A question about downloaded rounds.


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hammerklavier
February 8, 2012, 04:29 PM
I have a forensics question for a book I'm working on. If a round was severely downloaded, would the recovered bullet show some differences in severity of rifling marks that would indicate this? Assume an FMJ rifle round.

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MtnCreek
February 8, 2012, 04:36 PM
I don't see how it would; you're still forcing an object measuring x.xxx through rifling measuring y.yyy.

Bullets are manufactured to perform (on target) within a specific velocity range. Condition of the recovered bullet could tell you approx what the velocity was from the deformation, lack of or fragmentation of the recovered bullet.

kingmt
February 8, 2012, 04:38 PM
In rifling no but in bullet deformation yes.

SHR970
February 8, 2012, 05:23 PM
Rifling:No.

Deformation: Depends. If you are talking about Round nose FMJ you won't have significant deformation at most any range as the bullet won't tumble in tissue but makes a straight track.

If you are talking Spitzer FMJ, the lack of deformation during tumble would be construed as a round fired at a longer range at first glance. Example: 150 gr. FMJ fired from a 7.62 x 51 fired at 2065 fps. muzzle velocity hitting a victim at say 30 feet will act the same as a 150 gr. bullet fired from the same weapon at 2900 fps. muzzle velocity from 400 yards away. Speer #12 tell us that their 150 gr. bullet has a BC of .423, their table tells us that that bullet @ 2900 fps. muzzle has about 2065 fps. retained @ 400 yds.

So unless the forensics people can show that the bullet hit at an angle not consistent with that range or that a shot from that range was impossible, they would conclude that the shot was from around 400 yards.*

* Adjustments would be made based on their assumption that the round was mil spec and the velocities expected from a mil spec round. I used these numbers as they come straight from a book you can look at.

cougar1717
February 8, 2012, 06:05 PM
A little reality check never stopped a good storyline before...

918v
February 8, 2012, 06:11 PM
A severely downloaded FMJ may tumble in flight as it transitions from supersonic to subsonic velocity. It may strike the target cockeyed, if at all.

medalguy
February 8, 2012, 06:17 PM
Well, let me add that a "severely downloaded round" might get deformed when you drive it out of the barrel with a brass rod.. Other than that.....;)

hammerklavier
February 9, 2012, 09:40 PM
Thanks. That's what I suspected, just wanted to make certain I wasn't off base.

PapaG
February 10, 2012, 04:32 PM
O.K. I'll take exception to most of the answers here. A normally loaded round, to full velocity will tend to show slight "skidding" where the bullet enters and grips the rifling...very minor but it is there. Softer alloys show more than harder ones and jacketed ones very little but there is a slightly wider "land" pattern (the groove in the bullet). Downloaded, slower bullets, will have less deformation at the beginning of the rifling marks. Since the original question had to do with rifling marks and not expansion on target, I thought I'd add my two cents.

SHR970
February 10, 2012, 10:13 PM
PapaG: The skidding will be present even on downloaded bullets. As the bullet is in its initial acceleration phase and the powder is coming up in pressure, you will won't see much of a noticable difference if at all under the normal magnifications that a forensic tech. will be using to look at the slug. If you download using the standard technique for a rifle (using a faster powder like a pistgol powder), the downloaded bullet can actually have a bigger skid due to the quicker initial acceleration due to the faster burn curve of the powder along with faster peak pressure and thus faster initial acceleration of the bullet as it hits the lands.

But your point of skid marking at the front of the bullet is valid nonetheless.

FROGO207
February 11, 2012, 08:25 AM
Also I add that with a solid LEAD bullet the base will be expanded/deformed/gas cut with different rates in respect to the pressure produced by firing the round.

PapaG
February 11, 2012, 01:22 PM
Thanks, SHR....I have recovered quite a few lead and jacketet bullets fired through everything from a 44 mag, 06, 30-30, 357, and others, from oiled sawdust, sand, and snow (try that sometime) and have found that we are both quite correct..

My downloaded ammo for handguns is normally with a fast powder like BE (750 fps in 38, 44 spec, etc) and Unique or 4198 in rifles. They all skid somewhat.

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