Impossible? I don't know.
Your assumption would be wrong. ;-) If it were true, there would be a "one spine fits all". There isn't.
Due to shoulder issues, one year I had to drop my poundage down to 60lbs. In order to get optimum arrow flight, I had to shoot 500 spine Axis shafts.
Other years, I was able to bump up to 65lbs. In order to achieve optimum arrow flight, I shot 400 FMJ's.
This past year, I was finally able to get back to 70#. In order to get optimum arrow flight with 340 FMJ's, I had to use a 100gr tip. Even though that combo tuned and shot great, my FOC was below 9%. Although I don't overly concern myself with FOC, I wanted a little more. 340 Axis shafts with 125gr tips is what I ended up with. This put me a little above 12% FOC, and still gives me above the minimum weight I feel comfortable with for elk and moose. This combo shoots like darts.
As you can see, spine difference can make a big difference. Beyond that, even the same spine in different material can also make a difference. As I've said many times, it all boils down to what works best for you, taking into account the animals you hunt, and your effective distance you shoot at those animals.
The main reason they make weaker shafts is for the weight savings for the high speed crowd. It's also possible there are other issues, with the bow (I don't know how you could easily verify perfect nock travel, for instance) or the shooter, that haven't been or can't be tuned out. More like one error canceling another.
If you want to maximize penetration, it's worth starting with stiff shafts, and heavier heads and see how they fly.
Wyo. You said 340s work great, but you don't say if you tried 300s.
I did not. No need, I found what I wanted. Another combo I tried was the 340 FMJ with 150 tip. Optimum arrow flight, but for me the loss in trajectory wasn't worth the additional weight. As I said, whatever works for you.
"The main reason they make weaker shafts is for the weight savings for the high speed crowd."
That broad brush statement simply isn't true. If it were, why isn't your wife shooting 300's?
Spine should be selected based on what tunes best our of YOUR bow. Numerous factors go into determining the correct spine for YOUR bow. Draw weight, draw length, length of shaft, cam type, tip weight, etc, can all influence what shoots best.
Once you determine the properly spined shaft, that's when different options concerning overall arrow weight can be personalized, based on your needs/wants.
For example, let's say you find Easton 340 spined shafts shoot best out of your bow. Within that 340 spine category, there are numerous different models that vary in weight. They can range anywhere from Fatboys that weigh 7.8 gpi up to FMJ's that weigh 11.3 gpi.
As I stated in my earlier post, even this isn't set in stone. If you change any one factor, you should recheck your tune.
Even though arrow speed usually is dead last on my list of attributes, even with the 500s she is getting just under 200 fps. That's about my limit for compromising on trajectory. She's just over 9 grns/# of draw weight, and almost 17 % FOC. There's not much more we can do. Besides, she's not generating enough energy/momentum no matter what she shoots to worry much about arrow stiffness in regard to terminal performance, especially considering how short her arrows are.
I should have added that weaker spine/lighter shafts are also appropriate for energy challenged set-ups.
In general, pick a good COC fairly heavy BH with 2 or 3 blades. Choose a stiff shaft to start out and make sure it tunes. There is no reason it shouldn't. I like to be in the ballpark of at least 15% FOC, and 8 - 9 grains per pound of draw weight.
If I ever see my fletching sticking out of an animal (which doesn't happen very often) at least I know there wasn't much else I could've done to improve penetration.
I then tried a .260 spine arrow with 15 grain insert + 125 grain head. I figured these would be really stiff for my setup. They shoot way better. Group a lot tighter!!
This was all out of a 70 lb bow at 27" draw.
I think for a compound bow you can't be too stiff. No such thing.
cnelk's Link
According to the modern standards (ASTM F2031-05) an arrow's official spine deflection is measured by hanging a 1.94 lb. weight in the center of a 28" suspended section of the arrow shaft (not to be confused with the old AMO standard of 2 lb. and 26"). The actual distance the 1.94 lb. weight causes the shaft to sag down is the arrow's actual spine deflection. For example, if a 1.94 lb. weight causes the center of a 28" arrow to sag down 1/2 inch (.500"). Then the arrow's spine deflection would be .500". Stiffer arrows will, of course, sag less. More limber arrows will sag more. So the stiffer the arrow is, the LOWER its spine deflection measurement will be. The more limber an arrow is, the HIGHER its spine deflection measurement will be.
Read more - see link
Thanks for the continued info Spike Bull
"But the important question is, does it really make a difference? From a pure physics standpoint, yes. But again, few shooters have enough skill to notice. Small amounts of spine variance are realistically inconsequential to the weekend bowhunter and backyard enthusiast. But that doesn't stop the arrow companies from bickering about who has the best spine consistency and accusing each other of spine crimes. After all, every arrow company wants YOU to believe that their arrows will give you a technical advantage ... even if that's a little distortion of the truth. Again, we'll avoid the internal melee here by not pointing fingers, but if you wish to know more about spine variance, check the forums. There are a few forum regulars out there who own spine testing devices and Hooter Shooters. And they'll be happy to disagree with us on this issue."
With that said I think it's hard to be over spined with a modern compound bow and a release. Over spined would be tail right or broad head right. I have always been able to tune that out.
This years hunting set up I am going to try a 250 spine Victory VAP shaft. A 33 grain firenock outsert. And a 150 grain VPA non vented head. Out of a 70 lb Hoyt Nitrum Turbo. OT2 says I will be over spined by .036 but I think it will tune fine. Time will tell.
A decade or more ago, I bought the straightest shafts Gold Tip sold at the time and had a heck of a time getting them to group well even with field points. I couldn't hold 4", 3 shot group at 40 yards. I got some Beman hunters (same deflection, same length, same fletchings), and could regularly hold 3" groups at 40, and was able to put an entire dozen Carbon Tech (same spine, length, and fletchings) into 3 1/4" at 40 yards the only time I tried.
One doesn't need to be a pro or use a Hooter Shooter to be able to tell the difference between arrows with loose spine tolerances and those that are much better. Arrow quality is largely better across the board these days, but the proof is in the pudding.
