Quoted:
Have a JM 1894SS.

Can it safely handle 340gr +P+ 44 mag loads?

Marlin (for an unknown, not well thought out reason) is using a very slow rate of twist (1/38 inches) on their 1894 chambered in 44 mag. Because of this slow rate of twist, the heaviest bullet that the factory Marlin will stabilize is about 270grs. Other firearm makers that chamber for the 44 mag all use a much faster rate of twist so that their guns will stabilize and therefore accurately shoot bullets over 300grs. Many folks today want their 44 magnums to be able to utilize the heavy 300gr. and heavier bullets - Marlin has not figured this out. If you want decent accuracy out of our new +p+ load in a Marlin, it will need to be re-barreled with a twist rate of roughly 1/20 inch.