Check out the legal section on this forum, it has a lot of good info.
If you are building a postban you have to follow these rules, at least until September 2004 due to the AWB:
1. If your rifle will have a detachable magazine and is semiautomatic you can only have ONE of the following features or it becomes an illegal "Semiautomatic Assault weapon"
a) Pistol grip
b) flash hider
c} threaded muzzle
d) bayonet lug
e) folding or collapsible stock
f) grenade launcher device (such as the groove found on standard NATO flash hiders for rifle grenades - I am not referring to the 40mm M203 which is a Destructive Device {DD} and requires separate specific approval from BATF)
Most will choose the pistol grip as their one feature on a postban, but you can add a BATF approved thumb hanger if you must have one of the others.
2)In addition, the OAL (overall length} must be 26" and the OAL Barrel length from breech to tip of barrel (which can include permanently affixed muzzle devices such as brakes or comps) must be at least 16". This applies to Preban rifles as well and will not change if the AWB sunsets. This means you can have a 14.5" barrel as long as it has a permanently affixed muzzle device (such as using blind pin/weld method) and OAL is 16" or more. I have an 11.5" barrel on my preban, with 5.5" pinned flash hider, this is legal since it is 16" OAL.
M16 bolt carriers are a gray area you might want to avoid, and M16 fire control group parts are a definite no-no, you can't even own them without an NFA registered full auto receiver or you risk jail time.
Check out the M16-Full Auto section of this forum for NFA questions, there is an FAQ and a lot of good info.
Other than that, go for it and have fun.
you might also check out the build it yourself section on this forum.