240 weatherby twist rate

[moosie]

I am building a custom 240 weatherby mag and want to shoot 85 gr ttsx, 87 gr berger and have the option to shoot 105 gr bergers for big northern whitetails. What twist rate should I go with? my smith suggested 1:9. Is anyone out there shooting this combination of bullets out of a 6mm with success (in terms of accuracy) and with what twist?

 

[Joel Russo]

I have a custom .240 Wby with a 28",9 twist barrel, and I shoot a 105gr bullet at 3380 fps. The load is 52 gr of RL22, which is a max load for this combination. Accuracy is astounding as it has shot numerous .0" groups at 100 yds. The rifle has never had anything but the 105-107 grain bullets through it as it was built as a long range varmint and deer rifle. If you are building your rifle for long range application, then I suggest you use the 9 twist barrel as you will be able to shoot the 87-107 grain bullets. Not sure I would use the Berger on big northern whitetails, but that's your call...

 

[moosie]

Ok. 1:9 it is. Thanks Joel.

 

[Edd]

Berger recommends an 8 inch twist for their 105 gr bullets.

 

[moosie]

Well I've done a lot of reading through various manuals and I would like to hear from shooters with real life experience. There's no substitute for real life experience regardless of what the books say!

 

[Edd]

Well I've done a lot of reading through various manuals and I would like to hear from shooters with real life experience. There's no substitute for real life experience regardless of what the books say!

I reckon so.

After all, what could the people who make them and test them know about the best twist to stabilize them.

 

[Lefty7mmstw]

You don't need quite the twist with the 240 roy in a long barrel at 10% more velocity than the 243. Even in a 24" bbl. 243 win a 105 could barely hit 3 K fps and they will list twist for the slower rounds that can use the bullet. You can back off the twist 10% and still be at the same rpm.

 

[Edd]

When using 3000 fps as the baseline, the Berger stability calculator says you need an additional 3000 fps to compensate for the difference between an 8" and 9" twist.

 

[Lefty7mmstw]

When using 3000 fps as the baseline, the Berger stability calculator says you need an additional 3000 fps to compensate for the difference between an 8" and 9" twist.

twice the fps eh.. had to wake up first on this one as I worked over-night last night

Bullet RPM = MV X 720/Twist Rate (in inches) so a 3000 fps bullet from an 8" twist is spinning 270,000 rpm

a 3300 fps bullet from a 9" twist is spinning 264,000 rpm, or less than 1% difference.

I'll dare you later to get a 105 in a 243 later to 3000 fps even , most likely would be 2900 to 2950, so you are actually talking fewer rpm from the slower cal. with the faster twist....

 

[Edd]

78% more, not quite double.

Using the same Berger calculator.........

3300 fps in a 9" twist gives a stability factor of 1.24
2900 fps in an 8" twist gives a stability factor of 1.50

A minimum of 1.5 is recommended. Seems there may be more to it than RPM. Perhaps higher velocity makes a bullet more unstable.

 

[Kevin Thomas]

Moosie, do you actually believe those books are assembled without "real world experience"?

Berger knows what they're talking about. Go with a 1x8", at a minimum. Dancing too close to the edge otherwise, and that's where the troubles begin.

 

[moosie]

Thanks for the input guys. I Know they recommending a certain twist for a certain bullet. I guess I should rephrase my question, if I go with a 1:8 twist will I be able to shoot both 105's and the 85, 87's with that twist? Or would 1:9 be better? It would be much simpler for me to just choose one bullet weight and build the gun for that one bullet weight, then I could just go by what the manuals recommend. But I really wanna twist that'll shoot both. So what twist will let me shoot both, accurately?

 

[Kevin Thomas]

moosie,

You ALWAYS choose a twist based on the longest/heaviest bullets you plan on using. The lighter, shorter bullets will take care of themselves. You can spin a bullet faster than what's "needed" without any problem. You won't "overstabilize it" (no such thing), you'll merely have a higher Stability Factor than what's actually required to keep it nose on. Going the other way, with insufficient twist and too low an SG is likewise ALWAYS a problem.

My caution here is based on Berger's recommendations, and the fact that the scenarios they calculate their SGs on still have a few wildcards that can come up. I've mentioned before that buttoned barrels can easily (and routinely do) vary by as much as a half inch from their stated twist rate. Faster or slower, purely luck of the draw in the case of your particular barrel. I just like to have some edge for such little bugaboos . . . just in case.

 

[TracySes23]

moosie,

You ALWAYS choose a twist based on the longest/heaviest bullets you plan on using. The lighter, shorter bullets will take care of themselves. You can spin a bullet faster than what's "needed" without any problem. You won't "overstabilize it" (no such thing), you'll merely have a higher Stability Factor than what's actually required to keep it nose on. Going the other way, with insufficient twist and too low an SG is likewise ALWAYS a problem.

I suppose then, we only need to be concerned about, is what RPM they begin to disintegrate at.
Having absolutely no experience in this area, I'm curious if accuracy ever suffers at the extreme RPMs, assuming they don't disintegrate.
I have one 22-250 with a 1 in 14 twist and another with a 1 in 9 twist. I also have nearly 3000, 22 caliber bullets from many years ago when I had an FFL. They vary in weight from 50 gr to 63 gr. All being either Hornady or Sierra. The 50 gr Hornady SX, I can shoot in my 221 Fireball. I should be able to shoot the remainder in my new Savage LRPV with a 1in 9 twist. I assume the 52 & 53 grain Sierra HP will no blow up on exiting the barrel.

I apologize if this is too far off the path. I'll search for another thread if I am.

Spencer