Does anyone know what would cause a bullet too tumble

Usually, not enough spin from a too slow rifling twist for the bullet length. This applies to the bullet in air. You can't spin a bullet fast enough to keep it straight once it hits meat, although some bullets get lucky and stay straight.

An undersized bullet may not get enough grip on the rifling to get spun up.

Have you got a specific problem bullet or gun?

Bye
Jack

Thanks for the reply

I was a little rushed when I posted the ?
thought id put it out there & see what came back.
This is what I know
A friends older Marlin3006, he reloaded two different lots one used Hndy 150g
the other Nosler 150g both used same powder charge& type
hears the weird thing Nosler flew strait Hndy tumbled
so then we tried a few different factory loads & they tumbled the only one that did not was Nos?
IM thinken the gun is shotout but why the Nos fly strait?

Were the Noslers Partitions? The open base may have let the back of the bullet swell up enough to grip the rifling. Just a theory.

Bye
Jack

thanks for the input turns out after almost giving up on the gun I suggested he REALLY clean the bore he did and looks like there flying strait now thanks again Ledft