While I have seen barrels give velocities differences from published , or expected results, I am always suspect of the chronograph/conditions. When this had happened to me, a very high percentage of the time it turned out to be the chronograph readings being effected by lighting conditions. I almost always go their first when I'm suspect of the velocity is very efferent from expected. It appears this issue may be resolved with a Magnetospeed
I have been using for the past few months. IMHO.