The 551 was an utter death trap, especially when it hit a mine. It had (essentially) caseless ammunition sitting out in the open in the fighting compartment and when hit the thing would just go up.
Saving myself typing by linking a prior post on a similar subject: http://www.freerepublic.com/focus/f-chat/3604214/posts?page=25#25
Should have put this comment here (posted it over there):
Father of a good friend growing up was a West Point Armor Branch O-6 (bird colonel) with 2-3 Vietnam tours. This in the mid-70s. He *hated* the Sheridan. There were still some of them around at Fort Knox in that era, more in Germany in the Fulda Gap.