You are confusing size with spine. The 400 spine(.4 inches of deflection) will allow for less draw weight and still spine correctly for your set up. A 340 spine will be starting on the stiff side(less deflection) and if you lower the DW it will be WAY too stiff. So if your arrows are actually cut to 27" from the throat of the nock, to the end of the carbon, a 400 would allow you to adjust the DW of the bow to aid in tuning it to shoot. If you use 340, the only way to make it work would be to add length to the shaft, and after they are cut that is pretty tricky to do.
The aggressive nature of the Destroyer cams could dictate that you need the 340 rather than the 400 because they are launching them stick crazy fast, and have a very powerful power stroke. That could over flex the less stiff 400 shaft and make tuning and safety become an issue. IMO buy 2 340s and try to tune it with those, and if not, buy 2 400s and see if it gets better. That way you are not out a ton of money and will be able to tell what is going on fairly quickly.
The aggressive nature of the Destroyer cams could dictate that you need the 340 rather than the 400 because they are launching them stick crazy fast, and have a very powerful power stroke. That could over flex the less stiff 400 shaft and make tuning and safety become an issue. IMO buy 2 340s and try to tune it with those, and if not, buy 2 400s and see if it gets better. That way you are not out a ton of money and will be able to tell what is going on fairly quickly.
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