Tanks

Never watched this movie, but TNT is showing it and I am watching it now. Very realistic battle scenes, but there is one that I don't understand. The tanks show for the Americans were not the tanks that you see in most war movies. These were mostly lumbering giants, pretty slow and they have those "stubby" guns rather than the 88 that were in other tanks. I think the ones they showed are Pershings, but at Saipan, didn't they have Stuarts?? Just curious if anyone knows.