Bullet Spin and Performance

[Lightvarmint]

In my opinion, if your using a big game bullet or any bullet using a relatively heavy jacket, RPM levels really will have no effect on penetration or expansion as long as they are not high enough to cause bullet failure in flight. If that happens, there will be no penetration or expansion in the intended target anyway!!!

I have tested this quite a bit, no scientic testing, just what we all can do. In my opinion, If the bullet will survive the launch velocity and RPM levels at the muzzle, down range effects will be nearly identical.

The only thing I have seen is that bullets seem to penetrate in a slightly straighter line when they are fired from a faster twist barrel. This is espeically true when dealing with long solids or bullets that when expanded still have a relatively long bullet length after expansion. Bullets that expand to the point that they are less then 50% of their original length, they seem to penetrate pretty darn straight.

There is one area of shooting and hunting where rpm levels can make a dramatic difference in expansion. Varmint hunting. Here we are taking bullets with very thin jackets and driving them as fast as we can. I have tested on several occasions with two 22-250 rifles, one in 1-14 twist, the other in 1-9. Shooting several different bullets. With conventional bullets that would handle the rpm levels in the faster twist barrel, those bullets produced noticably more "POP" on game such as gophers, prairie dogs, jack rabbits and rock chucks compared to the same bullet at same velocity in the 1-14 twist barrels.

I had to be careful to limit velocity to the same in both. Bullets such as the Speer TNT, Sierra Blitzking and Hornady V-Max had the occasional bust cloud if I pushed things to hard. The Ballistic Tips took anything I could throw at them in either barrel with ease.

I also found there was a more dramatic "pop" using tipped bullets in the fast twist barrels, again, driving them as fast as possible. With these situations, I believe these bullets are on the ragged edge before coming apart anyway, except perhaps the Ballistic Tip. When they impact, they literally come apart. Those that are tipped simply dust. Now some will say why are we seeing some exit wounds on larger varmints? Well, if you shoot a yote at 20 yards with 00 buckshot, you will likely see an exit wound.

Same idea here. Even though the bullet has fragmented violently, all the pieces are still in close proximity and still traveling at very high velocity. They shed velocity quickly and in game such as a coyote, most will not exit but on say a jack rabbit or rock chuck, most often even though the bullet has fraged darmatically, they still exit.

Again, in my opinion, for big game bullets, its hardly worth talking about at long range other then perhaps straight line penetration that may be improved with higher RPM levels.

Just my 2 cents.

Kirby,

Expansion is somewhat dependent on bullet spin as per your above post. It must be properly stabilized to travel straight and therefore to impact properly on the bullet tip and cause proper deformation and expansion. Non stable bullets sometimes make right and left hand turns and do not expand because they do not impact properly upon the tip to cause expansion..

GS Custom bullets of South Africa design their bullets to "lose" the petals and become a flat nose bullet upon penetration of the media that it is shot into.

You can take a full metal jacket bullet and make it behave like the GS bullets just by "adjusting" the tip with a serrator. Furthermore, you can take an unformed bullet with a heavy jacket with just the core seated and perform an internal serration process and use the same bullet profile for say a 338 Allen Magnum or even as slow as a 338 Federal and get proper performance out of each one using the same bullet die..... Basically, they do not have to be the same on the inside to be the same profile on the outside and perform properly at the extremes on the velocity scale. Just think of it two or more levels of performance using the same heavier jackets, core, and tip for different starting velocities........... Heavier jackets tend to behave more like bonded bullets without the hassle of the chemical process.....

So, for the pistoleros, you can even have the same high BC bullets that are set up for HV out of a big rifles and use the exact same bullet (adjusted) in the smaller cased pistols and still get good performance out of both...... Just like Kirby said, all it has to do is to survive the launch and, that is easily adjusted by experienced bulletsmiths.....

Questions of the Day:

Additionally, how does the number of engravings by the lands in the barrel affect both accuracy and expansion performance of hunting bullets?

Lightvarmint

 

[Fiftydriver]

LV,

Your question about how groove count is a very interesting one.

