ahhhh shiny crowns, never mind that they come new with a black ground, you will scrape and damage the most important part of the rifling !
Nope, gotta scrape them till they shine, usual practice is to scrape them out with the female end of a cleaning rod, but if you're real keen, get a hardened steel screwdriver, and really hack the dirt out. You can tell you've got all the carbon out when you start to come up with metal splinters...
Pro-tip, the lower receiver is an alloy, fairly easy to cut, a few minutes with a drill and some high speed steel drill bits, and you can make plenty of lightening cuts, no problem to drill out up to half a pound. You don't really need the *entire* mag well....
