No gas, at least I hope!

[The Shrink]

Marsh,

If BP gassed off, we would be loosing effeciency and power over time when it is not in a gas-tight container.  I think what you're seeing is fine powder, the same that shakes out of the charcoal bag when it's empty.  

I remember a couple of threads on Shooters that concluded that it's a lot of work with no discernable effect.  I've never done it and don't plan to.

[Charlie Detroit]

but it seems like a lot of work for no result. If you did it enough, you might wind up with a can of 5Fg (or finer). Some guys like that for priming their sparktossers.

[fredj]

Marsh- I bought one of those Cal-Graff BP screening devices several years ago and it's really an excellent device, old timer bench rest slug gun shooters have socked thier BP for years where you put some BP in
a cotton sock and slosh it around which will remove excess graphite, carbon and some of the fines, the Cal-Graff and other screening devices
biggest benefit is consistency as some BP mills have different amounts of
fines in different batches, as well as removing excess graphite and carbon dust both of which inhibit clean burning. I think with the better late lots of Elephant which was factory socked or the Swiss it doesn't seem to make much if any difference. For really critical applications like 1000 yd target shoots or BR shooting where every little thing helps it's likely helps
to some degree, depending on the BP you're using. For me a BP gadget
was something I couldn't resist. fredj

[fredj]

Marsh- I bought one of those Cal-Graff BP screening devices several years ago and it's really an excellent device, old timer bench rest slug gun shooters have socked thier BP for years where you put some BP in
a cotton sock and slosh it around which will remove excess graphite, carbon and some of the fines, the Cal-Graff and other screening devices
biggest benefit is consistency as some BP mills have different amounts of
fines in different batches, as well as removing excess graphite and carbon dust both of which inhibit clean burning. I think with the better late lots of Elephant which was factory socked or the Swiss it doesn't seem to make much if any difference. For really critical applications like 1000 yd target shoots or BR shooting where every little thing helps it's likely helps
to some degree, depending on the BP you're using. For me a BP gadget
was something I couldn't resist. fredj

[Kenneth L. Walters]

I bought a Cal-Graf a couple of years ago.  Most of my blackpowder is GOEX cartridge.  When I screen it I get varying amounts of the two larger sizes.  Didn't really expect that.

Now I screen all my powder.  Though I haven't made a great effort to study this, my gutt belief is that at the worst it didn't do any harm.  At the best some gun's group size seem to be cut in half.