For warranty purposes you would have to be dead on probably, but how will they know? As far as what is safe to shoot it depends on several factors of your bow. You can't just set a minimum weight for every bow and configuration. If you shoot a slower cam, less draw length or have more weight on your string you can get away with lighter arrows and still be safe. A person shooting a round wheeled target bow with 26 inches of draw can get away with much lighter arrows than someone shooting a bowtech Black Knight at 30 inches of draw length. The amount of power one produces is vast compared to the other. And this in turn effects what your minimum safe arrow weight would be.

Also having good string silencers, Limb Savers and vibration reducing stabilizers helps quite a bit.

Bowjackson's site has an AMO minimum weight chart if you want to look at. Some of my set ups have been lighter than 5 grns per pound and that is an AMO limit, not IBO. Simply because of how innefecient the set up was.

Paul