Wikipedia and popular military books provide a lot of information on tanks, but they don't really explain what the information means or it's significance. For example: A T-72 has a 125mm cannon, and an M1A2 has 120mm cannon. Is the extra 5mm on the T-72 gun significant? There are three anti tank rounds that I am aware of: SABOT, HEAT, and HESH. Why have three different rounds that do the exact same thing? A book will tell you that tank X has a "spall liner", but not why HESH rounds are still used (even though it seems that every tank I have ever read about has a spall liner to protect against HESH). Tanks often have less armour at the rear and top - but the rear and top is armoured. I would love to know what the armour is suppose to protect against as it seems that a tank can be easily taken out from behind and on top by very basic weaponry. What is rear and top armour suppose to protect against if not from RPGs? Is it just to protect against small arms? Where should I go to get more indepth information on tanks? In particular, I would like to be in a position to make an informed comparison between an M1 and a fully upgraded T-90. I am not just concerned with which tank is better, but with how much better one tank is over the other and in what environments they excell in. Thank you for any help.