The first version I recall from Centuri that was similar to the Viking was the cub scout Akela 1 kit. It used the thicker ST-7 tube and the heavier two-part injection molded Centuri cone for the ST-7. The heavier cone made it much more stable than the new lightweight cone Estes Viking. The reason I mention this is that a Viking is perfectly stable with the heaviest motor (a C6-7) IF you are using the fins in the orientation that extends out into the airstream. If you glue them in the 'close to the body tube' orientation - especially if you only use 3 fins - they are often unstable off the rod, but after the propellant burns up they become stable and zoom directly in whatever direction they are pointed when they stabilize.
If kids show up with 3 fins close to the tube, we only allow them to fly on A motors (or mini motors in an adapter) to keep the c.g. foreward OR we hand them 3 more fins to glue on in between the other 3 fins to make the rocket stable with the heavier B6 and C6 motors.
And if anyone show up with one or more fins glued at the top of the rocket, we have them cut them off. And we hand them a copy of the attached sheets.
View attachment cmrdm04.pdf
View attachment cmrdm05.pdf