Bullet weight selection

[BoomFlop]

Speed doesn't change the twist, but it does change the spin/rpm of the bullet after it leaves the barrel.

RPM=MVx720/twist (in inches)

180,000 rpm=2500x720/10
252,000 rpm=3500x720/10

So to spin the bullet the same amount of times, it would require a 7 twist at 2500 fps.

So to answer your question...72,000 times more per minute.

Steve

 

[Deleted member 8840]

So a 140 6.5 bullet out of a 6.5 CM at 2950 fps is just about as stable as a 140 in a 26 nosler at 3300. But a 160 in a CM 8 twist at 2700 will not be stable but will be at 2950 in a 26 nosler with a 7.5 twist. Speed doesn't help much. Yes!

 

[mwkelso]

Boomflop and Muleystocker,

I personally don't know what kind of rev speed it takes to separate a jacket off a bullet. However, I did pencil out a couple scenarios in twist rate versus speed.
I'm pretty confident in the math shown below, but please let me know if you have any questions about it. I could always be wrong...
Comparing it to the equation you listed, the numbers definitely do not match. What does the "720" represent in that equation?

 

[Deleted member 8840]

I built a 7mm 378 weatherby with a 36 inch barrel and tried to push 180 grain bullets at 3600 fps. The first shot was a little over that. The second shot I could not read because the Chrone didn't read it. The reason why there was a small hole in the backside of it. When I took the chronogragh apart I found the base of the jacketed bullet went into the backside of it. I not sure which bullet it was but the jacket blew up before it got to the chronograph. The Accubond and solid cooper bullets were the only ones that stayed together. So not all thing go the right way.

 

[mwkelso]

I built a 7mm 378 weatherby with a 36 inch barrel and tried to push 180 grain bullets at 3600 fps. The first shot was a little over that. The second shot I could not read because the Chrone didn't read it. The reason why there was a small hole in the backside of it. When I took the chronogragh apart I found the base of the jacketed bullet went into the backside of it. I not sure which bullet it was but the jacket blew up before it got to the chronograph. The Accubond and solid cooper bullets were the only ones that stayed together. So not all thing go the right way.

What was the barrel twist rate?
And do you think a slower twist would have prevented that?
I had read somewhere in this site that barrel/powder temperature can affect the jacket on cup and core style bullets. I wonder if the extra barrel length got the bullet hot enough to soften the jacket too much... I'll see if I can find the thread.

You're right, not all things go the right way.

 

[muleystalker]

Here is an example of how faster velocity for a given bullet doesn't have that big of affect on the required twist for that bullet as most people think. The parameters for this sample are all the same except the velocity was changed. A change in altitude has way more affect than adding velocity. There is only a 0.05 difference in SG for a 400fps change in velocity.

Bullet:100gr .243 Badlands Bulldozer
BC:G1 0.575
Length: 1.430"
Twist: 1:7.5"
Temp: 59*
Altitude:0'
velocity; 3000fps
SG (gyroscopic stability) is a 1.07 Which is right at the unstable to marginal on Berger's barrel twist calculator.

Using the same above bullet and all the above perimeters only changing the velocity to
3400fps you get an SG of 1.12 , that is only a 0.05 change on the stability scale for a 400fps change in velocity.

For reference the SG scale is:

0------------------1.0|1.0-------------------1.5| 1.5---------------------greater
------ unstable------|------marginal-----------|-----------stable---------------

 

[muleystalker]

So a 140 6.5 bullet out of a 6.5 CM at 2950 fps is just about as stable as a 140 in a 26 nosler at 3300. But a 160 in a CM 8 twist at 2700 will not be stable but will be at 2950 in a 26 nosler with a 7.5 twist. Speed doesn't help much. Yes!

Its not so much the weight of the bullet but more the design/length of the bullet, it just so happens that added length is a byproduct of making a heavier bullet for a given caliber, unless you change the design such as going from a VLD to a round nose. In your example there are 6.5 160gr round nose bullets that will stabilize out of a 1;8 twist at 2700fps.

 

[Deleted member 8840]

It was a 9 twist Benchmark barrel and after talking to them about it we came to the decision that the barrel was just to long. The jacket was not holding up to the heat that it was making by the time it got to the end of the barrel. I shorten the barrel to 28 but didn't do much after that. A project for sometime down the road. Just finished a 338 excalibur with a 32 inch barrel pushing 245 grain bullets at 3400.

 

[muleystalker]

I have had best results with 5r barrels, it's seems most the times I have seen or talked to people that had bullets blowing up it was with traditional 6 and 4 grooved rifling. Not to say a 5r can't blow up bullets.

 

[Dirtrax]

S

Would you mind explaining the three-legged stool for bullet selection?

Sure...one selects either a bullet weight based on twist or a twist based upon bullet weight desired. Then you factor in a reasonable velocity achievable with the chosen cartridge. Assuming you have an idea what altitude and environmental conditions you will likely experience you do the math. Will this work or not? I don't mean to be crass but compromises and choices have to be made reflecting real world circumstances.

 

[RockyMtnMT]

The longer for caliber bullets get the less accurate the stability calculations get. This is my experience.

 

[mwkelso]

S


Sure...one selects either a bullet weight based on twist or a twist based upon bullet weight desired. Then you factor in a reasonable velocity achievable with the chosen cartridge. Assuming you have an idea what altitude and environmental conditions you will likely experience you do the math. Will this work or not? I don't mean to be crass but compromises and choices have to be made reflecting real world circumstances.

That works, makes sense. And not received as being crass. I would prefer an honest, common sense answer over a "here is your perfect world scenario" answer.
I think a lot of the angst and confusion that myself, along with every other shooter new to LR and studying ballistics in general, is that we over-analyze EVERYTHING. I've heard it referred to as analysis paralysis.

 

[doubledoc]

What of the Greenhill formula? Barrel twist rates have always depended on the lenght of the projectile, more than any other factor. Look at the twist rate for a .44 Mag. slug!

 

[Dirtrax]

Monolithic bullets throw a monkey wrench into the equation. Much longer bearing surfaces and quite frankly the length over all changes things. I'm close to ordering a batch of Hammer Hunters just for sh*ts and giggles.