Question. Why in the name of all that is holy, does a sniper rifle need tactical rails? Rails are for entry guns... not sniper guns. Sheesh.
Easy. Versatility.
http://www.thehighroad.org/attachment.php?attachmentid=56995&d=1177279923 The gun in this picture is PvtPyle's DPMS SASS.
During the night shoot, using those rails, he quickly popped on his night vision device, and his IR laser, plopped down prone, and first shot, in the pitch black, nailed an 8 inch gong at 300 meters. Silent ninja death baby.
But during the day time, it is kind of pointless to have a giant invisible laser and night vision device on your gun.
Plus the rails are on a free float tube. That gun is a heavy pig, but that is mostly because of the barrel. So they don't really hurt anything, but add a lot of versatility. Which is what the SASS is all about.
As for how it shoots, first time out, putting it on paper, just screwing around, with regular ammo, he did a .6 inch group at 100. And I know it can do better than that with a little work or some better ammo.