RE: can i shoot 70# with 300 grain arrows?
It wouldn't hurt it anyway because of your draw length. They base that on IBO specs that are 70 lbs, 30 inches of draw and a 350 grn arrow. If you lower your draw length then your bow is less efficient letting you shoot a lighter arrow.
And most bows made now a days should have no problem shooting lighter arrows anyway. Besides that thing is just a guideline and sort of a crock considering bow speeds and efficiency ratings are all over the place.
For instance compare two bows, like a target bow with a deflexed riser, lots of brace height, smooth low energy cams and an IBO around 260 or 280 fps. Then compare it to a speed bow that can deliver near 350 fps. Obviously the target bow produces much less energy and could safely shoot a much lighter arrow than the high energy speed bow could. I'm not even sure I would shoot 350 grn arrows out of a bow that fast.
And any time you alter the power of the bow like lowering the draw weight or changing the draw length it effects this as well. I can safely shoot 300 grn arrows out of a bow with 60 lbs of draw weight, but I can't out of bow set at 70 lbs but with a 27 inch draw length? Sure you can, you basically did the same thing to the power level of the bow.
Bowjacksons site has a minimum AMO arrow weight chart if you want to look at it, I would go by that rather than the IBO minimum thing. I'm not at my computer or I would look it up for you.
Paul