I know they are hard to find in unbroken condition. My brother picked me up a set when I was "restoring" my Colt SP1 back to it's original configuration. He hooked up with a buddy who had over 100 of one side in unbroken condition but only 2 of the other side that weren't cracked. I don't know why one side is more prone to cracking than the other. My brother purchased an M16 directly from Colt when he had his Class 3 and it came out of the box with a cracked hand guard. If you can find a pair in uncracked condition buy them.