How do you determine the arrow spine with heavy tips? I have a 60lb long bow at about 29.5 inches, the arrows I have are easton 340 FMJ with a 250gr point and they are 30'' long. Is this arrow too stiff for my set up? they seem to fly really well
I would think your spine would be fine. If they are flying well for you than that is all that matters however I would think if your drawing 29.5 inches and the arrow is only 30" long you might need a longer arrow for broadheads and hand clearence. My general rule is an arrow can be no shorter than an inch longer than your draw length.
Is it possible for the spine to be incorrect but still fly good?
Maybe they aren't flying as well as you think they are? 340 is purty stiff, take a look at O L Adcocks site and see what he has to say about bare shaft tuning. There is also a spine calculator somewhere here on Tradgang and I think it's the bees knees!!
Just google stu miller's arrow spine calculator. It's an excel-based macro program that lets you put in all the info for your bow and arrows, and fiddle with the inputs. It usually gets you in the ballpark, and it's free!
I'd use the bareshaft tuning method to see how close they are. You'll know right away if they're tuned.
Fletching covers up a lot of flaws and we don't always see everything in an arrow's flight.I got rid of my 60# bows but I believe I would need at least 350 grs on the front to make those work.Fletching could make that much difference seem OK when it is not.I agree with the bare shaft testing.I would strip the feathers off at least one arrow.Go to bowmaker.net and follow the tuning information.The bare shaft,shot along with your fletched will quickly answer your question.