To clarify..
I have observed the phenomenon in three rifles:
two were in 7.62x54 Mosin Nagant rifles (not mine) using .311 174gn FMJ bullets and N140 - I don't know the specific loads, but assume 45 - 47 gn range...
The third was my own 8 x 50 Lebel - I have given the details of the loads, however they were .323 bullets as that is the only 8mm bullets I can obtain.. They fly fine, no yawing or tumbling and group reasonably well at 300 yds. The Frenchfactory loads I pulled were .323 as well!
My comments about .303 were simply to point out that it is a roughly similar load using the same components that has never shown these symptoms, hence my curiosity..!
Working on the "first is unusual, second is coincidence, third is enemy action" theory I decided to ask around....!