7.62x39 sections - air pockets


#1

While scrounging for steel core section images online I came across this interesting image of several commercial 7.62x39 sections showing various air-pocket sizes in the tips (lead cores):


from left to right, the current 7.62x39 rounds: Ulyanovsk EM1 match (accurate and yaws quickly) Klimovsk FMJ, Klimovsk “Silver Bear” FMJ (Identical), WOLF FMJ (note gigantic air gap), Barnaul FMJ, Uly FMJ, Early Klimovsk FMJ, Rare early brass cased TCW fmj. -

(description key comes from website of above photo)


#2

What an interesting picture. I would imagine small voids are pretty common because the swaging process must be inclined to trap air but to see it like that really illustrates the point.


#3

Vince–Those voids are not the result of swaging. They are designed voids for the purpose of expansion.


#4

Ron, I think you’re mistaken about the purpose. My understanding is that the voids result from the requirement to keep projectile weight in the range of 120-124 grains. With those bullet shapes and lengths, if the voids were eliminated by filling the jackets completely with lead, their weights would be well in excess of 124 grains.

Since they are FMJ bullets, they’re unlikely to expand in soft tissue. But, the rearward weight bias caused by the voids would enhance the tendency to yaw after impact.


#5

I had thought that the voids were for the dual-purpose reason of creating a projectile that would tumble upon impact/penetration, and also achieving a certain grain weight.


#6

I go with stanc, since the jackets of the regular steel core projectile were used (but no steel core) the available space could not be filled with lead because the projectile weight would have become too high.


#7

OK, OK, I accept the whipping. On reflection, I was confusing the “air-space” in the nose of the bullet with a discussion of the “Effect” bullet on the old forum. Just the perambulations of a Senior mind.

However, the fact still remains that the “air pocket” is not the result of air being trapped during swaging, but is a design feature. I just had the “wrong” feature.


#8

I still go with my original theory that the voids are a defect caused by air getting trapped when the core is rammed into the jacket. You can’t design in an air gap because it would be too variable from bullet to bullet. Thats why things like aluminium tips are used if you want to shift weight towards the rear.

If the core is too big in diameter it acts like a cork trapping the air inside the jacket before it has time to escape.
Once the core has started to expand under pressure the air has no way of getting out and remains in situ with a FMJ projectile. The whole process will take a fraction of a second on an automatic machine, the air has no time to get out. The size and shape of the lead core is fairly critical if proper venting is to occour in this process.

On the fourth bullet from the left you can see wrinkles down the sides of the core which I would take as a very clear indication of air being trapped and trying unsucessfully to get out. That core would not grip the jacket on firing and you would probably get a spinning core inside the jacket.

You may well see the effect of trapped air on the outside of the jacket as a little wrinkle in the jacket when the other problem occours which is air getting trapped between the jacket and the die but that is more easily dealt with by making a tiny vent hole at the very point of the die. you can sometimes see a little mark on the tip of the bullet left by the vent hole.

When hand swaging you are told to apply a slow steady pressure to prevent air getting trapped but it still can. thats why you should weigh the bullets afterwards. The swaging manuals talk at length about the avoidance of trapped air so its obviously a recognised problem.

Much the same happens with casting voids when casting lead bullets. The molten lead flows around the air and won’t let it out.

The voids in the bullets shown are quite exreme in some of them and would have a marked effect on trajectory. It looks to me like someone has been batch weighing loaded rounds (an old match shooters trick) and sectioned those that were under weight to find the cause.

Still very interesting IMO. I would like to see other bullets from the same batches sectioned for comparison.


#9

Vince - if that was the case, all lead-core bullets would have such a gap I would think, and yet they don’t. I go with EOD and the orthers. The Russians are not strangers to a gap at the nose of the bullet jacket - it was a design feature of the the 5.45 x 39 cartridge to increase yaw and the wounding effect. Also, the gap results in a different center of gravity that can, when done correctly, increase the accuracy of the bullet in flight. As said, it also reduces the weight of the finished bullet, which probably was not desired when the steel-core military bullet jacket had to be maintained for bearing surface and overall length (to insure proper feeding in magazines) but with a lead core to meet the importation requirements of many countries, including the major market - the USA - for “sporting ammunition.”

I am scientifically challenged, but I feel sure that the others are correct in their explanations.

Just my opinion, and admittedly from one who is a little short of the technicalities of manufacturing processes.

