6.5 PRC 156gr Berger EOL twist dilemma

Here's how to calculate bullet rpm.
Hope it helps.

Calculating Bullet RPM from MV and Twist Rate

The lesson here is that you want to use the optimal RPM for each bullet type. So how do you calculate that? Bullet RPM is a function of two factors, barrel twist rate and velocity through the bore. With a given rifling twist rate, the quicker the bullet passes through the rifling, the faster it will be spinning when it leaves the muzzle. To a certain extent, then, if you speed up the bullet, you can use a slower twist rate, and still end up with enough RPM to stabilize the bullet. But you have to know how to calculate RPM so you can maintain sufficient revs.

Bullet RPM Formula

Here is a simple formula for calculating bullet RPM:

MV x (12/twist rate in inches) x 60 = Bullet RPM

Quick Version: MV X 720/Twist Rate = RPM


Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.

Example Two: What about a faster twist rate, say a 1:8″ twist? We know the bullet will be spinning faster than in Example One, but how much faster? Using the formula, this is simple to calculate. Assuming the same MV of 3000 FPS, the bullet makes 12/8 or 1.5 revolutions for each 12″ or one foot it travels in the bore. Accordingly, the RPM is 3000 x (12/8) x 60, or 270,000 RPM.

riflingx142.jpg
Implications for Gun Builders and Reloaders
Calculating the RPM based on twist rate and MV gives us some very important information. Number one, we can tailor the load to decrease velocity just enough to avoid jacket failure and bullet blow-up at excessive RPMs. Number two, knowing how to find bullet RPM helps us compare barrels of different twist rates. Once we find that a bullet is stable at a given RPM, that gives us a "target" to meet or exceed in other barrels with a different twist rate. Although there are other important factors to consider, if you speed up the bullet (i.e. increase MV), you MAY be able to run a slower twist-rate barrel, so long as you maintain the requisite RPM for stabilization and other factors contributing to Gyroscopic Stability are present. In fact, you may need somewhat MORE RPM as you increase velocity, because more speed puts more pressure, a destabilizing force, on the nose of the bullet. You need to compensate for that destabilizing force with somewhat more RPM. But, as a general rule, if you increase velocity you CAN decrease twist rate. What's the benefit? The slower twist-rate barrel may, potentially, be more accurate. And barrel heat and friction may be reduced somewhat.

Just remember that as you reduce twist rate you need to increase velocity, and you may need somewhat MORE RPM than before. (As velocities climb, destabilizing forces increase somewhat, RPM being equal.) There is a formula by Don Miller that can help you calculate how much you can slow down the twist rate as you increase velocity.

CLICK HERE for Miller Formula in Excel Spreadsheet Format

That said, we note that bullet-makers provide a recommended twist rate for their bullets. This is the "safe bet" to achieve stabilization with that bullet, and it may also indicate the twist rate at which the bullet shoots best. Though the RPM number alone does not assure gyroscopic stability, an RPM-based calculation can be very useful. We've seen real world examples where a bullet that needs an 8-twist barrel at 2800 FPS MV, would stabilize in a 9-twist barrel at 3200 FPS MV. Consider these examples.

MV = 2800 FPS
8-Twist RPM = 2800 x (12/8) x 60 = 252,000 RPM

MV = 3200 FPS
9-Twist RPM = 3200 x (12/9) x 60 = 256,000 RPM


Of course max velocity will be limited by case capacity and pressure. You can't switch to a slower twist-rate barrel and maintain RPM if you've already maxed out your MV. But the Miller Formula can help you select an optimal twist rate if you're thinking of running the same bullet in a larger case with more potential velocity.
 
Here's some information from Berger on rpm and over stabilization.

What do the results of the Twist Rate Calculator mean?


If you've used our twist rate stability calculator and found the Specific Gravity (SG) of a given bullet being shot from your rifle under specific environmental conditions, you might be wondering why an SG of 1 is unstable, an SG of 1.5 is recommended, and why SG's in between will not provide you with "optimal results on target."
SG is the gyroscopic stability factor. It is the measure of gyroscopic stability applied to the bullet by spin. There are 3 ranges on the table below: unstable, marginal stability, and comfortable stability.

twist rate unstable
Unstable: SG is less than 1.0. Bullet is unstable. Other indicators are bullet "key holing" (bullet going sideways through target instead of point forward. And extreme inaccuracy of bullet at all ranges.

twist rate marginally stable
Marginal Stability: SG IS 1.0 TO 1.49. Note the box above the line graph: Your bullet is marginally stable. This does not mean that the bullet will be unstable in its flight, in fact you may be able to shoot good groups under these conditions. However, it does mean that the bullet Ballistic Coefficient (BC) will not be optimized. This information can be found on the top right area of the example above. It first shows the BC the bullet would have when stable, then what the adjusted BC would be for the given twist rate, the percentage that the bullet BC is compromised, and then the minimum twist rate we recommend for you to get optimal results with the given bullet, load, and environmental conditions.
At short ranges this effect on the BC will usually be too small to notice. At mid-ranges out to 600 yards you'll begin to see the effects. At long range (600 yards plus) your groups will grow even more. That being said, in most cases a marginally stable bullet will not tumble and will be flying point forward to the end of its flight.
To see just how big the effect on BC can be in your case, visit the Berger Ballistics Calculator then fill in all the required parameters with your data. First, calculate your range card using the full BC of the bullet you are using. Then calculate your range card using the compromised BC value you got from the twist rate calculator. Now compare the drop at various distances. This gives you a rough idea of how much the reduction in BC will affect your shooting experience.

