I'm thinking of taking off the rail on one of my classic swatt 556 guns and replacing it with the 551 hand guard. I dont mind the rail I just don't see any need for it. If I wnat to hang things off something I've got a Christmas tree from last year.
First you have to separate the upper and lower halves of the rifle.
Next you slide the lower part of the handguards to the rear about a half inch or so and they should be able to come down and away from the upper part of the handguards. Mine was really tight and I had to tap them to the rear with a rubber mallet. There are little tabs that lock the to parts together and if you slide the bottom half back too far you won't be able to get them apart.
To remove the upper part of the handguards you will have to take the 3 screws our of the top rail on the receiver. Be careful not to lose the parts to the rear flip up sight as they will fall out of the bottom as you take the rail off. Once the screws are out, the top half of the handguards should come right off.
If you want to use plastic handguards like the 551 handgurads, you will need to get the shorter rail for the top of the receiver as the one on the SWAT model is contiguous with the handguards.
This is what I did with my 556 Classic SWAT and am happy with the change.
Also don't over tighten the screws for the receiver rail and use some blue loc-tite on the threads. If you over tighten you can break off the threded stud on the receiver and there is no easy/inexpensive fix for this.