I have done some studying on this as well, more by accident then intentional but none the less. I shot a whitetail at 750 yards with the 200 gr ULD RBBT 7mm wildcat bullet. The deer was hit on the onside shoulder on a slight quartering toward me angle. Ran 30 yards and fell over dead.

I got full penetration on the deer but she was standing on a game trail with a relatively steep bank behind her and luckily it was pretty clean dirt. As luck would have it, I was able to recover the exanded bullet.

I found parts of the core and most of the jacket which looked kind of strange as it had 4 major petals on the expanded bullet. When I got to looking at this, the bullet jacket expanded and seperated along the lead edge of each of the land engavings..... Being a 4 groove Lilja barrel.

Seeing this I started checking to see how this effected expansion on different bullet.

On the Berger, Wildcat and A-Max bullets it seemed to be pretty common for this to happen. With the SMK, it never made any difference from what I could see and on the Accubond there was also no sign of any predictable jacket cutting locations, instead it appeared to roll back over itself more like a sock.

Seeing this, I believe that with the thin jacketed bullets, this can be an issue.

I took this a bit further and tested in a 6 groove 1-9 as well as a 3 groove 1-7.

In most cases, out of the 7mm AM, the 1-7 twist ripped the Berger, A-max and Wildcats apart. The Accubonds performed the same way as the 4 groove barrel.

The 6 groove barrel shot all bullets very well and there did not seem to be any distinct jacket expansion along the land engraving edge.

As far as accuracy and expansion, there was not a dramatic difference that could be measured, at least in the bullets that could survive testing in all twist rates and groove counts.

One interesting point, when the 1-7, 3 groove barrels got around 250-300 rounds down the barrels, they started dusting all the thin jacketed bullets no matter what velocity they were driven to. I tested from 2800 to 3400 fps, did not matter all dusted.

From new to that point, these barrels were shooting WELL under 1/2 moa with all bullets so this tells me that the bullets were on the ragged edge and when the bores got just a bit of wear on them to rough them up a bit, it was more then these bullets could take.

The same bullets in the 1-9 4 groove barrels, have never had this problem at all unless you push them over 3300 fps. I am referring to the 180 gr Berger and 200 gr ULD RBBT.

 

[Fiftydriver]

My only concern with trying to duplicate long range impacts with lower velocity is the fact that RPM levels are dramatically less. Does this make a huge difference, again, with a big game bullet or SMK, more likely then not, not enough to measure in the test.

I do feel that it certainly does effect the stability of the bullet, if not in flight, certainly after penetration. If a bullet, say a 300 gr SMK impacts but begins to tumble or at least yaw and this will not allow the bullet to expand consistantly, at least to my way of thinking compared to a higher RPM impact where the bullet will be far more stable as it impacts and penetrates through the animal.

Again, this is more theory on my part then anything and in most cases its not practical to do actual testing at true long range so this may be the only way to do this other then to get a faster twist barrel to match the RPM levels which again, just is not practical for us to do financially.

 

[Michael Eichele]

Thank you for the graphic.

Would you please explain the concept of 'stability factor'?

Stability factor is a number calculated by formulas to determine the optimum twist for our bullets or optimum bullets for our twist. A factor of under 1.0 is concidered unstable. 1.5 is concidered perfect. Over 1.5 is concidered stable, not neccesarily optimal. Greenhill had a formula but there are much much more accurate formulas used today.

Many things go into calculating the stability factor. Some include barrel twist, bullet demensions (including meplat width, nose style and length, ogive radius, bearing surface, base diameter, boat tail length, and overall length, bullet diameter and weight), velocity, air density and a given bullets "specifec gravity". Such as lead, copper jacketed lead, solid lead, ect...Zinc has a very low specifec gravity and tungsten has a very high specifec gravity. The lower the number, the more it takes to stabilize it. This is why solid copper bullets need a bit more twist than do jacketed lead. Jacketed lead has a SG of 10.7 where solid copper is 8.96.

Obviously some factors above are more critical than others. The 3 most critical elements for the materials we typically use are twist, length and diameter. All others are factors just not as big as those 3.

