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Old 09-18-2017, 07:07 PM
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Default Velocity vs. Twist Rate

I have found my velocities in a HVA 1640 30-06 exceeding the expected velocities from the reloading manuals by up to 10% with Varget, Win 760 and H4831. My only explanation is the 1 in 12 twist rate as compared to the normal 1 in 10 rate is giving less resistance to the bullet as it passes down the barrel. Note the can of A4350 is now fertilizing my garden as it was previously giving a problem of very low pressures in my 6.5x55. If you see something different please let me know. Thanks

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Old 09-18-2017, 07:33 PM
Nyksta Nyksta is offline
 
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I find that differences in jump to rifling and tightness of the chamber and headspace are a bigger factor than twist rates. And these dimensions are going to vary between every individual rifle. I see about 70 fps between 3 308s all with the exact same load.

44.5 gr Varget 2.800" COAL 165 Accubond 308 Win

2718 fps for tightest chamber with 0.054" jump. 20" barrel 1:10 twist

2732 fps for almost as tight chamber, 0.119" jump, 22" barrel 1:12 twist.

2666 fps for largest chamber, 0.109" jump, 22" barrel 1:10 twist.

Last edited by Nyksta; 09-18-2017 at 07:42 PM.
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Old 09-18-2017, 07:38 PM
Arty Arty is offline
 
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I've found that I get deviation from published specs in most rifle and powder combinations, and in the same rifle after rebarrelling. I'll just look into the specs for powder recommendations, and work up loads from the minimums with stuff I've already got or is locally available. I'll try to peg MV at 2900-3000 fps subject to group size progression and pressure signs.

There's lots of things which will affect velocity between some test rifle and test conditions, and what you're using that day. Twist might have some effect.
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Old 09-19-2017, 06:20 AM
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There are many factors that affect velocity .

Loading manuals are only a guide and some firearms may surpass the listed max load and some may NOT achieve listed maximum load. That is why you should not start at maximum listed load.

The barrels used to pressure test are held to a much tighter standard than production barrels .

Reloading is an interesting hobby and just when you think you have things figured out you will have an incident that will make you wonder ????😏
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Old 09-19-2017, 07:37 AM
bobinthesky bobinthesky is offline
 
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I've never seen evidence where twist affects velocity but if it does then I'd imagine the difference to be very small.
All barrels are different and the published velocities are that of the test barrel, yours may produce a completely different velocity.
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Old 09-19-2017, 09:53 AM
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I remember reading one of the studies done about twist rates and velocity loss/gain. They found that it didn't make hardly any difference! This really surprised me. Kinda like the whole length of barrel debate that goes on all the time. Doesn't make much difference with most cartridges. It makes sense since there is a wide range of twist rates from calibre to calibre. My new 6.5mm barrel uses 1:8 twist. I had a gain twist from Ron Smith that was 1:16-1:8. He told me they didn't really shoot any faster. It was more a novelty thing. One thing i did notice is that it handled pressures better. That initial spike was less noticeable. I'm still breaking i the new barrel and will know what velocity changes there are once that's been done
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Old 09-19-2017, 11:43 AM
Salavee Salavee is offline
 
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I remember reading one of the studies done about twist rates and velocity loss/gain. They found that it didn't make hardly any difference! This really surprised me. Kinda like the whole length of barrel debate that goes on all the time. Doesn't make much difference with most cartridges. It makes sense since there is a wide range of twist rates from calibre to calibre. My new 6.5mm barrel uses 1:8 twist. I had a gain twist from Ron Smith that was 1:16-1:8. He told me they didn't really shoot any faster. It was more a novelty thing. One thing i did notice is that it handled pressures better. That initial spike was less noticeable. I'm still breaking i the new barrel and will know what velocity changes there are once that's been done
Exactly the same scenario here. I have not found any difference in velocities with an RKS gain twist but the difference is notable regarding pressure spikes with certain load combinations.
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Old 09-19-2017, 04:00 PM
cody c cody c is offline
 
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I have a barrel from Ron as well, it's a really interesting science, my understanding of it:


As for the twist rates and pressures, think of it this way, after the projectile engages the rifling it's accelerated to its spin rate, this is a very short distance to the lands in most cases, the energy to accelerate a projectile to a 1:10 twist at 2800 FPS is a 201680 RPM acceleration, in fractions of fractions of a second.

For a 1:12 rate 168000 rpm at 2800fps

A faster twist rate should therefor create more resistance, or make it slightly more difficult to push the projectile through the rifling.

