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Generally, a .400 spine shaft is weak out of a 70# bow unless the DL is very short (less than 26"), and even at that a 75-gr point weight is usually required. It is possible to tune for bullet holes
with a weak spined shaft IF form is consistent (no bow hand torque, no string or fletch contact with face/facemask), nocks are precisely indexed to stiff side of spine if there is one, spine consistency within the dozen is very good, etc., but even if tuned for FPs, weak spined shafts will not generally group broadheads well, especially fixed-blade broadheads. Carbon Express, Gold Tip, Victory, Alaska Bowhunting, and Easton all recommend a spine between .350 and .320 with 100-gr points for your setup. Check out the manufacturer selection charts for your arrows since recommendation vary depending on weights of components, shaft length, shaft construction (parallel, tapered), etc.
Carbon Impact and Carbon Tech, recommend a .400 spine shaft for your setup with 100-gr points, but they are the exceptions.
Check out OnTarget2 free demo software if you want a more precise estimation of the correct spine, especially if you are using illuminated nocks, wraps, insert weights or insert adapters, etc.
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