24" barrel at 040 grains = 0952. With a 28" barrel =1022. difference 70 fps

10 grain net gain------------084--------------------------074

24" barrel at 050 grains = 1036. With a 28" barrel =1106. difference 70 fps

10 grain net gain------------085--------------------------085

24" barrel at 060 grains = 1121. With a 28" barrel =1191. difference 70 fps

10 grain net gain------------117--------------------------085

24" barrel at 070 grains = 1238. With a 28" barrel =1276. difference 38 fps

10 grain net gain------------118--------------------------086

24" barrel at 080 grains = 1356. With a 28" barrel =1362. difference 06 fps

10 grain net gain------------071--------------------------070

24" barrel at 090 grains = 1427. With a 28" barrel =1432. difference 05 fps

10 grain net gain------------072--------------------------071

24" barrel at 100 grains = 1499. With a 28" barrel =1503. difference 04 fps

10 grain net gain------------066--------------------------061

24" barrel at 110 grains = 1565. With a 28" barrel =1564. difference -1 fps

10 grain net gain------------066--------------------------062

24" barrel at 120 grains = 1631. With a 28" barrel =1626. difference -5 fps

Those are results using a 350 grain conical from an old Black Powder Hunting article (thanks to Rattus)