Re: 44 mag 300 gr xtp using W296 or H110
What kind of firearm are you shooting this load through? I'm tempted to assume a Redhawk or Super Redhawk since you can seat the bullet at the second cannelure (although some Dan Wessons and Anacondas will accommodate the longer OAL cartridges). If you are using anything but a Ruger, I'd highly recommend sticking to book loads, and even then don't exceed them by much without doing some research on the "10.9mm" load (44 mag/ 300 grain seated in the second cannelure).
For my Super Redhawk I load the 300 grain XTP over 20 grains of 296 and seat it in the first cannelure from the nose. Fired cases <span style="font-style: italic">fall</span> out of the chambers with primers displaying rounded edges. My groups are consistently 2" at 100 yards (from a rest, scoped). Velocity from my 7 1/2" barrel is a tick over 1300 fps and shoots clean through tough hogs (Piney Woods Rooters) weighing up to 475 pounds. With performance like this, I see very little reason to overload.
Do yourself a favor and get a 454 Casull if you feel the 44 isn't enough gun. While it is unlikely you can manage to get enough 296/H110 in a 44 mag case to actually blow up a gun, you are still going to dramatically accelerate the wear on your firearm and shorten case life.