Dents on bullet bottoms, and how they puzzle me


#1

I have been reading a book on forensic techniques and have found something that puzzles me.

The book is titled Gunshot wounds: Practical aspects of firearms, ballistics, and forensic techniques, by Vincent J. M. Di Maio, second edition, CRC Press, 1999.

The author says:

[i]Base Markings

On discharge of a weapon, powder grains may be propelled against the base of the bullet with sufficient force to mark the base. Such markings are most evident in bullets with a lead base, that is, lead bullets or full metal-jacketed bullets whose lead core is exposed at the base. The shorter the barrel, the more numerous and the deeper the powder marks. 5 Different forms of powder produce different marks: spherical (true) ball powder produces numerous
deep circular pits; disk powder produces shallow, circular imprints as well as linear markings (powder flakes striking on edge); black powder produces a characteristic peppered appearance (Figure 2.7).
Powder marks are more prominent on the exposed lead base of full metaljacketed bullets than on the base of all lead bullets. Bullets with a jacketed base (partial metal-jacketed bullets) may show very faint powder markings on the base.[/i]

Now here’s figure 2.7:

What puzzles me is that I am sure that base dents like those on (b) are caused by the bullets travelling tip-to-base in the chutes when they are manufactured, and later when they are to be loaded into cartridges. I have seen this to happen only in pointed, jacketed, exposed lead bottom bullets, and I have seen it in unfired bullets.

The author says, as we saw before, “powder marks are more prominent on the exposed lead base of full metaljacketed bullets than on the base of all lead bullets.”

Why this would happen? Why the powder will mark or dent the exposed lead base on jacketed bullets more than the base of an all-lead bullet?

I think this is because only jacketed rifle bullets have a sharp point that can dent the base of the preceding bullet while on the chute.

I am worried to disagree with such an authority as Mr. Di Maio, so I am asking for your opinion. What do you think? Can a cartridge be so packed with propellant as to dent the base of the bullet? Or do flying grains of powder have the force to mark those bases upon firing?

Cheers,

Schneider.


#2

Sounds a bit far-fetched to me as well.

But it is quite simple to test with the proper setup. Pull some rounds, document the base, stick them back, fire them into a container that allows them to be caught and then re-examine the base.

Or call ‘Mythbusters’ and let them figure it out :)


#3

Schneider: I think your statement that jacketed pointed bullets can (and will) dent the bases of other bullets with exposed lead bases during the loading process is correct. My own example–admittedly from a very small sample–is that 7.9m/m round nosed bullets with base marks (like the “M” seen on earlier German examples) are more often seen undented than 7.9m/m spitzer bullets having similar base markings. In both cases I’m speaking of unfired specimens. Jack


#4

At the moment that the primer is popped on a pistol cartridge the flame squirts through the flash hole and ignites the powder which starts to burn. It doesn’t explode, it doesn’t even all start to burn right away.

It is a very fast but progressive burn releasing the gasses which drive the bullet along the barrel. more and more powder burns faster and faster accelerating the bullet along to its final velocity.
It all takes around two thousands of a second, but as the pressure builds at the primer end of the case some of the still unburnt powder at the bullet end is pushed into the soft base of the bullet by the pressure further back…

Rifles and pistols, but pistols more so, eject some unburnt powder from the end of the barrel. Hopefully not a lot, but some. It all depends on the type of powder and its characteristics, but it is normal.
The bullet “b” in your picture shows a coarse grained slow burning powder and clearly, almost graphically, demonstrates the effect.

It is true and well known.


#5

Well, maybe both things are true. The powder thing and the bullet tip thing.

Here are some unfired bullets, all pointed. Unfortunately the light angle in the scanner is not the best one for highlighting surfaces.

5,56 x 45, SS109, spanish
7 x 57 spitzer bullet, CNCS
7,35 x 51 Carcano, SMI
7,62 x 51 SS77, FN
7,92 x 57 S RA S 1936
7,92 x 57, sS type, spanish

And here’s the base of a spanish 7,62 x 51 bullet. It looks to me much like (b) in the book’s picture:

Cheers,

Schneider.

P. D. Now these scanner screens really attract dust, don’t they…?


#6

It may be that the marks in unfired bullets are the result of surface defects or debris on the face of the tool that presses against the lead core during manufacture?


#7

Could these powder marks also be caused by compressed loads?