Velocities and twist

[SWRS]

Seeking a technical question regarding twist and it effects on pressure and velocity. Consider the following:

22CM in a 24in barrel shooting a 60gr bullet in a 8 twist and a 12 twist. Same powder charge. Will one be faster than the other? Would their highest powder charge be different to make the same max pressure?
Help me break this down, the more technical science language, the better.

 

[DLT]

I have wondered the same. Should be a interesting thread

 

[TAJ45]

I do not know the answer to your question.
I do have a gut feeling that the 8 tw will show throat wear sooner.
Brian Litz has pretty well negated the idea of significant velocity differences. Single digit variation.

 

[Yote Hunter]

I do not know the answer to your question.
I do have a gut feeling that the 8 tw will show throat wear sooner.
Brian Litz has pretty well negated the idea of significant velocity differences. Single digit variation.

I also remember Litz had an article testing pressure and velocity differences between a slow and fast twist barrels and from what I remember there was virtually little to no difference

 

[wildcatter]

I think the difference in slower twist working is more obvious in altitude than fps.

 

[357Mag]

Seeking a technical question regarding twist and it effects on pressure and velocity. Consider the following:

22CM in a 24in barrel shooting a 60gr bullet in a 8 twist and a 12 twist. Same powder charge. Will one be faster than the other? Would their highest powder charge be different to make the same max pressure?
Help me break this down, the more technical science language, the better.

SWRS -

Howdy !

IMHO - The larger consideration is the rifle's " expansion ratio ".

Ballistician Homer Powley gave this simple definition for " expansion ration " : total gun volumne divided by case volumne ". Powley also tells us that expansion ratio ( ER ) can be used when comparing chamberings, to get a better idea of how efficiently the chosen configuration can convert chemical energy of the "burning powder" into useful velocity. Guns that have lower expansion ratios are less efficient that those with higher ERs.

For a given calibre and barrel length... the way to change a rifle's expansion ratio is to change the chamber size.

For a given calibre and chamber size.... the way to change a rifle's expansion ratio is to chamge the barrel
length.

Using this guidance, we can see that the longer-barrelled version of a given calibre / chambering will result in a higher rifle' expansion ratio.

Comparing two rifles w/ the same chambering and shooting the same load but in 2 different barrel lengths
will reveal ( in most instances ) an observable velocity increaase when the longer barrel configurartion is chosen.

Changes in the rifle's ER will then likely result in the opportunity to change powder selection(s).
For example: a somewhat slower " burn rate " powder might be a better fit for a 28" barrel, than it would be for a 24" rifle w/ the same chambering; because the longer barrel length allows increased opportunity for powder combustion gases to " expand ".

Along with this.... the slower" burn rate " powder could facilitate higher " load densities "; while still maintaining safe load pressures.

Powley did also speak to potential throat errosion, when he said about internal ballistic and powder combustion: " ... High temperature is high pressure. It's as simple as that " .

As regards throat errosion..... since rifle powder is oft times not all " burned " until a few inches beyond the chamber, some throat errosion can be caused by un-burnt powder ( tubular for example ) " kernels " being forcefully propelled through the barrel's " leade " / throat.


Lastly-
Classic Expansion Ratio computation is made independent of things like bullet wt and barrel' twist rate.


With regards,
357Mag

 

[SWRS]

Given this information, I now have a new inquiry. This involves bullet construction and bullet spin, since we have concluded that twist rate has minimal bearing on velocity, consider this:

Same bullet traveling at the same speed, one from a 12 twist, one from an 8 twist. Which bullet will have the deepest penetration before fragmenting? (talking fragmenting bullets, not bonded or solids)

 

[357Mag]

Given this information, I now have a new inquiry. This involves bullet construction and bullet spin, since we have concluded that twist rate has minimal bearing on velocity, consider this:

Same bullet traveling at the same speed, one from a 12 twist, one from an 8 twist. Which bullet will have the deepest penetration before fragmenting? (talking fragmenting bullets, not bonded or solids)

SWRS -

Howdy !

