These are my citations for the work I found on the cannons.

Imrie, Risley. "Six Pound Fieldpiece." Imrie Risley Miniatures. N.p., n.d. Web. 6 Jan 2011. <http://www.saratogasoldier.com/6_Pound_Fieldpiece.html>.

"History Of The Trebuchet." Trebuchetstore. N.p., Thursday, January 20, 2011 . Web. 20 Jan 2011. <http://www.redstoneprojects.com/trebuchetstore/trebuchet_history.html>.

"Tiger Tank." History Learning Site. Ask, 2 Feb 2011. Web. 2 Feb 2011. <http://www.historylearningsite.co.uk/tiger_tank.htm>.

http://en.wikipedia.org/wiki/M4_Sherman