Calculating them is tricky. This is why most here use ballistic calculators either on a PC or online.

I have been lucky enough to have some loads with a 1.5 SF. I believe they have been my best overall performing loads ever. I have had above published BCs, AND they always seem to drift in the wind less than predicted, minimal spin drift and increadiby accurate from 0-1000 yards. I dont really have any "scientifec" proof that these are better than a 1.0 or a 2.0 but the results in my opinion are enough to convince me that 1.5 really is a perfect number. Anybody will have a very hard time convincing me that any other number is better.

Clear as mud right?

 

[BryanLitz]

I have a program that calculates the stability factor for a bullet based on Don Miller's formula. For those unfamiliar with Don's work, he's been publishing it in PS magazine for years. I believe his stability formula is very accurate, and a huge contribution to exterior ballistics as it updates the old Greenhill formula which is still often used, but incorrect for modern bullets.
Anyone who would like a copy of the program can email me at:
[email protected]
and I will send the program to you in a email attachment.

meichele states above that a stability factor of 1.5 is 'optimal'. I won't argue that, but will point out that it isn't a fixed rule. I've seen stability factors from 1.3 to 1.5 suggested as 'minimal required', meaning optimal. Using a faster twist than that required to achieve an Sg of 1.3 to 1.5 isn't as consequential as many think in terms of 'overstabilization'. You do get more spin drift from a bullet with a higher Sg, but it's no less accurate, and doesn't erode the BC as much as some think.
The worst thing that happens when you spin a bullet too fast is that you exaggerate any dispersion components that are caused by spin. For example, a bullet with a cg offset will have more dispersion if it emerges from the muzzle with greater spin. In this case, the extra dispersion isn't a consequence of higher stability, but simply a byproduct of spinning faster, regardless of Sg. Using well made bullets, and minimizing in-bore yaw will mitigate the dispersion related to spin rate.
-Bryan

 

[royinidaho]

LV,

I know nothing about the two videos I posted. I found them on youtube by searching on "slow motion bullets". That long bullet appears to be a berger???. Put the cursor on the tip and you'll see that the yaw is at least a half a caliber. One might even think the yaw was caused by the paper?????

The bullet passing through the paper appears to be about the correct spin rate for distance travel and appears to almost thread itself through the paper.

The second pic with the gelatin was very revealing to me. The bullet seems to have spun all the way through the gelatin with less velocity but the same spin rate.

 

[RockyMtnMT]

I have a program that calculates the stability factor for a bullet based on Don Miller's formula. For those unfamiliar with Don's work, he's been publishing it in PS magazine for years. I believe his stability formula is very accurate, and a huge contribution to exterior ballistics as it updates the old Greenhill formula which is still often used, but incorrect for modern bullets.
Anyone who would like a copy of the program can email me at:
[email protected]
and I will send the program to you in a email attachment.

meichele states above that a stability factor of 1.5 is 'optimal'. I won't argue that, but will point out that it isn't a fixed rule. I've seen stability factors from 1.3 to 1.5 suggested as 'minimal required', meaning optimal. Using a faster twist than that required to achieve an Sg of 1.3 to 1.5 isn't as consequential as many think in terms of 'overstabilization'. You do get more spin drift from a bullet with a higher Sg, but it's no less accurate, and doesn't erode the BC as much as some think.
The worst thing that happens when you spin a bullet too fast is that you exaggerate any dispersion components that are caused by spin. For example, a bullet with a cg offset will have more dispersion if it emerges from the muzzle with greater spin. In this case, the extra dispersion isn't a consequence of higher stability, but simply a byproduct of spinning faster, regardless of Sg. Using well made bullets, and minimizing in-bore yaw will mitigate the dispersion related to spin rate.
-Bryan

Bryan,

Is this a program that a computer dummy can run? Can I just plug in the needed #'s and hit go? If so, yes I would very much like to have it.

Thank you, Steve

 

[BryanLitz]

Steve,
It's VERY easy to run.
Send me your email address ([email protected]) and I'll fire it off to ya.
-Bryan