I agree that bigger is better but 1-inch on those smaller shells would nearly core them!
It would be so much easier to wash them out though.
I use a 1/4-inch nozzle to ensure adequate wash and you need room around that for the water and powder to pass going back out of the shell at a good rate. Ideal is a 1/2-inch hole but 3/8-inch seems to work well enough. It also looks more disarmed to any authority who is questioning it.
I've found the same as you regarding powder taken from a shell. Much of it won't burn if it's been damp or wet. Some does burn but seems to me that it doesn't burn with the ferocity that I'd expect so I think it's at least weakened. But, of course, there is still some powder with enough power to blow up shells which we've unfortunately come to know in recent years.
How do we keep it dry? The obvious... The driller doesn't put the shell in water or use a drip when drilled.
That way the powder is left dry and some can be shaken out and then you can wash it out when ready to do so and dressed appropriately. As I'm sure you know the powder stinks when wet and stains skin and clothes and ground so best done somewhere appropriate.