Re: .357Mag Marlin: Unburned Powder After Firing
The "reasonable" taper crimp is probably part of the problem. Pick up one of Lee's Factory Crimp Dies, and lock that bullet in there, tight. You may also want to play with soem other primers and see if that helps, but I'd go straight to the crimp die first.
Not much of a Lee fan myself, but this is one tool that I have to say works better than anything else on the market.