nashorn & jagdpanther have the exactly same gun
maybe you are thinking of the "sturer emil" self propelled gun for example, which is basically like the nashorn, but has the pak 44 mounted on it (jagdtiger gun)
I was sure that nashorn and elephant used longer gun, just becouse the jpanther one is mounted in front of vehicle. But it looks like you are right, this is different version of same gun, with same barrel length. This must have been a nightmare to go through any obstacles with such a vehicle, sounds almost as terrible as ISU-122.
Also I don't like the design of jtiger and emil. These 2 were too heavy and had way too low manouverablility to be used efficiently. Aditionally the 128mm gun was too expensive and 3rd reich couldn't deliver enough of them even though both vehicles were build in tiny numbers. The heaviest sensible design was ferdinand/elephant and this was only thanks to hybrid engine that gave it good acceleration and thus manouverability (for a super heavy ofc).