Have been meaning to post some observations on this topic for quite some time.
About 3 years ago I was preparing a rifle to take to Africa (Mod 70; 416 Rem). Becoming completely familiar with the rifle and cycling my cartridges prior to the trip to ensure proper feeding I would on occasion have the cartridges in the magazine stove pipe. Quite concerning..It did not happen all the time. Perhaps 10%.
I took it to a few local gun smiths that were as confused as I.
I began tinkering with it (Tendency to do these things as I am a Engineer; but no gun smith).
In my case what was happening was the magazine follower was sliding back and forth on the leaf spring it was attached to. When the magazine follower was in a forward position it fed fine but if it slipped back to the rear of the magazine box the cartridges would stove pipe.
I remedied the problem buy placing a thin sheet of brass shim stock between the leaf spring and the female sleeve on the bottom of the magazine follower holding it snug/tight in position.
Have had no problems since.
Hope this might help with others that have experienced this issue. Maybe an easy fix!

EZ