Bullet Penetration and Tumbling


For years I have been fascinated by the performance of bullets both in the air as well as once they hit their target. As a hunter at heart, a bullets terminal performance inside flesh and bone are particularly interesting. Today’s subject is about Bullet Penetration and Tumbling.

Recent events have lead me to investigate the subject in a little more detail, and today I am jotting down a few theories based on experience and the albeit anecdotal evidence I’ve been exposed to.


I’ve seen quite a few bloody bullets in my hunting career, a subject I’ve gone into great detail in another discussion titled: Is shot placement more important than cartridge and bullet selection?

Expansion is no guarantee of a kill, this bullet would have probably worked great had it gone through vitals (.270 Nosler ABLR)

The main theory behind that article is that shot placement is more important than the cartridge or bullet you choose. That’s not to say that cartridges and bullet selection are not important, just emphasizing that making a good shot is MORE important.

Today’s subject seems to be a little bit of an offshoot from that discussion, and more relevant to bullet selection.

Different bullet designs utilize different methods to transfer their energy to the target. Some use raw energy to simply peel open into a larger frontal face (meplat) creating a larger diameter to plow through tissue. Others are designed similar to a mechanical broadhead that open to a consistent pattern that also generates a larger path of damage through the tissue.

There are other designs as well, our forefathers hunted with simple lead balls with little to no increase in size upon impact.

The bullet that got me thinking

For the last few seasons I have been using bullets quite different than I had used during the many seasons prior. Some of you may have already guessed that I am talking about the Cayuga Solid bullets manufactured by Patriot Valley Arms.

I was approached by the manufacturer to test the bullets performance on various big game animals at varying distances and conditions. For all the information about that subject you can read the whole thing in: A Solid Season.

Dont consider this a sales pitch for any specific product, I share information on products I use and I include information where they can be found. As always, I encourage you to use what you shoot best.

After shooting nearly a baker’s dozen of Utah big game animals (and one unfortunate Wyoming coyote), I had become quite pleased with the bullets performance. We used the bullets in almost every popular caliber, but mainly in 6mm, 6.5mm, and 7mm.

A Mule Deer heart that took a 122 grain 6.5mm Cayuga

The animals hunted with the bullets were Pronghorn Antelope, Mule Deer, and Rocky Mountain Elk. All the animals shown in A Solid Season were taken as close as 200 yards and as far away as 1000 yards.

How do bullets work?

As the seasons passed, and the meat stacked in my freezer, I was curious what exactly these bullets were doing. We had yet to recover a single one, even the 6mm bullet had zipped through a Mule deer at 1000 yards. This piqued my curiosity as most other styles of bullets had at one point or another come to rest in our game animals for inspection.

There are other solid copper bullets from other manufacturers that likely perform in a similar fashion

The Cayuga is turned on a lathe from a solid copper bar, with a hollow point cut into the tip of it. There is no other structural facets or features to it, which had me imagining that they simply mushroomed open somewhat at the front.

Discussing the topic with the manufacturer of the bullets, we discussed the probable eventuality that the bullets were tumbling within the tissue. Something I hadn’t considered beforehand.

Wound Channel Analysis

Inspecting the various animals that we had shot over several seasons, the damage was what I would consider a textbook wound channel. That is to say, everything looked exactly as I would have wanted and expected to see.

Broken shoulders when applicable, pulverized vital organs with large holes preventing their further use, and the almost exclusive anchoring of the animal upon being hit. This was the norm when shooting these bullets.

As I mentioned at the beginning, I’ve seen a lot of bullet holes. And for all applicable purposes the solid bullets I had used seemed to perform in a near identical way. Compared to all the cup and core bullets I have used over the twenty years prior.

This set of elk lungs took a solid bullet (and some bone fragments)

But do bullets tumble?

The gyroscopic energy of a bullet induced by the barrel’s twist surely keeps the bullet stable and moving in the right direction while in flight. But how far into the target tissue does this stability remain? Does bullet penetration and tumbling happen immediately?

Watching ultra-slow-motion of bullets impacting a ballistic torso you can clearly see that many bullets tumble. Particularly towards the end of their travel through the dummy torso filled with fake bones and organs.

the almost perfectly preserved tip of a Hornady 135 A-tip does it look like it bent to one side?

I expect this is due to the reduced stability caused by the loss of rotation induced by friction and bullet deformation within the target. Despite its tumbling, it continues to pass through the target. We know the bullet is expending its energy as it passes through the target. So it would make sense that running into bones and other “stuff” within a body could cause it to turn or tumble.


The increased diameter of bullets as they expand, as well as the reduced weight as some of them shed their mass is also likely to slow their rotation. Much like a spinning ballerina extending their arms slows their rotation.

I also think its a reasonable assumption that many bullets do the same as they mushroom out. The overall shape of the bullet becomes more of a round shape than a long cylindrical one. This too could make the bullet change its orientation as it passes through the target.

