One source of difficulty with the Mauser design is that it was originally conceived as a military weapon and since the German military believed in clip loading and rapid fire in all situations, no provision was made to facilitate single loading other than through the magazine.

American designers, who were guided by the American Army's view that combat fire should be conducted by single loading the rifle, using the magazine only as a reserve, when designing the M1903 Springfield rifle changed the Mauser flat breech system to one with a conical breech, which guides a loose round dropped into the receiver into the chamber more surely than the original Mauser design.

More important, the Springfield extractor was designed in such a way to permit it to snap over the rim of a cartridge loaded other than through the magazine, in contrast to the Mauser design, which in its original condition will stubbornly refuse to allow the bolt to close on such a round.

The M1917 Enfield rifle, as well as its predecessors the British P13/P14 rifles, share these features with the Springfield, as do the Springfield's and Enfield's sporting rifle successors, the pre-64 Winchester Models 54 and 70 and the Model 30 Remington.

Attempting to single load an original Mauser action rifle other than through the magazine can result in a round stuck in the chamber with no immediate means to remove it.

I leave it to the user to decide whether or not it might be advantageous to be able to single load a cartridge under stressful conditions, where taking the time to press the round into the magazine might be undesirable or the need for doing so might be overlooked or forgotten.