Ballistic gelatin test results : 9x19mm Cor-Bon DPX and FMJ

[Brass Fetcher]

Cartridge : 115gr Cor-Bon DPX and Winchester 115gr FMJ

Firearm : 4.0" barrel, recoil-operated semiautomatic

Block calibration : 10.4cm @ 573 ft/sec

Shot 1 - Cor-Bon 115gr DPX. Impacted at 1153 ft/sec. Penetrated to 14.5" (corrected) and expanded to 0.538" average diameter.

Shot 2 - Winchester 'White Box' 115gr FMJ. Impacted at 1172 ft/sec. Penetrated nose-forward to 5.9", at which point the bullet tumbled. This yawing continued on until 13.6", where the bullet apparently righted itself and turned nose forward for the remainder of the travel through the gelatin block (16.0" total travel distance through gelatin). Bullet was recovered undeformed at 0.354" diameter.

 

[daysleeprx]

Interesting result with the FMJ!

 

[loplop]

Good performance for the DPX.

The FMJ tumbled, very interesting. Any idea why it tumbled, John?

 

[dispatch55126]

Its most likely caused by the differant densities in the gel.

 

[Brass Fetcher]

I would have to say it was due to the instability of the bullet. My understanding is : the faster the twist of the rifling (the bullets rotating speed), the less likely the bullet is to turn end-over-end (bullet is more stabilized). Despite being stabilized by the rifling, the bullet is still 'wobbling' slightly about its center line. While this may result in a slight inaccuracy in a bullet traveling through the air, the density of gelatin (which is mostly water) is far greater than air... some 700 times greater - so any wobbling is magnified by the gelatin.

Once the bullet nose is sufficiently tipped, the pressure of the fluid on the moving bullet then pushes the nose further down/up/sideways and causes the bullet to rotate about its centerline in that axis.

This tumbling must be strongly affected by bullet velocity - I've not seen this happen with most 22lrs, all 25acp, 32 and 380ACP. Everything else seems to have tumbled significantly.

 

[loplop]

Very interesting. I did not know that anything outside of rifle FMJ would tumble.

 

[dispatch55126]

Just as you said, it deals with the density of the media. Take the gel and push a pin through it. Now do the same thing with a pencil. The larger the object, the more media it has to push out of the way. That in turns can/does cause material to pile up somewhere on the nose which pushes it the opposite direction, ie. flip.

 

[dispatch55126]

A rifle bullet is basically the same thing. However, in the case of the 5.45, the long slender bullet becomes unstable when passing though anything denser that air causing it to nose in smoe direction and tumble.

 

[Brass Fetcher]

Well, I posted this in a AK74 thread, but it should fit here pretty well too. The name I was looking for is 'aerodynamic moment' - the bullet goes through minute 'wobbling' about its axis of travel and this is magnified when the bullet hits gelatin or BG due to the greater density of the fluid.

http://www.nennstiel-ruprecht.de/bullfly/fomo.htm#header_forcesmoments