I'm posting an email from Brady to further clarify the whole issue.
Brady's Email
OK, in a way I am glad you asked since it has been a real head scratcher for me as well I did some testing a long time ago and came up with around 20 shels to sink one set at 50% of MA hardnesss. So I called HTC and just asked:) Well turns out that:
CA(8inch) and the SB shell weight is between 250 to 500 pounds depending on range, so the father out you get the less damage it does, hince the varance in the number of shels to sink it.
Repair time works like this: It is half the Down time. So if the down time is 30 min and it is damaged in 15 min it will be fully repaired, it is not incremental, and goes from the time of the last shell hit, so as long as damage keeps ocuring withen the 15 min windo it will not reset.