First off, hello all. I thought I might tap the wealth of first hand experience here on the capabilities of a 16" rifle. I have recently purchased a new Bushmaster Dissiapator and a Aimpoint comp mxd. My first AR by the way. It is a really sweet setup for quick shots out to 100yds. What I am debating over is a magnified scope. Does anyone here know general distance limitations of 16" versus 20 or 24"? Will a 6 to 20 power be a waist of money? Maybe a better way of putting it is. Do you think a 16" rifle is capable of tight groups at 300yds. and beyond. Everyone's input will be most helpful.

Simply put, a 16" barrel is not going to do much for you out past 300yds.  The bullet's velocity is about kaput after going that distance.  

As far as a scope goes, I have a 50mm Bushnell 6x18 on a Frankenbush flattop.  I like the size/magnification because it helps me see the holes in the paper better.  

Good idea on the Aimpoint!  I have one for my carry handle on a pre-ban Oly and I love it!

Oh, and by the way, welcome aboard!  You will learn all kinds of stuff at this place.  I can even almost guarentee that you will have another AR-15 within 45 days.

Welcome!  In my (limited) experience, your new Bushy will shoot better than you can.  Flat top?  Optics will be easier.  Some might argue that 6-20X will be overkill on a 16" barrel, but the important thing to consider is what works best for you.  From what others have posted here, any magnification greater than about 3.5X or so should "hide" the front post, so consider that, too.  (Many have also posted extremely happy results w/ lesser powered optics, too, though.)  It's all going to boil down to what you will be most comfortable w/ shooting.  I found the most ergonomic setup for me was an EOTech mounted as far forward on my M4's RAS as it will go, backed up by a KAC 600m flip-up rear.  Sure, there are pros & cons to this, but I like it; so I'm sure you're experiences will be similar.  Try as many configurations as you can - be careful, though, as you've embarked on an expensive hobby.  Soon you'll have a couple lowers, 4 or 5 uppers, a couple red dot type sights, a couple more scopes...