[Brian Pfleuger]

Small charges of slow powders can be dangerous, due to "flash over".

There's really not a powder that fills a reasonable amount of the case and yields the speeds you're looking for that is anything like efficient.

If you use small charges of very fast powders (pistol powders), you'll be maxing out at 50% and under case fill.

That means using a filler or having to tip the gun up to get the powder back against the flash hole after chambering.

If I were really wanting to do this, it looks like a god candidate might be Vihtavouri N130. Depending on barrel friction, QuickLoad predicts an 80% case fill (30.0gr) will yield a max of 1234fps ignoring barrel friction, or as low as 1083 fps with friction. A charge of 24.9gr would yield between 830 and 1042fps and still be at 65% fill.

This isn't terribly efficient. You'd be blowing 10-20% unburnt powder.