To get the M21 you go to the directory OperationFlashpointDemo/Users/Yourname/Saved/Missions/__cur_sp.demo/ and open weapons.cfg. Look down for name="yourname" and change every M16 below that to M21 (so Weapon0 name="M21", Magazine0 type="M21" etc). Then save and SET AS READ ONLY. Restart the game and you should have an M21

You can substitute M21 for 'SVDDraganov' (sp?) 'HK' (HK MP5/SD6, my personal favourite

.
If you want night vision goggles, just look for the line under your name:
class Weapon0
{
name="M21";
};
and change it to
class Weapon0
{
name="M21";
};
class Weapon1
{
name="NVGoggles";
};
I've not got much free time at the moment, but what I have I've been spending mostly on Flashpoint!
Oh, you can get an extra mission for the demo from
this thread