People shoot clean thru grizzly bears with bows inside 30 yards, but comparing arrows to bullets is like comparing apples to oranges. The arrow kills by hemorage not shock.
As to your 300gr XTP outpenetrating a 180gr TBBC I would have to see that to believe it, I haven't been very impressed with 300gr XTP penetration out of an 454 cassul or magnum muzle loaders. You definately welcome to use whatever rifle you may want, and lucklily you will have a guide that will back you up if things go wrong. I never hunt with a guide so I use a more adequate gun from the start. My bear guns are 338 RUM w/ 225gr Accubond @ 3150, 9.3x62 w/286gr partiton @ 2400, 375 H&H with 270gr TSX @ 2700fps, or 300gr partition @ 2550. For backup work or really thick cover I use a guide gun w/ 405gr kodiak bonded @ 1925fps.
Aside from a good hard cast I have yet to see a 429cal bullet that I would call adequate for big bears when driven to 444 velosities.