As per the last post, he shows less signs of pressure with a gain twist where the projectile starts at a slower twist, I believe he can therefore load a little hotter to gain a bit more speed, as most of the time the limits are case pressure, but still have the benefits of stability with a faster twist by the time it leaves the bore.

I really think gain twist cut rifling is the way to go especially for reloaders, if I had money I'd own more of them.

RPM= FPS(2850) x 60(seconds) X 1 (for 1:12) or (divided by .833 for 1:10) if your wondering where i got the numbers
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Old 09-19-2017, 05:21 PM
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By staying away from gain-twist, I avoid the concern I have regarding the width of the groove etched into the bullet caused by that change. I expect a very minor change in the rate of twist would be inconsequential, but at some point...it just doesn't make sense.
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Old 09-19-2017, 05:46 PM
Salavee Salavee is offline
 
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Originally Posted by cody c View Post
I have a barrel from Ron as well, it's a really interesting science, my understanding of it:


As for the twist rates and pressures, think of it this way, after the projectile engages the rifling it's accelerated to its spin rate, this is a very short distance to the lands in most cases, the energy to accelerate a projectile to a 1:10 twist at 2800 FPS is a 201680 RPM acceleration, in fractions of fractions of a second.

For a 1:12 rate 168000 rpm at 2800fps

A faster twist rate should therefor create more resistance, or make it slightly more difficult to push the projectile through the rifling.

As per the last post, he shows less signs of pressure with a gain twist where the projectile starts at a slower twist, I believe he can therefore load a little hotter to gain a bit more speed, as most of the time the limits are case pressure, but still have the benefits of stability with a faster twist by the time it leaves the bore.

I really think gain twist cut rifling is the way to go especially for reloaders, if I had money I'd own more of them.

