Welcome!.

Pictures please..

Otherwise it didn`t happened..:LOL

With the info provided it seems the gun has been Rebarreled. Otherwise I would be concerned shooting .509" Woodleighs in a .500cal blackpowder gun.

The .500 express was loaded with so many different load combinations, caselength etc more than any caliber books could be written about that caliber alone. Most .500ex were marked "38" before 1887 then they were marked ."500ex".
A friend of mine has a .500 H&H from 1883 similar to yours in .500cal marked "38" with a 3" case. The rifling is shallow suggesting is was for a Paperpatch bullet.

Congratulations with your H&H