AR-15's come and came with all kinds of bolt configurations. In order for an M16 lower to run full-auto you must have a full-auto bolt carrier. Some AR-15's came with a full-auto bolt carrier, many came with semi-auto bolt carriers and those come in several configurations.
Like someone said above, you will also need to make sure the Upper receiver has the auto-sear relief cut in it. Most aftermarket uppers are made this way. Many (maybe all) Colt AR-15 upper receivers do not have the auto-sear relief cut.
If the M16 lower is an older Colt type (and I'm not sure if anyone other than Colt was making M16 lowers prior to '86) then you will need an offset pin to marry up the older M16 lower to a newer flat-type upper receiver. This is not a big deal - you should be able to find a pin set for this purpose.
Other than the offset receiver pin, full auto bolt carrier and relief cut in the upper receiver, there really is nothing else to worry about. You're ready to rock-n-roll.