... your experiment brings up a great question why the difference?
Here is my hypothesis.... Like amounts of BH, being a smokeless progressive burning powder, - creates and maintains a greater pressure for a longer period of time in the smaller confines of the bore of the 45 cal vs the larger space in the 50 cal barrel, where the pressure will be reduced quicker as the projectile moves up the barrel.
"10-4" on that one!
The gasses formed by a given amount of fuel reacting to oxidation is always going to try to expand at an also given (constant) rate.
Narrow the
escape hatch (smaller bore) and the expanding gasses will create more pressure whilst trying to expand at the 'given' rate resulting in a higher velocity venting of the gasses.
Water from a garden hose exits much faster from a constricted nozzle than from an open hose end... not
exactly the same geometry, but close enough for argument.
As you stated, in the larger bore the greater volume of space for the gas to expand into by a given distance of movement of the projectile which allows for the pressure to lessen more quickly... lower velocity resulting.
Jeez, I like .45's!
