ELR ballistics (AB) vs barrel twist ??

[MRogersII]

Looking at different twist options for a new barrel. I entered appropriate data into AB app to obtain trajectory tables for a specific load. Then used the exact same data to setup a 2nd configuration with the only difference being the barrel twist (9 vs 10).

Both twist rates yielded the same ballistic solution. I would have expected twist rate (and, subsequently, rpm) to have affected the ballistics, but it did not.

What am I missing here?

 

[dkhunt14]

Looking at different twist options for a new barrel. I entered appropriate data into AB app to obtain trajectory tables for a specific load. Then used the exact same data to setup a 2nd configuration with the only difference being the barrel twist (9 vs 10).

Both twist rates yielded the same ballistic solution. I would have expected twist rate (and, subsequently, rpm) to have affected the ballistics, but it did not.

What am I missing here?

Are you sure you Put in the right numbers? I ran some in Bergers and if I changed velocity, twist or bullet, it changed it some. At least it did on mine. Sometimes it wasn't much but it changed. Matt

 

[MRogersII]

Are you sure you Put in the right numbers? I ran some in Bergers and if I changed velocity, twist or bullet, it changed it some. Matt

I ran the same bullet, velocity and rifle settings, but the two twists yielded the same ballistic table. I think I did it twice just to be sure. I thought perhaps I was missing something and that AB did not adjust for rpm.
Michael

 

[MRogersII]

I ran the same bullet, velocity and rifle settings, but the two twists yielded the same ballistic table. I think I did it twice just to be sure. I thought perhaps I was missing something and that AB did not adjust for rpm.
Michael

I will try it again though.

 

[MRogersII]

I will try it again though.

Just tried it again, all same except barrel twist and it yielded the same data table.

 

[VaRandy]

I
will give it a try. You had the same ballistics arc for two different twists at the same speed. At some point, such as maybe less distance and definitely more distance, the difference is twist WILL show.If your are using the heaviest of the bullets it will show at the furtherest distances for sure.

 

[MRogersII]

I would have thought that as the angular velocity increases, so also would the bullet's tendancy to "right itself", to use very crude terminology. Perhaps that is not the case. Or, perhaps it occurs on a scale that is not easily measured within the confines of the data used. Or, maybe I just need to go into the settings and turn on a switch whose default setting is "off". That would be easier.

 

[Ballisticboy]

Changing the twist rate should mainly change the spin drift and the vertical effect of a cross wind. It will change the yawing behaviour of a bullet, too little twist and the bullet will continue to yaw and too much twist will make the bullet slow to correct yaw and may produce dynamic instabilities at long range. Just right will make the bullet yaw damp out quickly and delay dynamic instabilities.
To see the effects of spin rate on yaw you would need a six degree of freedom model and all the data associated with it which will not generally be available. A four degree of freedom or modified point mass model should show up the spin drift and may give the vertical cross wind effect. A point mass model will not show any effect.

 

[6ShotsOr5?]

Changing the twist rate should mainly change the spin drift and the vertical effect of a cross wind. It will change the yawing behaviour of a bullet, too little twist and the bullet will continue to yaw and too much twist will make the bullet slow to correct yaw and may produce dynamic instabilities at long range. Just right will make the bullet yaw damp out quickly and delay dynamic instabilities.
To see the effects of spin rate on yaw you would need a six degree of freedom model and all the data associated with it which will not generally be available. A four degree of freedom or modified point mass model should show up the spin drift and may give the vertical cross wind effect. A point mass model will not show any effect.

I agree with BallisticBoy. I don’t think the 4DOF solution will change much, except pay attention to the calculated Miller S.I. and make sure you are > 1.5 with the bullets, MVs, and density altitude conditions you will be shooting in. If your stability is too low, your groups will open up.

 

[dcali]

Twist is not a terribly important effect in external ballistics. It's main purpose is to stabilize the bullet, and generally speaking, bullets are either stable or they're not. There are some benefits to being a "little more stable" than absolutely required, but that's a small effect and it has the effect of slightly reducing drag due to bullet yaw.

Because of that fact, most calculators simply ignore spin, and assume yaw drag is rolled into the BC. Some of the more interesting ones like AB account for rotation in specific ways and add them back in to the solution, but those will not have any impact on the basic wind deflection or elevation calculations.

 

[Bart B.]

Damoncali gave a good response.

I don't think any software calculates the increased drag out of balance bullets have. The faster they spin, the more they'll yaw. More yaw means more drag.

One of Sierra's earlier manuals had a table listing velocities bullets had entering a 50 yard range band. Their velocities leaving that 50 yard span were not proportional to their entry velocity. They concluded those loosing more speed had enough imbalance and yaw to slow them down a little.

Velocity losses were calculated to BC numbers showing a 1% spread in BC for a lot of match bullets was normal. Hunting and lesser quality bullets had more spread in BC. They lost velocity at a faster rate.

 

[chromatica]

Negatori damoncali, twist is important to external ballistics. Ballisticboy has it right. Bullets spinning faster have a higher Miller Stability and because of that they are more subject to spindrift. That will show up in your drift column.

