Question about Tumbling vs Expanding

[Valjean]

Looks like the GMX 120g is too long to adequately stabilize in a 1:8 twist barrel. The projectile is long. Does the inability to adequately stabilize and its length make it more prone to tumble once it hits deer? If so, would it's propensity for tumbling make it an effective hunting projectile out of a 1:8?

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[A5BLASTER]

Perrty sure this bullet has been tested by members from 8twist barrels and it stabilized it just fine.

[grayfox]

If the GMX is ballistically unstable (and I have not shot it but just referring to your diagram), then it means that it will "tumble" or yaw in flight... the result will be a highly inaccurate shot. If it does happen to hit the animal it could just as likely hit it broadside, or "keyhole" on contact. I don't believe you should necessarily infer tumbling vs expansion on the internals of the deer.

[Fess]

Doing a quick search on this site shows mixed results with the 120 GMX. Copper alloys are less dense that lead and therefore tend to be longer than bullets with a lead core. This affects stability. Some site members have gone so far as to cut some of the tail off this bullet to assure stability and increase speed. Shorter lead-free 6.5 bullets are around now, so that would be a better bet.

Greyfox has a point in that the angle of a yawing bullet is always changing during flight, so you never know where it will be pointing when it hits the target. This makes its lethality unpredictable. That was a big problem with the military's M855 ammo. The effectiveness depended upon at what part of the yaw cycle it was in when hitting the target. If it happened to be at an angle when it hit the target, it would yaw or "tumble" rapidly and fragment to create a big wound. If it happened to be straight-on to the target when it hit, it could pass through a skinny opponent before it began to yaw.

Accurately predicting the stability of a bullet in flight is complex and requires a lot more info than a simplified one like the Berger calculator requests. Online calculators tend to be conservative for this reason. The shape and construction play a big part as does the rpm at which it is spinning. Long ago I remember Bill Alexander noting that a the short 6.5 Grendel barrels he sold needed a tighter twist than the longer ones because rate of spin (in rpm's) of a bullet leaving a barrel varied with the muzzle velocity. In the end, the most reliable way to test stability is to actually shoot it. The CHEAPEST way is for SOMEONE ELSE to shoot it.

[Kswhitetails]

Any bullet traveling over 1500fps hitting the vital area broadside is going to be absolutely, devestatingly, lethal.

The disadvantage is in the yaw creating obscure projectile path and uncontrollable accuracy. Inside 150 yards I doubt it?d be an issue of lethality.

[VASCAR2]

A good example of how rifle bullets perform when they tumble but don’t fragment is the 100 grain Wolf FMJ 6.5 Grendel. Andrew with the you tube channel the chopping block and does videos for the AR-15.com Youtube channel tested the 100 grain Wolf FMJ in gel.

For FMJ in gel the 100 grain FMJ does very well compared to other FMJ bullets. The problem is inconsistent performance and the wounding mechanism is not as devastating as fragmenting high velocity or expanding ammunition. DNS hog videos describes this very well and you can see his results are much better with the 123 grain SST and 90 grain TNT. Incapacitation is a result in enough organ damage, blood loss or central nervous system shut down to stop the target.

Head shots or CNS shots are effective where a FMJ or bullet with yaw can provide incapacitation but for serious use it would be my last choice. It sounds like the Military 5.56 A1 bullet is proving very capable but that bullet has high velocity and fragments. High velocity can cause additional damage but incapacitation is determined by bullet path and the damage.

I think this short video is worth watching. This information conforms to what I?ve seen testing ammo and actual street performance as a LEO.

https://m.youtube.com/watch?v=T6kUvi72s0Y

Here is the 100 grain Wolf FMJ gel test.

https://m.youtube.com/watch?v=b_WxJoCU7Ps

[Fess]

It should be noted that, at least for M193 and M855 a very slight yaw at contact makes a big difference in how quickly the bullet acts in a body. One study done with high-speed photography showed a marked difference between a bullet hitting at 2.5 degrees and one that hit at 1/2 degree*. One report showed that when a large number of tests were done, roughly 1 in 6 bullets travelled far enough before turning sideways that it would "ice pick" through a skinny human, causing little damage unless it pierced a vital organ. This is, of course, one of the big reasons that the 6.5 Grendel was created. The report did state, however, that these bullets were more sensitive to entry angle than most. M855A1 and Mk316 5.56 ammo were specifically designed to be insensitive to entry angle.

