My arrows are just about 30inches carbon to carbon.was wondering if I cut them down to 27 inches and put a 50 grain brass insert with a hundred grain head ,would my arrow be to weak? Set up is parker sidekick 27 draw 65 lb arrows are Easton axis traditionals.will the 27 inch arrow still have broadheads in front of riser? Like wieght of arrow so don't want to go lighter ,but even if I cut arrow down to 28 inches and go to a 125 head would this work.
Can't help the spine question you have sir. But if you have some one to help you, Mark your arrow every half inch for four inches, then place the arrow on the bow and come to full draw and half them mark the mark that is even are just ahead of the riser and that will tell you how short you can cut your arrow and still have the broadhead in front of the riser.
As you shorten your shaft you increase spine. Now best accuracy will come some where when you get the weight, spine and FOC right. Bareshaft tune your arrows to your bow and have the best accuracy possible. But 340 spine should be plenty.