My feeling is that your right about the 400 gr bullet being too heavy. I think it is to. I have a early Pape 450-3 1/4 double and it shoots 300 gr bullets very nicely. With yours being the shorter case version I'd make a 300gr. the maximum weight to send through it. Also I'd put good money on it booting the crap out of you with 400gr. bullets as well.
To clean the engraving, just simmer the parts in a caustic solution. You need a steel pot of some kind to do this as alum. etc. will get eaten up. You can make great pots for this job by going to your local tip and selecting a few different shaped Primas type gas bottles. After taking out the bleed plug, cut the top off and your good to go. After simmering in the caustic, boil in clean water a couple of times and everything will be as clean as it can be with all the shit out of the engraving. A light brushing while in the caustic will help move the crap. I just use old tooth brushes. It won't effect any remaining bluing or case colours. DON'T clean the barrels with this method as the caustic solution will eat the solder that of course holds the tubes together.