Hi well I am in Canada too, Alberta and have only seen the 300gr xtp in Bullets not loaded ammo. I would stay away from that XTP as it was a Bomb in my Hawken with a sabot. I was pushing it at a even 2000fps and it made a huge mess of the Deer I hit with it, but must say it did shoot through them just it came out in more than one hole!!
Funny how shooting the big Hawken for Years with a hard cast ball or a pure lead Minnie Never a problem then I switched to a Sabot and Ya they are as good as any .444 the way I load my front stuffer ;D
Ya I said hard cast! Why bother to load pure lead the ball rides in a patch?
I do not have a .444 but load one for a friend I use the Heavy Lee Mold the slugs come out at 305gr then I add a check and my lee sizer sizes them to .430 wish it was a tad bigger. It's a older Marlin 1976 year but shoots great with the cast Slug. 1.5'' at 100yds for 3 shot's is nothing to sneeze at.
The older Marlin also handles the long slug great not sure about the newer ones I herd they do not like 300gr+ slugs? Not sure on that as I am still looking for my own Marlin to play with.