I'm looking to use a bushy 16" barrel, do I need to purchase a barrel extension?  Sorry for the ignorance.

The barrel extensuion is the portion of the barrel that includes the locking lugs that engage the bolt lugs to lock the action.  Think of it as a perminant part of the barrel.  Never remove it and never buy a barrel without one.  Because of the way AR barrels are manufactured different barrel extensions are not interchangable without messing up head your space.  Chances are that any barrel you buy from a reputable source will have a barrel extension already installed.

Thanks!  that set my mind at ease.  I'm buying a bushy barrel and I'll ask whether it's included when placing my order.

It is and they are head spaced at the factory,even barrel only assemblys have the extension.

well that does sound like exactly what I'm looking for.  thanks again guys.  has anyone tried out olympic barrels?  they're cheap, I'm wondering if the quality is cheap as well.  Though I'm pretty drawn to the bushy barrels.

Bushy and colt and CMMG are the best colt is pricey but bushy and the CMMG M4 is chrome lined 1/7 like colt,colt is 1/7 but some times they are not chrome lined and they are pricey bushy has every variety and they are lined,both colt and bushy use 4150 stell MP"magnetic particle inspected"and ive heard the colt M4 bolts are the strongest if you can find one.