As far as "handy" goes, I've been a little uneasy about the pistol grip on the DPMS. It's a plastic version of the luxurious walnut target grip for the G3 series, but plastic can only go so far.
The palm shelf wasn't helping my palm, and the lumps and bumps didn't match anything on my hand. So I switched to the much plainer, but more customizable Magpul MIAD grip system.
I realized the old grip was also more vertical, and needed a bit more angle than it was willing to give me. Now at least I'm not distracted in figuring how to manipulate my hand into doing what the grip wants.
The next two gadgets should be in the inventory of every series shooter. This is the JP Enterprises receiver bore guide, and the TMA dry fire adapter.
The TMA dry fire adapter is made of Delrin, and slips inside the bolt carrier. The ends (either end works) are relieved so nothing can touch the firing pin. The underside has a ramp that gently wedges the hammer to a stop before it hits anything square on.
This dry fire device works in regular M16 types, too. It's also available in orange, to remind you to remove it. But the black one has the potential of being a quick-and-dirty was to immobilize a weapon. Everything seems to work, but you pull the trigger, you hear the hammer hit, and nothing happens. The device fills the entire cavity of the bolt carrier, but unless you pop open the rile and look closely at the bolt carrier, you might not see it at first. But the main purpose of this gadget is to let you dry fire all day without fear.
The other item that every series shooter should have is a bore cleaning guide, like this one from JP Enterprises. It replaces the bolt carrier, and is designed to provide precise alignment of the cleaning rod (I prefer Dewey coated rods) so it can clean the barrel without risk to the chamber or rifling. Everything is aligned properly by the O-rings, and there's a little port for adding the solvent as the patch is on the way to the bore.
Even before I get the scope mounted and take the rifle to the range, there's lots of gadgets needed for the care and feeding of a series target rifle.