Crosswinds give rise to aerodynamic jump which causes an initial vertical deflection - upward for wind from the right, downward for wind from the left. The amount of this deflection is a function of Miller Stability. You should see these differences in the drop column.

MRogersII, if you are not seeing any difference, something is wrong with your simulation. Why don't you give us all of your input parameters and we will duplicate your scenario.

 

[SilverEagle]

I may have passed over this in the previous comment. I would not pay too much attention to the gyroscopic stability at the muzzle, but more importantly to the stability at the target.
My ELR rifle is in .300 win mag, and I’m running a 1-8” twist barrel as opposed to the more accepted 1-10” twist. Yes it gives me more gyroscopic spin drift, but I feel that that is a small trade off for bullet stability

 

[284winner]

I may have passed over this in the previous comment. I would not pay too much attention to the gyroscopic stability at the muzzle, but more importantly to the stability at the target.
My ELR rifle is in .300 win mag, and I’m running a 1-8” twist barrel as opposed to the more accepted 1-10” twist. Yes it gives me more gyroscopic spin drift, but I feel that that is a small trade off for bullet stability

For the 230/240 bullets, you would do better in a 9 than a 10 twist barrel. Your 8 twist isn't off for the heavy bullets for ELR in your 300 Win mag. You'll lose a bit of speed and again some rpm but stability will be strong.

 

[D.Stone]

I do not think the AB calculator adjusts for stability.

 

[Mr.Knowit all]

Looking at different twist options for a new barrel. I entered appropriate data into AB app to obtain trajectory tables for a specific load. Then used the exact same data to setup a 2nd configuration with the only difference being the barrel twist (9 vs 10).

Both twist rates yielded the same ballistic solution. I would have expected twist rate (and, subsequently, rpm) to have affected the ballistics, but it did not.

What am I missing here?

How far was your solution set up for? You won't notice any spin inside 5 or 600 yards between the 9 and 10 twist.

 

[Bart B.]

Changing the twist rate should mainly change the spin drift and the vertical effect of a cross wind. It will change the yawing behaviour of a bullet, too little twist and the bullet will continue to yaw and too much twist will make the bullet slow to correct yaw and may produce dynamic instabilities at long range. Just right will make the bullet yaw damp out quickly and delay dynamic instabilities.
To see the effects of spin rate on yaw you would need a six degree of freedom model and all the data associated with it which will not generally be available. A four degree of freedom or modified point mass model should show up the spin drift and may give the vertical cross wind effect. A point mass model will not show any effect.

Here's an interesting "twist"

 

[Ballisticboy]

Here's an interesting "twist"

That looks like an empirical relationship created to match measured values. It would be interesting to know how the measured values were obtained and analysed. In real life the spin decay is related to the skin friction drag, the bullet inertia and any surface inperfections which may increase spin damping.

 

[Bart B.]

That looks like an empirical relationship created to match measured values. It would be interesting to know how the measured values were obtained and analysed. In real life the spin decay is related to the skin friction drag, the bullet inertia and any surface inperfections which may increase spin damping.

I agree that bullet skin friction drag can influence bullet spin decay. It can also influence BC. Groove angle, width, depth and count all influence skin friction. Geoffrey Kolbe of Border Barrels developed it.

It's not part of any exterior ballistics formulas I know of. How would the downrange spin rate be measured precisely? Wind speed varies with terrain and height above ground and it can be measured. There's formulas to calculate it but they're not part of bullet trajectory software calculation for wind drift.

 

[Longrange57]

Here's an interesting "twist"
View attachment 1084847

Bart B...I know you have plenty of knowledge...Old thread...just a few questions and observations.
The above spin decay chart would be a little suspicious. 17.9% rpm loss on the bullet in 1.7 sec seems to a bit much. There is another bit of info floating around as well about an Aberdeen Proving Ground test where a .308 with 146 gr and muzzle vel of 2750 fps lost 25% in 1,000 yards. I've also seen other numbers posted from the Geoffery Kolbey formula. 1st question, where is the real data to backup these claims?? If the lower value at 17.9% in the 1st 1,000 yds is correct....what is the next 1,000 yards.....and the next 1,000 yards? I have a buddy shooting the KO2M with a 9 tw .375 cal.... he is regularly on at 2,500 yd ranges. I have shot this rifle at 1,400 yds and its a laser. Many who compete never get on at 3,500 yds. The ones who do have a flight time of 6.7-6.9 seconds. Many other atmospheric factors come into play other than spin rate which we all know. The rounds go subsonic around 2,600-2,700 on average at about 4.8 seconds. Most shooters have a muzzle velocity of around 3,000 fps. The above data would indicate that from 4-6 seconds most bullets would not have enough spin to be stable and some bullets would likely have no spin at all! Wondering about your thoughts??
Also, is there any data to backup what the real spin decay is? If the decay is as bad as we think then I have many theories why BC is drastically affected past 1,500-2,000 yards.