Greetings Three O' Three,
I think that the use of Dacron as a filler is frowned on these days. There are better options including AR2206H and Trail Boss for the .303 depending what velocity level you are after. Pistol and shotgun powders were used at one time for cast and reduced loads but it can be hard to get uniform velocities. If you would like to post what your aims are with reduced loads I and I am sure others can likely help you get started.
Regards Grandpamac.
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