I have a metal colander I put the brass in. I fill the tub with fairly warm water, far enough to submerge the colander, add some dish soap, agitate, and let it sit. Every 5-15 minutes, I'll repeat the process, for a total of 1-2 hours. For me, it's more of an "agitate, and let it sit" proposition, rather than hands-on.
Once it looks like most of the dirt is loose, I'll dump the cases out of the colander, and swish them around the tub, as I rinse with clean water (don't use cases small enough to go down the drain
).
Then, it's onto a towel for some water removal, and a day or so to air dry.
I don't have to do it very often, since my tumbler generally removes all the dirt. ...But it works well, when you have to do it.