There has been discussion about the topic of your question, before....
Let's use varmint shooting as the background for our discussion....

Some varmint shooters are thouroughly convinced the rate of spin on the bullet imparts an extra messure ot lethality to the terminal effects equation. One individual making the extra effort needed to try an better quantify the effects of spin rate on bullet fragmentation.

The tester was able to induce bullet fragmentation consistently enough to where he was able to show spiral fragmentation patterns seen target paper.

None-the-less.... the tester was not able to say what percentage of the original bullet wt was represented by the multiple holes in the paper. MORE to the point.... the tester had no means of determening how much of the intact bullet's kinetic energy was delivered by those X number of fragments. This basic challenge to the whole testing exercise would extend to potentially lethal shots taken on live varmint.
The basic question that endures: how much energy ends up being deliverd by all the the bullet fragments retained within the carcass.... and that questions assumes all bullet fragments are retained. Lacking an answer to that last question, there is no practical way to answer the energy per each fragment question.
See more, below....

That leads us to the penetration question:
You mention individual bullet design, and then of course should also be considered its construction.
IMHO - Given the wide variety of bullet calibres / weights / " styles " / construction et al, the " Which bullet will have the deepest penetration " question cannot be reliably answered.... with regard to the twist it is shot from. Quite simply... because muzzle velocity imparted to the bullet has not also been considered.
Varmint rifles ( and others ) plus their cartridges, are principally designed to be able to deliver lethal energy into a target ( animal ) at distance. Muzzle velocity imparted to a specific bullet weight, form the cornerstone
of the delivered energy equation. Only a relatively small percentage of the available energy is consumed by the process of imparting spin to the bullet.

And for simplicity sake, most discussions taking place on the topic pre-suppose the bullet in question is retained inside the animal carcass. In other words.... the energy dump from the bullet is imparted into the carcass, and no " shoot through " takes place. Again, that is an assumption; for simplifying the discussion.

My point:
It'd be pretty tough to provide empirical evidence that would prove one twist rate to be more devestating by imparting an additional lethal effect than would another. More likely, experienced shooters will provide circumstancial evidence " based on " splat patterns " exhibited by things like PD's .

Similar challenges exist if attemtping to quantify bullet penetration in the target animal.
How would this be done for say.... groundhogs ? 10 same wt live animals are shot in the exact same spot at the exact same distance; using the same cartridge / bullet / load... but shot from different twist rate rifles ?
Could such a test be practically performed using 10 same wt already dead but not otherwise damaged groundhogs ? How could that be accomplished ?

A thought:
If you have cartridge / rifle combo that has proven lethal for the ( varmint ) animal you are shooting, at what reduced velocity does lethality become unreliable ? At some point, excessively low Mv will introduce unreliabilty in terms of terminal effects. Alternately, the distance the " load " ( bullet ) is tested at can also introduce unreliability in its terminal effects.

- Make an informed choice when it comes to catridge and especially ( varmint ) bullet selection.

- Make sensible decisions when it comes to practical distances to apply the chosen bullet to the varmints you are going after.

- Make it a point to not start out w/ a combo that is already behind the curve on usefull energy, or in other words... unecessarily constricts the ranges you can apply a ballistics solution to the varminting equation.

-Use enough energy along w/ that well-consructed ( varmint ) bullet. This might even result in some circumstances where you are applying an over-abundance of KE to the target animal.

Do these things..... and spin rate will not loom quite as large, as regards bullet lethality.
But ...in regards to bullet stability, rifle barrel twist rate is of course a big consideration.

Hope this helps ?!


With regards,
357Mag

 

[1lessdog]

I have two 6MM AI one is 1 - 10 and other is 1 - 14 and shooting 55 gr Sierra Blitzking and the 1 - 14 is over 100 fps faster than 1 - 10 twist and both have 26 inch barrel. The only difference is 1 - 10 is a Krieger and 1 - 14 is a Brux.

