use an Alpha Mk10B when flying ex-mil jets, and an HGU-5/P when aerobatting in civvie types.
You need to change the earphones and microphone when using them for civvie types (different impedance etc).
You will find quite a few good examples on ebay.
Hope that helps.
Last edited by Squeegee Longtail; 18th Sep 2013 at 11:05.