Paul;
Note the 1250 fps given in the Lyman book is 94% of the normally stated 1330 fps for a 3 3/4 DE - 1 oz load. This does not of course mean that modern day Black Powders are Bad, just they they often do not quite reach the potential of those from a century ago.
I can offer no explanation as to why the velocity fell with plastic wads. I am old enough that I began reloading in card & fiber days. In those days loads were simply listed as X grains of a given power for Y oz of shot. Hull brand or primer was not listed, other than back then R/P hulls used a different size than the 209 everyone else used.
When plastic obturating wads came on the market we were simply advised to reduce powder charge by 10%. It was several years before loads began to be listed by Specific components.
Black Powder does have the unique quality of burning at essentially the same rate regardless of outside influnces so I doubt this 10% reduction would apply with Black. Plastic wads are not generally recommended for use with Black as its higher temps tend to melt the plastic which can cause a sticky mess in the bore. Perhaps something related to this caused the lower velocities.
