Many semi auto rifles, like the civy version of the M-16 can be easily modified to full auto weapons.
Unless you know
EXACTLY what you're doing with it, you're more likely to kill yourself than modify those weapons into full auto rifles. The easiest and cheapest (and therefore, most common) way is to file away at the part of the receiver that keeps the weapon from firing the second round with out another pull from the trigger. When you do that, the next time you just let go of the bolt to load the weapon, it
will go off if you're not careful, and it will
not stop until the feed of ammunition is disabled.
Unless you actually have the parts to do it, and know very well how, those civvy versions will never be reliable full auto weapons.
Tl;dr: Don't. You'll just hurt yourself. And I guess kind of a "No, it's not easy" comment.