Every week I speak with hunters considering jumping from a 100 grain head to 125. Almost every broadhead company makes a 125 grain head now, and hunters have become more cognizant of their arrow’s front of center (FOC), kinetic energy (KE) and momentum (M). If you want to nerd out on the physics of arrow flight and penetration check out Ashby on momentum, KE and arrow penetration. Most of the stats and calculations in the article are taken from this work.
I made the switch this spring, shot 125 grain field tips for a few months and switched back last week to 100 grain points. As with so many things in archery the change provided positives and negatives. In this article I will describe what I view as the advantages and disadvantages of increasing to 125 grain heads.
|Grain||Overall Arrow Weight||Speed||Kinetic Energy||Front of Center||Momentum|
|100 grains||510 grains||264 fps||78.947 ft-lbs||10.5%||.598 Slugs|
|125 grains||535 grains||258 fps||79.095 ft-lbs||12.1%||.613 Slugs|
I shoot an Easton Axis 300 spine (10.7 GPI) at 70 lbs with a 30.5 inch draw. I also have a 50 grain brass insert, which brings my overall arrow weight over 500 grains.
Front of Center
Let’s begin with front of center. Front of center is a measure of how far the balancing point of an arrow is ahead of the halfway mark. A higher front of center means that the arrow has more weight towards the front of the arrow. The sweet spot lands between 12 and 18%. Prior to 12% the fletchings have a harder time steering the arrow, and beyond 18% the arrow loses accuracy at greater distances. When I switched from 100 to 125 grains my front of center increased from 10.5 to 12.1%. I expected improved accuracy, but I was also playing around with different vane combinations at the time and never isolated that variable. You can calculate your own front of center and KE here.
Kinetic Energy vs Momentum
Hunters talk a lot about the kinetic energy of their arrow. Kinetic energy is calculated by taking ½ (mass x velocity squared). Momentum, on the other hand does not square velocity. Momentum calculates the amount of force it will require to stop an arrow. This is why momentum matters more to hunters. Hunters likely get hooked on speed when calculating kinetic energy because each additional foot in velocity creates an exponential increase in kinetic energy. This stuff gets complicated, but what I took away from it was that heavier arrows traveling slower will have greater penetration than lighter arrows flying faster. The measurements get a little funky with ft-lbs and slugs. Here’s a chart for KE thresholds to harvest different sized game. Essentially, my thinking when I switched to a 125 grain head was that I would get an arrow with both better penetration and greater accuracy.
The biggest pain of switching to 125 grain heads was increased pin gaps. At 20 and 30 yards my pins moved very little, but my 40, 50 and 60 pins spread out considerably, which made it much more difficult to shoot at 45 and 55 yards effectively. Last year I shot my bull at 35 yards with my 30 yard pin, the arrow was several inches low, but would have been much farther off with a 125 grain head.
Lastly, I have had some peep rotation throughout the summer which makes seeing my 60 yard pin in the bottom of my sight housing difficult. As a result I switched back to 100 grain points last week and will hunt with 100 grain Montecs this year.
I couldn’t measure the increase in accuracy or arrow penetration with the 125 grain tips, though I would like to do more research in future summers. Mostly I felt the effects of greater pin gaps that would make a miscalculated distance a far worse outcome.
I plan to range my elk this year before I take the shot. However, I am solo hunting and it can be difficult to call and find a shooting lane at the same time. Over the last few years I have taken three shots and haven’t ranged any of them. I believe there is a good chance I take a shot without ranging the elk this year and following along that line of thought I’ll likely misjudge by five yards.
As far as getting better penetration with 25 more grains, though it would improve penetration, I don’t believe it has a high likelihood of impacting the outcome of my hunt. In a perfect world I would shoot the 125s at every animal, but thats not the case. My preparation for hunting largely centers around being as pragmatic as possible. How can I build a durable set up and shoot effectively at the most likely shot distances? By switching back to 100 grain heads, shooting flat and calling it good enough.
Thank you for reading,