I don't see where tumbling really affects the ammunition adversely, as long as it's not tumbled for an excessively long time. My concern might be more with the primers than the powder becoming affected. I have lightly tumbled finished ammo over the last 40+ years with no ill effects. I don't tumble the stuff I use for long range accuracy, but never tested the idea of any adverse effects as a result. I have never heard of an incident where ammunition has discharged as a result of tumbling. That does not mean it can't happen, but I don't think it's a common occurance.