Because pressure curves are not linear, acceleration rates down barrels are not constant numbers of feet per second per inch. It comes close to linearity in the last inches of a long barrel, though. A velocity vs. distance down the tube graph from QuickLOAD is provided below for the .45-70. The bullet is 405 grain Remington soft point over 4198 loaded to SAMMI maximum pressure. The blue line is the velocity (scale on right edge), while the red line is pressure (scale on left edge). Inches of barrel is the X-axis along the bottom.
As a point of information, SAMMI specifies a standard test barrel and test barrel length for each chambering. For example, for .308 Winchester, it is a 24 inch barrel with a 12" Right Hand twist, 4-groove rifling with grooves 0.176+0.002,-0.000 inches wide, and a copper crusher piston hole 0.206" wide. If a SAMMI member manufacturer specifies a velocity for .308 ammunition without specifying the barrel length, you may assume it was shot in one of these.