 

[David Milisock]

Seeking a technical question regarding twist and it effects on pressure and velocity. Consider the following:

22CM in a 24in barrel shooting a 60gr bullet in a 8 twist and a 12 twist. Same powder charge. Will one be faster than the other? Would their highest powder charge be different to make the same max pressure?
Help me break this down, the more technical science language, the better.

Two barrels with identical twist can and many times do exhibit different velocities with the same powder charge.

The logic certainly falls that the faster twist rate (with all other things equal) would produce more friction and therefore more pressure.

Faster twist rates (with bullets of equal caliber) allow you to shoot bullets of higher sectional density and higher ballistic coefficient at slower speeds.

Slower twist rates allow you (within the limits of pressure, required stabilization and case capacity) to push a more fragile projectile faster. Reducing time of flight, ergo, drop, wind drift and gyroscopic drift. It also improves expansion performance.

In the book the heavier bullet produces better numbers and on the with those experience with the range conditions most times better groups, but field experience will be different.

I have a Howa 1500 in 223, 1 in 12 twist, at 350 yards pushing 40 grain VMAX at 3,900 FPS, it nearly removes the head of a chuck, time entrance acserious mess on the other side. Swopping out the barrel for a 1 in 8 twist will not improve that.

Would the barrel swop affect velocity? Sure but it would change if I stopped for anotherv1 in 12.

 

[wildcatter]

I have had long talks on both speed and accuracy talks concerning twist rates with Randy Robinet! A man consider well educated on the performance of projectiles!

This came about when I inquired buying his 104 grain 6mm bullets for a 6mm Rem. AI rifle I was building with a 1-8 twist. When his first remark was "I am not sure you would be happy with them" I was stunned! Then he continued to tell me, he wasn't even sure they would make the trip?

His concern was as has been mentioned, 1-8 stress at velocities above 3300 fps, along with the heat it would produce. as the conversation carried on, for more reasons than the speed!

The more rpm's created, does not suggest fps. as a 1-14 twist at 3600 fps, encounters one half the rpms that a 1-7 twist generates at 3600 fps.

Then barrel travel is also increased by a significant distance as it revolves twice as many times in the 28" barrel I was going with, if I were to use the 1-7 just for ease of computing the math.

This all boiled down to not just stress that the rifling's used to create the spin and produce these ridiculous "RPM's" but as those rpms increased, the distance the bullet encountered the surface area of the barrel, and this also increased the heat it generated on the jacket which transferred to the core?

Yes he was getting very technical, and actually mostly just enforcing the obvious to me. But with the heat now he continued that a 55 grain 6mm, which is aprox, 1/2 the surface area of a 108 grain 6mm bullet, also had to be considered in producing more of all the contributing factors a bullet has to overcome not just to make the trip, but to retain its ability to perform accurately!

Trust me This is just touching on the points that were spoken, as well as the difference in Jacket material, that different bullets are made from. Not just the alloy, but the thickness as well, then how much stress difference between a 3 or 4 groove, to a 5 or 6 groove spinning the same projectile?

I am no authority, and had to more than once ask him to speak in less technical terms, as anyone who has held these kinds of conversations with Randy knows, how patinate, and well educated he is on this subject, so I will stop at this and just give some food for thought on the subject not being an authority myself.

What I can tell others is,,, yes I was able to spin the 108's to over 3300 fps with both IMR-7828, and H-4831. I also was able to get under .250" MOA accuracy with it. But, not anywhere near the 3300+ mark. After 2 months, and burning a more than a couple pounds of powder and trying everything I could, right around 3050 to 3100 fps was where these bullets shot best.

I hear people all the time tell others my rifle shoots 62 grain XX great with a 1-8 twist barrel and so do 105s, for example. But I have to think, their idea of great may be settling for less than optimum accuracy capability of a bullet.

I also think as Fred Sinclair told me decades ago, your best accuracy "potential" will always come when the bullet is spinning just fast enough to stabilize it! Over spinning it will not let it shoot to it's maximum potential.