What is the difference in rails. I saw one that said free installation... Do the rails have to be mounted  permanently ? I just bought bought a 6920 and I want to put accessories on it.
The factory hand guards just pop on and off which is what I want to do, or relatively easily.

any suggestion ? feel free to point out the obvious stuff, because my knowledge is  very basic...( my first AR was an A2 i bought 2 months ago)

Yes and No. There are railed handguards out there that are two piece. Like your factory plastic handguards. Take them off, install new ones.

Or the other option is a free floating set, usually these styles require the removal of the muzzle device,FSB and in some cases, the factory barrel nut is replaced with one that is supplied with the handguards. I would look all over this site( ARFCOM) Look at vendors that avdertise here( Look below your user name )

Handguard suggestions are tricky, kinda like suggesting shoes, or motorcycles. Everyone has different wants or needs. Some may want the most expensive, just because, others may look for function, fit and quality. YMMV

Easy installation is relative, but the easiest to install are non-free float types.

YHM makes a 4-railed foreend that can be installed like regular handguards.

Midwest Industries and Troy make handguards (that free float the barrel) that can be installed by removing the delta ring assembly (easiest way is to cut it off).

Daniel Defense (my favorite) and LaRue are a bit more labor intensive.

to the OP.  what do you want them for?  are you going for just the look?

if that is the case, just stick with a drop in 2 piece set like you want.  you can get a set from yhm, mi, or troy.  all these companies make quality products, and these would be three options.  

since you have an A2 i dont see you doing match shooting (but correct me if i'm wrong), so you wont need a free float tube.  you will most likely not even notice a difference in accuracy that you can attain with a FF rail.

if you want to go all out, you can get the daniel defense omega.  it is free float and two piece.  no harder to install than other two piece systems.  It is just about the lightest and slimmest rail you can get.

there is a tacked thread that deals with rails and weights you can also check it out

thx everyone for the great help ! ! I have a better idea now. It appears Daniels is the cream of the crop..
Yankee  seems to function well at a cheaper price.  After reading some more posts I think I will need the rail thats longer on the bottom so I can have a vertical grip with room for possibly something nice in front of it :o).

Still a lot more reading and pics to look at. I looked at the tacked thread with the weight comparisons, thank you all.

Surefire makes a nice two piece four position handguards. Very nice, super easy to install. Bottom rail is slightly longer towards the FSB.

http://www.ar15.com/forums/topic.html?b=3&f=12&t=172238

no, there are lots of rails that simply replace the handguards (YHM, MI, Surefire, etc) make em––these would be my choices for the price

most of the FF tubes do require you to disassemble the upper

I have the Troy MRF-DI handguards on my carbine, it is very solid.

Daniel Defense Omega Rail....  Seems pretty simple:   two Piece,  IS A Free Float, and you can take or leave your FSB.  You also don't need to disassemble the barrel, FSB, and no removal of the Delta Ring.

I'm about to buy the Rifle length for my Dissapator.

I have these on 2 carbines. Very nice drop in rails.

I would look at the Troy rails also.  It creates a more consistant top rail without a gap.  Very nice.