RPM= FPS(2850) x 60(seconds) X 1 (for 1:12) or (divided by .833 for 1:10) if your wondering where i got the numbers
Totally agree .. I have four RKS GT barrels and they are as accurate ,or better than any other standard twist barrels I use.. ie Lilja, Pac-Nor ,Jury and a number of real good Euro factory barrels.
While the bullet engraving theory appears to make sense, I have not seen any evidence of it at any distances I have shot with any of them.
I think when a bullet has been initially engraved it tends to stay on track with the rifling at any normal pressure/acceleration rate. It would be near impossible to cause a bullet "skid" against the engraved rifling ... I think.
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Old 09-19-2017, 06:10 PM
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The width of the groove is the result of the change in rate of twist...not "skidding". Bottom line is the amount of jacket displaced. I suspect if there was a demonstrated advantage to gain-twist, the SBr crowd would embrace it ... it is my understanding that few of the top dogs run GT, and when used, only a very small differential.
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Old 09-19-2017, 06:57 PM
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Originally Posted by 260 Rem View Post
The width of the groove is the result of the change in rate of twist...not "skidding". Bottom line is the amount of jacket displaced. I suspect if there was a demonstrated advantage to gain-twist, the SBr crowd would embrace it ... it is my understanding that few of the top dogs run GT, and when used, only a very small differential.
The only question I have is, has anyone has ever captured a GT engraved bullet in a media of some type?
It should be easy enough to measure the bullets engraving and compare it to the widest land diameter. Until then, I expect it will all be conjecture on our part.
As I mentioned previously, the only difference I can determine is a reduction in initial pressure spikes in some loads. That situation being rather rare, if I had to pay too many additional $$ for a gain twist barrel, I probably wouldn't do it.
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Old 09-19-2017, 07:05 PM
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As to the bullet "skidding ", would there not have to be some lateral movement of the bullet in order enlarge the existing land groove ? Maybe a "skid" is the wrong word but that's all I could come up with, using a two groove barrel as an example.
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Old 09-19-2017, 07:26 PM
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The only question I have is, has anyone has ever captured a GT engraved bullet in a media of some type?
It should be easy enough to measure the bullets engraving and compare it to the widest land diameter. Until then, I expect it will all be conjecture on our part.
As I mentioned previously, the only difference I can determine is a reduction in initial pressure spikes in some loads. That situation being rather rare, if I had to pay too many additional $$ for a gain twist barrel, I probably wouldn't do it.
Groove width is a matter of math ... but easy to imagine what is happening if you think of the engrave "angle" on a bullet that has been through 1:8 and another through 1:16 ...then superimpose. As the angle changes, the groove increases in width, displacing more jacket material.
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Old 09-19-2017, 07:37 PM
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Groove width is a matter of math ... but easy to imagine what is happening if you think of the engrave "angle" on a bullet that has been through 1:8 and another through 1:16 ...then superimpose. As the angle changes, the groove increases in width, displacing more jacket material.
I'm trying to get the pic .260 but all I can see is the bullet's initial land impression following the rifling land ahead of it while it's rotation rate is increasing. RKS GT barrels don't foul much either so that leads me to think there is not a lot of jacket material being displaced. For sure, I'll never figure it all out, but fun trying.
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Old 09-21-2017, 06:58 AM
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By staying away from gain-twist, I avoid the concern I have regarding the width of the groove etched into the bullet caused by that change. I expect a very minor change in the rate of twist would be inconsequential, but at some point...it just doesn't make sense.
Maybe there won't be any any grooves etched in bullet because the sucker has made a couple of revaluations and scrubbed its etchings. Or bullet will sport grooves of the final rifling pitch at the muzzle?
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Old 09-21-2017, 07:02 AM
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If you are seeing 10% higher velocity than listed in the loading data, the most likely explanation is that the load is producing more pressure in your rifle.
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Old 09-21-2017, 09:09 AM
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If you are seeing 10% higher velocity than listed in the loading data, the most likely explanation is that the load is producing more pressure in your rifle.
How is that? There is LESS pressure upon engaging rifling. As the rifling gets to a tighter twist, the gas seal is constantly maintained due to the changing twist rate. If there was higher velocity ( and I'm not saying there is) one would think it would be due to efficiency.
I dont know why you would think gain twist gives higher pressures. That's been talked about already. Load Data in the manuals and the bullet speeds they post are all over the map. I'd never use that info as a benchmark. One thing I'd like to try is 5R rifling in a gain twist.
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Old 09-21-2017, 09:36 AM
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I think velocity variations between barrels is like arguing "break-in" because it is not possible to make a direct comparison due to differences in steel composition, smoothness, bore diameter ...
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Old 09-21-2017, 09:50 AM
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If you are seeing 10% higher velocity than listed in the loading data, the most likely explanation is that the load is producing more pressure in your rifle.
There are no higher pressure signs showing up on the casings.
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Old 09-21-2017, 10:16 AM
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There are no higher pressure signs showing up on the casings.
Given that pressure signs often do not become evident on the cases until well over 70,000 psi, that doesn't prove that the load is not producing more pressure in your rifle. In fact an unusually high velocity is considered a pressure sign by many reloaders. If you are seeing a difference of 300fps, you can pretty much be sure that you are producing more pressure .
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Old 09-21-2017, 10:50 AM
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Given that pressure signs often do not become evident on the cases until well over 70,000 psi, that doesn't prove that the load is not producing more pressure in your rifle. In fact an unusually high velocity is considered a pressure sign by many reloaders.
X2 .. and this is where I have to put in a plug for Quickload and a good Chronograph. With some very simple tweaking of the QL software inputs, pressures and velocities for any specific load combination are easily determined, are very accurate.. and definately safe. One of the best investments a serious handloader can make IMO.
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Old 09-21-2017, 01:25 PM
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What do yous think about listed max loads and then extending the COAL. Do you guys feel that a longer COAL requires a drop from max powder load? What is the relationship? Do you think the listed max load velocity is a good indicator of hitting that listed max pressure?

Example. 308 win 165 gr load with Varget max load is 46.0 grains at 2.800" to get 2780 fps, which is consistent with my rifle speed on my chrony.

If I load 46.0 gr to 2.830" i get 2810 fps.

If I load 45.7 gr and 2.830 i return to 2780.

No noticeable pressure signs at any of these loads.

Do you guys think the pressure is the same if the final velocity is the same, even though they are different combinations.
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Old 09-21-2017, 04:34 PM
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What do yous think about listed max loads and then extending the COAL. Do you guys feel that a longer COAL requires a drop from max powder load? What is the relationship? Do you think the listed max load velocity is a good indicator of hitting that listed max pressure?

Example. 308 win 165 gr load with Varget max load is 46.0 grains at 2.800" to get 2780 fps, which is consistent with my rifle speed on my chrony.

If I load 46.0 gr to 2.830" i get 2810 fps.

If I load 45.7 gr and 2.830 i return to 2780.

No noticeable pressure signs at any of these loads.

