From the Winchester May 1, 1878 Catalog
MODEL 1876
The success attending the sale and use of Model 1873,
and the constant calls from many sources, and particularly from the regions
in which the grizzly bear and other large game are found as well as from the
plains where the absence of cover and the shyness of the game require the
hunter to make his shots at long range, made it desirable for the company
to make a still more powerful gun.
Retaining all the essential mechanical elements of the former model,
and adding such improvements as seemed possible, the result has been
a gun carrying a central-fire cartridge, capable of reloading, caliber 45/100,
with 75 grs. of powder and 350 grains of lead, being nearly double the
charge used in the Model 1873, about the same as that adopted by the
U.S. Govenment, and giving an initial velocity of 1,450 feet.
Here's another good book on the '76 Winchester ... Winchester Lever Action Repeating Firearms : The Models of 1866, 1873 & 1876 (http://www.amazon.com/Winchester-Lever-Action-Repeating-Firearms/dp/1882391055)