Bob:
Apologies for late reply, unfortunately I have no way to measure chamber pressure. I can make an educated guess though, based on ballistics and powder burning rate, compared with published data (both measured and archival data in Wrighty's book).

For the .577/650/100Varget load in a 2 3/4 case mentioned above, my guess would be somewhere around 10 tons, give or take. While this would be right at maximum for a BP-proved firearm, I trust that the risk is low in this instance because of the late-manufactured (for BP) hammerless screw-grip action and fluid-steel barrels. Also, I know this has been done locally with an identical rifle in the past: Dave L_'s "Stumpy" was initially re-regulated for a "hot" .577 load (virtually full nitro) after shortening the barrels following an 'accident'.

Please note: I would NOT load this close to maximum with a damascus hammer-gun!

Regarding the partially-burnt powder in the barrels after firing, I found that changing from AR2208 to AR2206 solved this problem completely in the Fat Lady, so I would expect a change from H4350 to H4895 would likely do the same in your rifle. Work up again with the new powder of course, and unfortunately chamber pressure with the slightly faster powder will probably be slightly higher at regulating ballistics, but both are medium burners so should be OK IMHO.

Hope this info is of some use.