they r for looks but they do change the balance of the rifle making them more "pointible" and according to hatcher it should help with accu. a bit but to be honest ive never done a back to back test
nope they are made to screw on and seat themselves at a predermand depth if you use a crush washer they will not go all the way to the handguard... if you want to use a crushwasher then say so i can modify it to be used with it...
What use would a crush washer be on a full length barrel shroud? You're not going to be able to hold onto the barrel with a vice so the washer ain't gonna 'crush' before the reciever 'cracks'