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Help me understand arrow selection

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I got my first compound bow a few months ago. I had my local bow shop set it up and they told me what arrow to get for my draw length (29") and draw weight (70lb) and that was a Easton ST Excel 340. Since then we have backed my draw weight down to 60lb. I downloaded Easton Shaft Selector 2011 software and according to that I should be shooting 400 grain arrow and now a 340. I did change the software to match the beginning setup and it does show a 340. My question is should I purchase 400 grain arrows now?

Thanks
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· wood scientist
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I would say that 400's would work. But if I was shooting broadheads I would still see if the 340's would work with both FP and BH. If I could get the same impact with each I would be tempted to keep them rather than buying a dozen new arrows. You would only be losing a few fps but would probably have a little better penetration with the heavier shaft. You might have to add a little poundage to move the two together, but then again, you might not.
 

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I got my first compound bow a few months ago. I had my local bow shop set it up and they told me what arrow to get for my draw length (29") and draw weight (70lb) and that was a Easton ST Excel 340. Since then we have backed my draw weight down to 60lb. I downloaded Easton Shaft Selector 2011 software and according to that I should be shooting 400 grain arrow and now a 340. I did change the software to match the beginning setup and it does show a 340. My question is should I purchase 400 grain arrows now?

Thanks
I used a raw shaft length of 29" for reference. Shorter will show slightly stiffer for both. According to this either arrow will shoot fine for you. When they split the sweet zone like this you can usually go either way and tune slightly from there if you need to.

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First of all the number on the arrow is the spine or stiffness of the shaft not the weight in gn.
The stiffness of an arrow is determined by supporting the raw shaft at two points x" apart and hanging x amount of weight in the middle. The amount of flex is measured in decimal inches.
Some companies like Easton call out the arrow by spine, most commonly 500, 400, 340, and 300. The larger the number the more the arrow flexes and the less stiff it is.
Other companies have a more arbitrary numbering system that relates to a chart that will give you the actual spine of the arrow.

How stiff on an arrow you need is dependent on several factors.
1. arrow length, the shorter you cut an arrow the more inherently stiff it gets
2. draw weight, the more draw weight the stiffer the arrow required
3. tip weight, the heavier the tip (resistance up front) weaker the spine will act.
4. Cam type, the more aggressive the cam the stiffer the spine will need to be.
5. Rear weight, weight at the back of the shaft will make it act stiffer.

Mitch
 
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