Here's my problem, I want to load  some reduced load .43 Spanish Remington loads to test fire my newly acquired carbine ( cut down from a rifle ) in this caliber. The B.P. will not fill the entire case and I would like to use a filler ( COW ) and a wad. There is not much of a shoulder to the .43 S.R. case, will a filler cause any problems ,and when I do load with a .439 bullet  can a filler be used in the reduced B.P. load. The chamber accepts a regular .43 Spanish brass case ( 2.5" long ).Thanks

Blacki

Blacki,
I personally wouldn't use any filler in a BM BP cartridge below the neck level - there's always the possibility of it getting stuck in the neck, and raising pressures through the roof.  If you want to go less, try using Fg and a wad column for the length of the neck.  Maybe I'm too cautious, but still....

Blacki
At the moment of ignition when the powder turns into an expanding gas the gas slams up against the COW. The COW is granular and as it is compacted it turns into a plug. The force pushes forward against the crartridge sholder untill it hits the sholder of the chamber. At that point the pressures rise until something gives. Usualy this is the COW. Sometimes the plug slams so hard against the sholder that it tears the cartridge at the transition between the body and sholder.

40 Rod

Thanks everyone for the replies. It helps.
Blacki