I am going 3-d target shooting for fun, no competition, with my 50 lb pull bow. Is there a certain grain arrow needed. If I shot an arrow that was "too light" would it ruin my bow.

You dont want to shoot an arrow that weighs less than 250 grains. IBO specs are 5 grains of arrow for every pound pulled.

Oh I forgot to add this target shooting is for fun. No competition. Just wondering if I can use any light arrow without messing up my bow.

Yes you could mess up your bow with too light of an arrow.

The lighter the arrow the more and more vibration and shock you will be adding to your limbs, cams and riser. So in effect, the lighter the arrow is that you shoot the closer you get to dry firing your bow.

It's suppose to be fun. I'd use the same arrows and equipment I hunt with. What better practice and experience for next fall?

Same here.