I bought a .44 Henry Big Boy a few weeks ago and ran about 150 rounds through it. All were handloads and most were 19.0 grains of 2400 behind a 240 grain jacketed bullet. The weight of the rifle made some pretty mild recoil. Zero problems and the 2400 loads were incredibly accurate.
The only way I could see a magazine detonating under would be either pointed bullets, or seating a high primer. If the bullets have a flat point and the primers are seated properly, then it shouldn't be a problem.
240 grain bullets are the standard for the caliber and shot well in my Henry. Now, the 300 grain bullets, I don't know if they rifling will stabalize them...a quick check of the spec sheet will tell the twist and will let you know. I don't load 300 grain bullets in the .44, so I never cared enough to find out.