The cracks in the tube are normal, and does not impact the performance at ALL. I've owned a bunch of ACOGs... they all crack with age/use. Bubbles are also present in about 1/2 of them I've owned, and again, ZERO impact on use.
It's just the acrylic (I'm guessing) substance they use to hold the fibre optic tube in place. The actual fibre isn't affected by the cracks in the surrounding material.
It's probably the least of my concerns when buying an ACOG. BUY ONE!!!!