Also, be aware that certain fields (particularly those with CQB) won't allow anything upgraded. i.e. stock internals all the way.
If you're curious why, it's just pussy factor. Some say safety, but until we start shooting each in the head with 5-600 fps guns (from five-ten feet away)... We're pretty safe.
(My friend had the honours of shooting my stock ICS from ten feet away at me... Hurt like a bitch, but I've hurt alot worse then that in my life.)