Following a thorough review of the literature involving training manuals,
scientific and professional journals, police agencies, and computer searches, the
author found that little is known about predicting the trajectory of bullets through
glass, particularly that of the .30. caliber rifle bullet most commonly used by police
marksmen. Currently, there is no available method to allow marksmen to predict
the effect glass has on the trajectory of a rifle bullet. Without a table of deflection
values or method of calculating bullet deflection with respect to glass angle,
shooting through glass is inaccurate at best.
http://www.dtic.mil/dtic/tr/fulltext/u2/a283575.pdf