I have a S&W 629-2, 6". Before I started loading the .300 grainer, I contacted Hornandy and ask them about the two cannulures on the bullet.
"The bottom crimp groove is for loading in .44 mag loads. The top crimp groove is for .44 special loads." I received this e-mail from them a day or so later.
ALSO ! In older .44 mags, such as my 629-2, Hornady, without the beefed up Recoil Package installed, they DO NOT recommend shooting the .300 grain bullet whatsoever !! Will not hold up to the pressures involved. USE AT YOUR OWN DISCRETION ! The newer .44 mag revolvers built, Hunter, etc., do have the recoil package installed, will handle this heavier bullet in reloading.