Look up Brian Pearce's data for the 300grn XTP in the 44mag in the Ruger SBH and SRH's.
You will not find this data in any manual, and it exceeds charge weights found in any published load manual. This data is provided for reference purposes only, and I assume no liability for the choice of readers in its use. Proceed with caution, and follow responsible reloading practices. I load the 300grn XTP to the second cannelure, landing the tip of the bullet exactly at the front end of the cylinder in Super Blackhawks, sitting over 22.0grn of H110/W296. Based on primer signs, I get lower pressure with this load than with standard max loads found in the Hodgdon or Hornady manuals. These tend to run a bit over 1325fps in my 7.5" SBH's. What does that mean? I can get better performance from my revolvers without exposing them to as much wear and tear of excessive pressure, spreading out the pressure curve, rather than simply spiking it higher with hotter and hotter loads. Recoil is stout, obviously.
I've not personally found the chamber throats or carrier in the Marlin 444 to be compatible with the lower cannelure, and the single shot 444's on the market do not require cannelures for crimping, so while Hornady will cite a "rifle cartridge" as the potential reason for the second cannelure, I personally believe this to be nothing more than a Red Herring, since the lower cannelure doesn't fit with the only possible rifle cartridge. I've also not found the lower cannelure to be compatible with the 445 Super Mag Dan Wessons. I suspect they make up these lame excuses because they can't publicly condone "off book" loading practices, and the lower cannelure is a nice "hidden feature" for us 44mag shooters who like to see how much H110 we can pack into a case.