I only own one lever action, a Winchester 1892 clone in .44mag/.44 spl. I don't know if this is common to other lever action models, but loading is somewhat awkward in that either the gate catches your thumbnail, or you have to be careful to push the round in far enough that the gate holds it by friction, allowing you to push it the rest of the way in with the next round. But, don't load it far enough and the gate won't hold it, allowing it to slip back out.
So this is the modified design that I think would make for a better tubular magazine lever action. Incorporate shell stops in the magazine tube like a shotgun that also hold the gate in the open position, allowing you to quickly shove rounds in the magazine without catching your thumbnail. Then, just slightly actuating the lever releases the loading gate so it performs the secondary function of keeping dirt and dust out of the action and magazine. Just my thoughts.