Tank guns is quite a generic term. If your talking WW2 Allied, you need a 1919A4 or .50 M2HB for an American/Canadian Tank and for British you need a BESA. There were other guns used on armour such as the Vickers .303, Vickers .5, Bren, Vickers K, 1917A1, .50 M2AC and M2WC etc.
post war British is going to be BESA, GPMG or Browning 1919A4/Later converted to L3A2.
what armour have you got?