twist rate example
Comfortable Stability: An SG of 1.5 or greater ensures adequate stability and optimal bullet BC.
Here is why the 1.5 number is used: "Typically, it's wise to aim for an SG of at least 1.5 when selecting a barrel twist for a particular bullet. Technically speaking, SG only has to be greater than 1.0, so why aim for the higher value? There are a couple of reasons. The more important reason is to have that extra .05 of SG as a safety margin. If you aim for an SG of 1.0 and conditions exist that cause it to drop to 0.99, you have a problem. The 0.5 safety margin in SG is there to account for non-standard atmospheric conditions, imperfectly balanced bullets, and errors in the prediction of the SG value itself". – Bryan Litz, Applied Ballistics for Long Range Shooting Third Edition (pg 160).

Is an SG greater than 1.5 better? Is a faster than needed barrel twist rate better yet for stability and accuracy? Let me quote again from Bryans book "the bullet does not experience significantly more drag if it is flying with excessive stability" Does that mean that there is no downside to over spinning bullets ? Not exactly. The relationship between stability and precision is probably where the bulk of stability misconceptions exist. Gyroscopic stability can affect precision in two ways. First if the bullet is not adequately stabilized it will emerge from the muzzle and fly with some significant amount of yaw until it stabilizes. This situation is bad for precision and adds significantly to the bullets drag. This problem can be solved simply by insuring that the bullet is fired with sufficient stability by choosing a proper twist rate.

"It's also possible for precision to be compromised if the bullet is spinning faster than it has to be for adequate stability. When a bullet emerges from the muzzle of a rifle, it's spinning very fast. Any imperfection in the shape, balance, or alignment of the bullet will cause it to disperse away from the bore line when it exits the muzzle. The amount of dispersion is related to how severe the imperfections in the bullet are, and also how fast the bullet is spinning. Higher spin rate produce more dispersion. This situation can create the illusion that the bullet's dispersion is caused by excessive stability, but that's not the reality. The actual stability level of the bullet is not what causes the dispersion. The imperfections in the bullet cause dispersion, and the dispersion is increased the faster the bullet is spinning. The more balanced the bullet, the less dispersion will result from spinning them faster. One of the reasons why short range benchrest shooters choose to shoot short, blunt, flat base bullets is because they require such a slow twist rate to stabilize". – Bryan Litz, Applied Ballistics for Long Range Shooting Third Edition (pgs. 167-68).
Let me add one more thing to this discussion. Excessive barrel twist rates by themselves or when combined with a long barrel (over 28 inches) and other factors can also cause bullet failures due to heat and friction. Especially when the bullet Rotations Per Minute (RPM) get close to or exceed 300,000 RPM. The formula to find what the bullet RPM is: Muzzle Velocity times 720 = X. X divided by your barrel twist rate = BULLET RPM.

This article has been a summarized version of bullet stability and only covers the gyroscopic part of this subject. There is also a dynamic part that extended range shooters (beyond 1,000 yards) should be familiar with. For more information on this subject, see Chapter 11 of the book, Applied Ballistics for Long Range Shooting Third Edition by Bryan Litz
 
Thx for your time drangoon300,
nailed it !

there is a reason that the twist in a 300 maxxy mag cal of your choice has a 1in11 or 1in 10 for 200+ gr bullets and a blackout barrel is a 1 in 8 or 1in 7 tw for the same 190 - 210 gr bullets
 
Order the 7.5 it should work for your description.
I have their gain twist in 1:8.35 to 1:7.35 5R Lefthand rotation that really works for my 6.5 PRC with 153.5/ 156 Bergers. I am sold on Bartlien and gain twist Lefthand rotation for the Northern hemisphere and the 5R rifling. I have not anything bad to report, great accuracy!
Wow.... that just took my brain to a whole new level.
Now maybe I can use this knowledge to really upset guys on Archerytalk. Some of them can't even understand that if your bow wants to spin a bare arrow shaft left handed, that you dont want to put right hand fletching on it. It then has to try to spin left and then stop spinning and then spin back to the right.
Now I can argue that left hand twist on arrows is better in the northern hemispere. ;)
 
Order the 7.5 it should work for your description.
I have their gain twist in 1:8.35 to 1:7.35 5R Lefthand rotation that really works for my 6.5 PRC with 153.5/ 156 Bergers. I am sold on Bartlien and gain twist Lefthand rotation for the Northern hemisphere and the 5R rifling. I have not anything bad to report, great accuracy!
So would that work for 143s and 147s too?
What about a 90 grain or 123gr?

Probabaly going to be trying to rebarrel a 6.5PRC soon. Maybe I just need a heavy barrel and a light barrel.:eek:;)🤣
 
Dragoon nailed it.

The only thing that I would add is generally try not to exceed 300,000 RPM with conventional bullets. Depending on the rifling style, number of grooves, or condition of the bore it can lead to bullets blowing up in flight.

Heavy Mono's love the high RPM's
 
Agree with 7 RUM, the Bartlein 3B is a little heavy in a carry hunting rifle for my taste. I think you made the right choice. I actually put a 2B, 8 twist on my Tikka 6.5 PRC and it shoots the Hornady 147 ELD-M bullets very well.
 
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