Is it possible that the solid copper construction of the Cayuga bullets has something to do with tumbling? The lack of any lead within the bullet means that they are particularly large for their weight. And the larger/longer bullets need a faster twist to keep them stable. Perhaps the bullets are close to the edge of unstable due to their size, and therefore easier to tumble than a traditionally constructed bullet.

Jacketed bullets

Many of the theories above can affect bullets regardless of their composition. Jacketed bullets also mushroom or expand and can change their orientation during pass throughs. The mushroomed tip of a jacketed bullet frequently has sharp copper edges. Together with the wider surface is likely why bullets come to rest backwards.

Once the centrifugal force of spinning is overcome by friction, the slowing bullet probably hangs up on the sharp edges and the drag of the wider mushroomed face pulls it to the rear. Much like the fletchings of an arrow keep your arrows traveling the same direction.

the remains of a Hornady 150 grain Interlock

The flattened meplat of a bullet causes a snow-plow effect as it pushes through tissue. Obviously the bigger surface of the meplat, the greater the effect. A bullet that travels sideways through a target also has greatly increased the forward moving surface area. Either of these scenarios seems to cause significant damage and wound channels.

A First Recovery

During a recent Pronghorn Antelope hunt, I was actually able to recover the very first Cayuga solid bullet. The shot was with my 6mm GT shooting the 100 grain Cayuga. The impact was from approximately 480 yards and crossed through the buck who stood at a quartering angle.

After shattering the left shoulder, the bullet passed through the ribcage tearing through one lung and the top of the buck’s heart. It continued towards the opposite corner of the animal passing through the liver, and a bit of the stomach (lucky for me it wasn’t messy).

The bullet came to rest in the soft belly skin. Just in front of the buck’s right hip and was poking out of the skin.

Inspecting the buck. Note broken left shoulder and exit wound low left (censored for my guy)

Perhaps striking a bone is all the bullet needs to start a spin. Or perhaps maybe hitting anything at all? Make sure you comment below to let us know your theory.

It is apparent from both the recovered bullet, and the damage that some of the bullet fragmented

After butchering the buck, I found that the bullet had broken the shoulder bone just below his shoulder blade. It was also apparent that either fragments of the bullet or pieces of the shoulder bone were sent thrashing into the chest cavity causing additional damage.

The broken shoulder, the shoulder blade is on the right

The Bullet

Perhaps the most interesting bit of all was the recovered bullet itself. Much of its weight had lost at the front. But it also had not mushroomed at all, in fact it really wasn’t much bigger at the increased meplat than the 6mm bullet diameter.

Discovering the bullet pointing against the direction of travel, didn’t surprise me. I’ve found many jacketed bullets in the same awkward stance. Weighing the fired bullet turned out to render almost a 10% weight loss, the remaining bullet weighed 90.2 grains.

the recovered bullet (left) compared to an unfired bullet (right)

There has already been some serious panties ruffled discussing this topic, and the usual suspects doing their armchair spotting and backseat analysis. I myself remain quite curious as to what happens when these and other bullets hit their targets.

While I can only offer my opinion on what might be going on in the milliseconds after impact. My experience killing animals with bullets that appear to be tumbling has been nothing but good.

I don’t know for sure what happens, only that the result is a dead deer, elk or antelope. And many of them have been the cleanest of kills. With only a couple of them taking more than a step.


Does a tumbling bullet do more or less damage? Do jacketed bullets tumble as well? Can bullets that come apart on impact effectively take down our prey?

Obviously I can only speak from my own experiences and those I have witnessed. But I think that all bullets will tumble at some point. Either when they slow down enough, or when they hit something significant enough to knock them out of their rotation.

this is another wound channel from a Cayuga solid bullet

We could all agree that a bullet that reliably opens to a larger size is probably a very safe bet for inflicting damage. However there is something that hasn’t added up for me here; We have been sold “reliable bullet expansion” as the most desirable trait a bullet can have for as long as I can remember.


After witnessing what this little 100 grain chunk of copper did to this antelope buck I am curious. If the bullet isn’t tumbling, and its spinning stability sticks with it through most of the animal, then how did minimal or perhaps no expansion cause such impressive damage? If a 6mm flat-point bullet can cause that much damage, is the expansion sales pitch oversold?

I don’t think it is, as I’ve seen FMJ performance through big animals as well, which proved to be significantly less damage. Granted, fmj’s aren’t flat-points but you get the idea.

this set of lungs was hit by a very fast FMJ bullet, though they seem barely damaged the animal expired in seconds

My theory is that these solids must be tumbling, or at minimum a combination. Otherwise their wound channels would look more like an FMJ wound channel than that of an expanding bullet. Furthermore they seem to produce just as deadly wounds as expanding bullets.


This is all of course just my opinion based on my experiences. Don’t get too excited or bent out of shape if it conflicts with yours. These questions are simply looking to create a discussion and see how my experience contrasts with yours.

While I don’t consider myself a forensics specialist, or a ballistician. I am very interested in both of these subjects as its related to hunting and the equipment we use for it.

Please feel free to add your comments below, and share your own experiences. Perhaps we might all be somewhat enlightened through discourse. I will likely update this post as additional information is added.


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