Yes 37 grains by volume is about all you can get in a modern .45 Colt case even with a drop tube. All bp loads must be compressed unless you use the Pyrodex pellets which are already compressed. Very bad dangerous results if not compressed. Elmer Keith always told new reloaders to start with BP cause you can't go wrong just filling up the case and seating the bullet.
And really that's what you must do. If you want lighter loads or to use bigger cases like the .454 with the same load, you've got to use wads to take up the space. Cabela's and Dixie Gun works both sell vegetable wads you can use to reduce the load and protect the bullet base. If you use wonder wads, dry them with paper towels first or excess lube will "kill" some of your powder. Hang fires! The Hodgdon Power Co web site has a lot of info available. If you use real BP you need to check out SPG lubed bullets. If you shoot standard lubed bullets, stick to Pyrodex or Cleanshot. DON'T mix shooting smokeless and BP rounds without cleaning. Results will vary with every variable you introduce. My favorite way to load BP 44-40 or .45 Colt rounds is to drop a .44 Pyrodex pellet in a standard primed case, seat a standard lubed lead bullet and crimp. The pellet will rattle but it's already compressed and will give you a load equal to 30 grains of Pyrodex RS.
More info can be obtained from the real BP experts who abound on various forums.