The Bob Semple only had machine guns, so the Big Bob improves that by having a 105mm cannon on the middle and a 75mm cannon on the rear (and a couple more machine guns for good measure), and the Bob Semple MBT improves on that again by having a 105mm Royal Ordnance L7 tank gun, capable of firing the L28A1 APDS munition that can go through 296mm of steel from 500 meters away, as for their armor though, good ol' corrugated steel would do