If you say you were about 10,800 AGL (12,000 less 1200 msl ground elevation)....it would have to be at least 20mm or larger.
.50-.51 caliber reaches about 5800 feet from what I recall....even a 20-23mm would have trouble I would think.
Trouble with the bigger rounds....they tend to be HE rounds....that go bang upon impact and thus it would do more than shatter the windy....
Any other thoughts guys?