I've been asked to get a gift for someone who collects almost all things military.
This person would like to get a US ACH(Advanced Combat Helmet) which you can buy from surplus stores. Is it possible to carry it with me on a plane out of the US?
There are two things you need to check:
As far as I know, there is no restriction on removing a helmet from the US. You need to check with the destination country whether the item can be legally imported. A good place to start would be contacting their nearest consulate, or you may be able to find the information online on their government website.
If in doubt, declare the item at customs on arrival. You may be required to pay import duty (i.e. tax). If it's illegal, you may be simply allowed to surrender it without penalty.