Vascar2 provided links to two articles that discussed balliistic gel testing. For those who wonder who about the relevance of gel testing, here is some background: In the mid 1980's, Dr. Martin Fackler, an Army Surgeon ran a series of tests to try to replicate and study the bullet damage he had seen in Vietnam. Many of the tests were done on live but anesthetized pigs. It seems that domesticated pigs do a good job of replicating human wounding. After some trial and error, he determined that 10% ballistics gel at about 4 degrees C (39 degrees F) provided the same results as shooting into the leg of a pig. There are links to some of this work on the wikipedia page for the late Dr. Fackler.
Like the guy from Federal in the first video said, it is not an exact match for all of the different tissues found in animals, but is a good, repeatable indicator.

*I would provide a link to those pictures, but it is in a presentation to the US Military brass urging the switch to the 6.8 SPC (and I don't want to be lynched).

[Kswhitetails]

If a sideways ( I don't remember what brand, or even what bullet, somewhere around 140 grains at 2500-ish fps - I THINK it was a Lapua FMJ) bullet hitting a 3/8 ar500 plate at 300 will put a nice oval slightly bigger than bullet hole in said plate, I cannot imagine what it would do to an animal. That same bullet disintegrates on the same plate when hitting point first. Physics is a cool study.

[Fess]

I guess the moral to the story is it is best to avoid getting shot with any of them!

[Kswhitetails]

I guess if I had to pick one, a well stabilized FMJ that is perfectly stabilized would be my choice in any case.

[BobinNC]

If you don't use the correct Miller Formula you will get the wrong result. To use Berger's Twist rate stability calculator you must deduct the plastic tip from the total bullet length (1.403"). Berger has no plastic tip bullets, so they don't mention it. Use the JBM Ballistic one here: http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

I used an estimate 0.200" for the GMX tip (it may actually may be a bit longer as I don't have one to measure). This yielded a Stability rate of 1.671, which is stable.

http://www.jbmballistics.com/ballistics/lengths/lengths.shtml#Hornady

[A5BLASTER]

If you don't use the correct Miller Formula you will get the wrong result. To use Berger's Twist rate stability calculator you must deduct the plastic tip from the total bullet length (1.403"). Berger has no plastic tip bullets, so they don't mention it. Use the JBM Ballistic one here: http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

I used an estimate 0.200" for the GMX tip (it may actually may be a bit longer as I don't have one to measure). This yielded a Stability rate of 1.671, which is stable.

http://www.jbmballistics.com/ballist....shtml#Hornady

X2 on this.

As I stated above that bullet has already been tested and shown to be stable from 8twist grendel barrels.

[Growler]

The way I read the first post, I think the OP might be confusing external and terminal ballistics. Tumbling or unstable flight leads to random inaccuracy. Tumbling terminal ballistics can result in fragmentation and meat loss. Even perfect terminal ballistics can’t fix a badly placed shot.

If you feel inclined, load some up and test them with your barrel. Just be sure to show us the results!

[drewsbow]

my 1 in 8 twist howa shoots these just fine

[Fess]

To BobinNC: good catch on info about the plastic tip and the JBM stability calculator!

The original post got me curious, so I started researching twist stability. I was aware of the Miller rule, but did not trust it with copper alloy bullets and plastic tips. As it turns out, some shooters did tests and in 2012 posted a modified Miller Twist Rule to account for the properties of bullets with plastic tips. That is the one being used by JBM, but not by many other sites.

Miller's equations made the assumption that the bullet is solid and made of materials with roughly the same density. It underestimates the stability number of bullets with plastic tips or hollow tips like some match bullets. It is important to be aware that the Miller formula is based on a muzzle velocity of 2800rpm. If the muzzle velocity is lower, the bullet is less stable because it is not spinning as fast. In other words, it is possible that a bullet that is stable from a 24" barrel will not be stable when shot from a shorter barrel. The JBM Stability Calculator asks for muzzle velocity to correct for this.