Yes. The ATF has a saying, "once a machinegun, always a machinegun."
You don't even need to have any trigger parts in it. If you have the lower of a M16 and it isn't registered to you in some way (on form 2 for a manufacturer, form 3 for a dealer, or form 4 for a pre '86 transferable) it is bad bad news.
See the pin above where it says SEMI? If you have a lower that has that pin or even the hole that that pin could go in, you have a machinegun. And if you are asking questions like "is this legal" you probably don't have a tax stamp and you definitely aren't a SOT. So if you do happen to have a M16 lower (and I'm not saying you do) turn it in to the feds before they catch you with it.