I use 16gr of IMR4227 with a Fed205 SR primer on a 158JHP bullet in my .357Mag Marlin Rifle (18" barrel) and still get unburned powder flakes in the barrel after firing. I use a reasonsable taper crimp. What else could be the problem?

Might need to try the mag primers if you haven't yet.
Maybe a different powder.

Use roll crimp die, not taper crimp, tighten down just until deformation of bullet.Primer will push out bullet before complete powder ignition on straight wall magnum cartriges if bullet is not super tight in brass.

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Originally posted by Reloader270:
.357Mag Marlin Rifle (18" barrel) ..

I have one of them too and really like it. But I use H110 and Mag Primers for the Jacketed Bullets. It also shoots 148gr Lead well with HP-38 and WW-231 with Standard Primers.

Very nice rifle which is a lot of fun. Haven't Hunted with it yet though.