With curiosity nagging I decided to try and work out the BC of the jackets. After some searching I found this site:
http://www.uslink.com/~tom1/calcbc/calcbc.htm#jackets
After putting in the measurements it came out with a BC 0.058!
I filled one with epoxy just what difference that makes and as it was slightly heavier it had a BC 0.061.
No idea, it they would be useful for anything, but it was an interesting exercise.
