Square Heel Experiment, anyone try it with 7mm bullets?

[Titan]

Has anyone experimented with removing the radius where the base meets the boat tail by turning on a lathe or grinding the base of a bullet? I was looking for anyone that had reproduced the “square heel experiment” presented in Bryan Litz’s Modern Advancements in Long Range Shooting. I am interested in performance gains to the form factor or BC with regards to the current crop of heavy 7mm match bullets (183 SMK, 195 Berger EOL, 197 SMK). If anyone is willing to share their experience I would appreciate it, I will share what I observed today.


I tried it out a limited test today with the 7mm 180 Hybrid and recorded velocities at the muzzle, 100 yards down range (Lab Radar) and 1000 yards (Shotmarker acoustic target). While the results were less than expected, I was hoping to get a 2%-4% increase to the form factor of the bullet and observed approximately 1.2% increase. This was a very limited test as I only tested 9 modified bullets against a control of 11 unmodified bullets. All 20 where all presorted by base to tip length and all were (1.528-1.529) inches an effort to uniform BC (the bullets were not pointed or trimmed) . The modified bullet’s bases were ground flat removing the radius edge and approximately 15 thousands from the base to ogive, this was also 1.26 grains of copper (178.74 down from 180). The test was shot alternating between the control and modified rounds and the difference between the modified and control average velocities at the three ranges were as follows:

CONTROL:

0 2862.8

100 2734.8

1000 1618.9

EXPERIMENT:

0 2869.556

100 2734

1000 1622.778

Difference:

0

6.755556

100

-0.8

1000

3.877778


If my math is right I believe this is a BC of .310 for the control (180 grain) with a 1.028 form factor and a BC of .3115 with a 1.016 form factor for the experimental group (178.74 grain)


Conditions

82F

13% Humidity

29.3 Barometric

The difference is so small it could be simply be chalked up to experimental error in the variance from the chronograph, but I figured I would throw out the results because there does appear to be a weak effect from squaring the base of this particular bullet. And I was interested if anyone else had ventured down this path with 7mm bullets and found any interesting gains.

 

[Ballisticboy]

I have not modified any bullets in the way you say but it is common knowledge in ballistics that curves are not a good idea at the backs of bullets or shells. This is because when the air is flowing past a curved base the point at which it breaks away will vary depending on the local conditions giving slight changes in the aerodynamic moments. With a sharp corner the air will always break away at the same point giving consistant aerodynamic moments. The affect of removing any rounding is that the yawing behaviour of the bullet is modified giving a drag improvement.
Of course, if your bullet has good behaviour to start with, any improvement will be marginal hence you may not see much effect while on other bullets with poor yaw behaviour you may see a greater effect.

 

[Brians356]

I have not modified any bullets in the way you say but it is common knowledge in ballistics that curves are not a good idea at the backs of bullets or shells. This is because when the air is flowing past a curved base the point at which it breaks away will vary depending on the local conditions giving slight changes in the aerodynamic moments. With a sharp corner the air will always break away at the same point giving consistant aerodynamic moments. The affect of removing any rounding is that the yawing behaviour of the bullet is modified giving a drag improvement.
Of course, if your bullet has good behaviour to start with, any improvement will be marginal hence you may not see much effect while on other bullets with poor yaw behaviour you may see a greater effect.

On racecars they sometimes create an abrupt edge using a spoiler. This short video is tangential, but sheds some light on the topic, and is poignant if you like backstories, and appreciate design geniuses of days gone by.

-

 

[boltfluter]

On racecars they sometimes create an abrupt edge using a spoiler. This short video is tangential, but sheds some light on the topic, and is poignant if you like backstories, and appreciate design geniuses of days gone by.

-

Brian, great video of days gone by. Thanks for sharing.

Paul

 

[barefooter56]

On racecars they sometimes create an abrupt edge using a spoiler. This short video is tangential, but sheds some light on the topic, and is poignant if you like backstories, and appreciate design geniuses of days gone by.

-

In my mind the most beautiful car ever made in a great era of racing car design by a great team.

 

[Titan]

Neat video it's a cool story.

On racecars they sometimes create an abrupt edge using a spoiler. This short video is tangential, but sheds some light on the topic, and is poignant if you like backstories, and appreciate design geniuses of days gone by.

-

 

[Canadian bushman]

I tested this a good while back. Shortly after that book was released.

It was with berger 215’s in a 10.66 twist barrel at 2580 fps. This bullet had somewhat marginal stability especially in winter time.

I could never get the adv. BC numbers out of this bullet so i turned the heels down perfectly square on 50 bullets.

Loaded them up with the same load as the other 50 with radiused heels. Zeroed with both and tried to observe a deviation in drops.

No difference.

 

[Titan]

I tested this a good while back. Shortly after that book was released.

It was with berger 215’s in a 10.66 twist barrel at 2580 fps. This bullet had somewhat marginal stability especially in winter time.

I could never get the adv. BC numbers out of this bullet so i turned the heels down perfectly square on 50 bullets.

Loaded them up with the same load as the other 50 with radiused heels. Zeroed with both and tried to observe a deviation in drops.

No difference.

Thanks for sharing your results Did you attempt to modify any other projectiles?

 

[Canadian bushman]

Thanks for sharing your results Did you attempt to modify any other projectiles?

No,
I started shooting everything out of tighter twist barrels. Way easier than squaring off bullets, and everything flies through the trans sonic zone better as well.