John Moss


#10

Those are my sectioned cartridges and the airspaces are in fact a design feature of these projectiles. In order to use the M-43 jacket with a lead only core and maintain a 123 gr. (+/-) weight, an airspace has to exist, since an M-43 jacket filled entirely with lead would weight around 150 gr. (this projectile exists too). There is no “wrinkle” where the air escaped, this is just an optical illusion from a poor scan.

In fact, on some examples, you can clearly see a “step” or “shoulder” inside the jacket tip where the lead core was intended to stop against in the manufacturing process.

I’ve sectioned more than what is shown in the picture and there is a bit of variation in the depth of the lead core in these jackets. Probably helps to explain why these projectiles are not known for their accuracy…

AKMS


#11

Lead cores are “pre-formed” at the cutting from wire stage, so that the “core seating” &“swaging” process is simplified. So a lead core for a Ball 7,62M43 Jacket will be “flat pointed” for weight considerations, and even if air does escape during core insertion, there will remain a “Pocket” at the Point because there IS NO LEAD there. No “compressed air” pocket or “un-escaped” air about it.
The Core and the Jacket are engineered to make sure the jacket is the standard length after seating and base forming ( Boat tailed and rollover of jacket)…the resultant air pocket may assist in expansion, tumbling or whatever for the bullet, but it is just an artefact of Bullet production.

The US, in WW I, made .303 “Mark VII” Bullets without the aluminium tip…in order to get the same Over-all weight ( 174 grains) the Bullet was actually a bit shorter than the British Spec. Mark VII ( with Fibre/Aluminium tips)… A proper mark VII jacket filled with lead is about 190 grains ( the Portuguese made some like this in the 1920s ( from samples in my collection)
but by adjusting Jacket length, brought the weight back down to 174-175 grains by the 1930s ( in 1928, they reloaded Old W15 and other US made projectiles from dismantled cartridges into new AE made .303 cases ( also from my collection of Portuguese .303)…The US made projectiles were shorter of course…see above.).

Forty years ago, I did an analysis ( 1922 to 1937) of AE made .303, from Powder weight and type, primer shape, Flashhole/Berdan style ( Roth and RWS) Bullet length and weight, and crimping style. I will have to dig it out and re-print it…I also still have the entire 1922-1937 collection of cartridges ( courtesy of a mixed Loose crate of Portuguese .303 from Mozambique…included as well all the US Contract dates, as well as British “z” loadings of 1917 and 1918…all ammo donated by Britain to Portugal in 1919, or otherwise dumped in the Channel and North sea as being “Defective”.

Regards,
Doc AV
AV Ballistics.

Sorry about wandering away from the original thread, but the “Bullet weight” question brought this other signal event to mind…


#12

Well said, Doc.

Since all of these cartridges are from commercial production for the US market to address the ban on steel core perojectiles, there would be no need or desire to manufacture them to expand, yaw, tumble or otherwise perform any other function than punch holes in paper. Placing the lead core to the rear of the projectile might make it yaw sooner, I have no reason to beleive it was designed with this in mind.

The Tula manufactured FMJ and HP both have the same identical construction with the large air space, save for the opening at the tip of the HP, and both are called “Sporting Ammunition”.

AKMS


#13

The hollow cavity for the 5.45 I was talking about was in military ball, and its yaw characteristic due to the weight distribution, and i’m sure other factors in the design of the bullet was proved at the Letterman Army Institute of Research, Wound Ballistics Lab, years ago. I know full well that total bullet weight was the primary motive in the commercial ammunition for the short core, but not in all ammunition. Design of bullets is not as simple and straight-forward as it seems it should be, and many factors affect bullet performance. I doubt whether the factories in Europe meeting laws against cores of steel, regardless of hardness, in their commercial ammunition give a hoot whether it yaws or not, or even what minor changes might improve accuracy 5 or 10%, in the type of calibers we are talking about. I am not talking about calibers that are gnerally commercial and made by factories that have a history of producing high quality hunting and match ammunition, but they way. I am talking about the type of ammunition sold cheaply intended for informal plinking (regardless of any advertising hype) in stock military arms. I think they do care when it comes to ammunition made primarily for their own military use, though.