Do you guys think the pressure is the same if the final velocity is the same, even though they are different combinations.
That V spread of 30 fps could very well be normal for any of those loads and/or your chronograph may have had a hiccup. Try a longer string of five to eight rnds of each load to get a better idea of you're std deviation. Often, as you approach max pressure/chge weight, velocity gains will plateau but an increase of a 1.7 gr powder charge and an .030 increase in COAL for only a 30 fps overall spread tells me you require more data points to work with.
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Old 09-21-2017, 05:31 PM
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Originally Posted by elkhunter11 View Post
Given that pressure signs often do not become evident on the cases until well over 70,000 psi, that doesn't prove that the load is not producing more pressure in your rifle. In fact an unusually high velocity is considered a pressure sign by many reloaders. If you are seeing a difference of 300fps, you can pretty much be sure that you are producing more pressure .
My calculator which may be rough shows the expected pressure for a .308 dia. 150gr bullet @ 3330 fps to be 76000psi. I would expect some visible pressure signs at that pressure, therefore I don't believe I am producing excessive pressures.
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Old 09-21-2017, 06:50 PM
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That V spread of 30 fps could very well be normal for any of those loads and/or your chronograph may have had a hiccup. Try a longer string of five to eight rnds of each load to get a better idea of you're std deviation. Often, as you approach max pressure/chge weight, velocity gains will plateau but an increase of a 1.7 gr powder charge and an .030 increase in COAL for only a 30 fps overall spread tells me you require more data points to work with.
Its only 0.3 grains difference and it is an average speed after 16 rounds each load

Do you think 0.3 grains and 0.030" seating depth coal trade the same pressure if they both have the same velocity under the seperate combinations.

45.7 and 2.830.
46.0 and 2.800

Last edited by Nyksta; 09-21-2017 at 06:59 PM.
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Old 09-21-2017, 07:41 PM
Salavee Salavee is offline
 
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[QUOTE=Nyksta;3627050]Its only 0.3 grains difference and it is an average speed after 16 rounds each load

Do you think 0.3 grains and 0.030" seating depth coal trade the same pressure if they both have the same velocity under the seperate combinations.

I didn't grasp that those velocities were an average. What were your SD'S? Increased velocity is always the result of increased pressure - originating from something . .. assuming all other factors are identical , those could be case length, neck tensions, temps and such, not to mention unreliable chrony readings.
It seems to me that a .3 gr powder chge increase won't yield much more than a +25 fps V increase with the load you were using - there should never be a decrease, but if your chrony is showing an SD of 20-30 for example, who knows?
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Old 09-21-2017, 07:57 PM
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My calculator which may be rough shows the expected pressure for a .308 dia. 150gr bullet @ 3330 fps to be 76000psi. I would expect some visible pressure signs at that pressure, therefore I don't believe I am producing excessive pressures.

Yes the calculations are rough, that is why the accepted method of estimating pressure is by using special equipment designed for that purpose. You can believe what you want, but if you are gaining 300fps, there has to be more pressure driving that bullet. 50 or 100fps is one thing, but 300fps is very significant
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Old 09-21-2017, 07:58 PM
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further to my above post, here is a quote from a precision rifle blog regarding Std Deviation ..

'If you had a standard deviation of 10 fps for your muzzle velocity, that means 68% of your bullets would exit the muzzle within 10 fps of the average velocity. That is + or – 10 fps, so if your average muzzle velocity was 3,000 fps, then you could expect 68% of your shots to be between 2990 and 3010 fps. Note that the extreme spread of those muzzle velocities would be 20 fps, not 10 fps … because it is always + or – the SD number. We also know 95% of your shots will be within 2 SD’s of your average. So 2 × 10 fps = 20 fps, and again it is + or – that amount. So with an SD of 10 fps and an average of 3,000 fps, you could expect 95% of your shots to have a muzzle velocity between 2980 and 3020 fps. That means you’d have an extreme spread of 40 fps for 95% of your shots. Remember, 5% of your shots would still fall outside of that range, meaning they’d be below 2980 fps or above 3020 fps."
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Old 09-21-2017, 08:50 PM
Salavee Salavee is offline
 
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My calculator which may be rough shows the expected pressure for a .308 dia. 150gr bullet @ 3330 fps to be 76000psi. I would expect some visible pressure signs at that pressure, therefore I don't believe I am producing excessive pressures.
Your calculator was pretty close . A MV of 3350 shows a pressure of 77K using H-414. SAAMI MAP pressure spec for the '06 is 60K.
I would back off a bit or there could be some visible pressure signs showing up soon.
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