Copper .30-06 ammo ballistic test

[gtita]

Marxman, the idea of using the sound recording is indeed interesting.

The principle is that being the sound speed through the air known (with due corrections for the meteorological conditions, i.e. temperature, humidity, altitude, pressure) you can easily estimate the average bullet velocity. This would be obtained by using the time lapse between the shot and the reception of the sound of the bullet that banged on the target. You will then subtract the time needed for the sound to travel between the target and the sound recorder placed besides the rifle. The residual time is the one employed by the bullet to travel from the muzzle to the target, which may be converted to a velocity value by taking into account the distance between the two points.

However, this gives you only an average velocity and no information on the BC, the MV, or even the trajectory. In order to measure these variables you need other measurements, that may be in part confirmed by the mentioned time lapse.

In this regard, I used the velocity data I generated thanks to the Chrony in order to estimate the drag coefficients (at muzzle velocity) for the two copper bullets, Barnes VOR-TTSX and Federal TC. This was obtained using the following formulas:

BC = SD/I
i = CB/CG

where

SD = sectional density
i = form factor
CB = drag coefficient of the bullet
CG = drag coefficient of the G1 model bullet

In order to obtain CG I used the calculator available at http://www.jbmballistics.com/cgi-bin/jbmgf-5.1.cgi where I entered the BC I derived from my velocity recordings, as well as the MV I measured with my Chrony.

Here are the input data and the results for Federal TC:

BC = 0.455
MV = 2,770 ft/s
CG = 0.541
CB = 0.322
…and Barnes VOR-TTSX
BC = 0.380
MV = 2,615 ft/s
CG = 0.553
CB = 0.394

As you can see, the VOR-TTSX bullet has drag coefficient (CB) approximately 22% greater than the TC. Unfortunately, I don’t have all the needed sizes of the two bullets to explain this difference. But I tried anyway, with some approximation. What it came out is that two key factors seem to play a significant role:

- the boattail shape: the TC as a slightly narrower base (~0.65 mm) compared to the VOR-TTSX (~0.7 mm);

- the bullet's nose length which is slightly longer in the TC (~1.8 mm) compared to its kin (1.75 mm).

I was able to confirm this hypothesis by entering the data I had in another JBM other calculator, i.e. http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi

That’s all for now!

[marxman]

oh. i assumed you would compare the velocity loss of your bullet to the velocity loss of the g1 over that distance at the same start speed and that ratio would be your basis for bc. the velocity loss of the g1 is known why cant you use it? am i missing something? why do you need trajectory? its a secondary effect that you could calculate from ballistic tables now that you have a true bc.

[gtita]

Marxman, you are right. What I meant in my last post was that one cannot extrapolate the real BC or the MV just from the time lapse measurements and from comparing them with those announced by the bulet manufacturer for a given range. This is because you may have equal time lapses but different BCs and different MVs. For instance, if you have a rifle zeroed at 100 yds and shoot a 180 gr bullet that reaches a 200-yd target in 0.24 s, you may have one of these two possibilities:

MV.........BC........Velocity (200 yds)....Elevation (200 yds)....Elevation (400 yds)
2,700.....0.5........2,351 ft/s...............-3.99 in (0.24 s)........-35.55 in. (0.55 s)
2,800.....0.3........2,221 ft/s...............-4.08 in (0.24 s)........-31.85 in. (0.51 s)

In other words, in addition to the time lapse you need at least one other precisely known variable among those here-above.

[marxman]

Originally Posted by gtita

Marxman, you are right. What I meant in my last post was that one cannot extrapolate the real BC or the MV just from the time lapse measurements and from comparing them with those announced by the bulet manufacturer for a given range. This is because you may have equal time lapses but different BCs and different MVs. For instance, if you have a rifle zeroed at 100 yds and shoot a 180 gr bullet that reaches a 200-yd target in 0.24 s, you may have one of these two possibilities:

MV.........BC........Velocity (200 yds)....Elevation (200 yds)....Elevation (400 yds)
2,700.....0.5........2,351 ft/s...............-3.99 in (0.24 s)........-35.55 in. (0.55 s)
2,800.....0.3........2,221 ft/s...............-4.08 in (0.24 s)........-31.85 in. (0.51 s)

In other words, in addition to the time lapse you need at least one other precisely known variable among those here-above.

yes i should not have thrown that computer thing in to confuse things. all good

[gtita]

I just wish correcting the 400-yds elevation values and velocities of my example here-above. I accidentally interchanged them:

MV.........BC........Velocity (200 yds)....Elevation (200 yds)....Elevation (400 yds)
2,700.....0.5........2,351 ft/s...............-3.99 in (0.24 s)........-31.85 in. (0.51 s)
2,800.....0.3........2,221 ft/s...............-4.08 in (0.24 s)........-35.55 in. (0.55 s)