John Moss


#14

Commercial Russian 5.45x39mm from Tula and Barnaul have very similar designs to the 7.62x39mm with both having the large airspace in the jacket tip. Amursk (Vympel) used a completely different projectile. Same weight but shorter overall and with a much smaller air space. Ulyanovsk used tracer projectile jackets and have a full lead core with no air space, resulting in a 70 gr. weight vs. the 59-60 gr. weight of the others. The Uly projectiles are interesting in that they appear to have been taken off of the production line at the point just before the tracer element would have been inserted. The lead “front core” is the same as that of the tracer, and a second lead “rear core” is inserted in place of the tracer mix. This must have allowed Uly to use as much existing tooling and machinery as possible to make these commercial loads.

AKMS


#15

AKMS - in regards to the commercial purpose of the bullets…

I have read that people occasionally use these rounds for hunting purposes. I seem to recall that some of the earlier Russian ammo that I have came in generic looking boxes that read “MADE TO HUNTING SPECIFICATIONS.” I suppose that some expansion and/or yaw might be desirable for hunting.

Who knows if that was really an intended design feature, though.

jjjxlr8


#16

This is interesting!
I was told the people here had a wealth of information, but I couldn’t have begun to imagine how much!


#17

Miles - the longer you hang around in this forum, the scarier it gets. There are some members in here, and other IAA members not on the forum, who have written volumes on different ammo subjects in the journal as well as their own books. There should be a member here somewhere who has all of the 7.62x54r and 7.62x39 steel cores with their specs on file for weight & length for the info in the other post you were looking for.


#18

Earlier in this thread there were comments about air being trapped in the nose of a FMJ bullet with a full length lead core. If such a thing does happen it is the result of poor QC, not the process. Think of a FMJ bullet being formed just the opposite of an open point, hollow point, or soft point design.

A lead core is made by first cutting a length of lead wire and then forming it to a pre-determined diameter, length, and weight in a core forming die. The weight is the most critical and is held to very close tolerances by “squirting” excess lead through a small bleed hole in the die body. The diameter is such that the formed core will fit easily inside the un-formed jacket which is still a flat ended cup. The two pieces are then placed in the pointing die which expands the two to the finished diameter, compresses the core, and points the jacket. There should be no free air anywhere inside the finished bullet.

Bullets made with intentional cavities, such as those shown, and/or multiple piece cores are made differently, of course.

All swaging steps result in an increase in diameter. This is because the two metals, lead and jacket, do not expand/compress the same and also tend to spring back at different rates. If a bullet is made by swaging down to a given diameter, the jacket will spring back more than the lead, resulting in a loose jacket. Diameter of a finished bullet is actually controlled by compressing and expanding the lead core. The jacket simply goes along for the ride.

Also, one other nit-picking detail. Using the word “yaw” to describe a bullet’s reaction to hitting a target isn’t really the best description. Yaw does mean a change in direction but it usually refers to a horizontal change such as in the path of a boat or airplane. It would be clearer to say that a bullet tends to make unpredictable or erratic changes in direction, or even tumble. This is true whether the bullet is unstable or not. Making a bullet nose or tail heavy only tends to exaggerate the phenomenom. If a bullet truly tended to yaw it would be very inaccurate in flight as well as in a target.

Ray


#19

Ray, I must respectfully disagree with just about all of that. Terminal ballistics experts object to the use of “tumble” to describe a bullet’s behavior in soft tissue. In fact, “yaw” is the preferred term since, as with an airplane, “yaw” describes a change in orientation relative to direction of travel; as best I can recall from my flight training, it does not mean “a change in direction” of travel.

A bullet yaws as a result of the transition from a low-density medium (air) into a high-density medium (ordnance gelatin, muscle tissue). A bullet can be perfectly stable in air, but quite unstable in soft tissue. By changing aspects of the design like the center of gravity and external shape, a bullet can be designed for early or late yaw in soft tisssue, but still be stable in flight.

Stan


#20

stan

I have always thought that yaw was a horizontal change in orientation while pitch was a vertical change. (The third motion is roll) If yaw is any change in orientation then I am wrong.

FMJ bullets can do more than yaw when hitting soft tissue. There are many documented examples of a bullet making a 180 degree turn and exiting out of the front of a target. There are also many documented examples of bullets tumbling in a target. Would you still describe those as yaw?

It’s interesting that this type of bullet behavior is primarily associated with FMJ. Expanding bullets will normally proceed in a straight line unless deflected by something such as bone.

As always, I am like the bullet mentioned above. I can do a 180 just like that. There’s an old saying, “In making theories, always keep a window open so that you can throw one out if necessary.”

Ray