Twist recomendations

[bushwackr]

Hi all

I'm kinda stuck here and need some assistance. I'm getting things together for a 6br norma build for my mrs. She wants this rifle to be around type coyote, deer and every now and again go shoot at a match. Well my dilema is there a barrel twist that would allow us to shoot light bullets say 58,87 vmax and larger say 100 sierra sbt so she can deer hunt and some match shooting. Would a 1:9 be about right or....... Barrel will be a cbi heavy sporter, or varmint.

[bushwackr]

I know there is not a magic number but it would be nice if there would be a happy medium. Any suggestions Please.

[bootsmcguire]

My 1:8" twist CBI in 6BR shoots 58's darn well, and 100 Seirra Spitzers do fine, and the 105's I tried did well also. In cases with relatively small powder capacity a faster twist is a safer idea. You may loose a touch of velocity potential with the lighter bullets, but you will still be able to stabilize the heavy stuff, so IMHO the 1:8" seems to be the best compromise for a wide range of bullet weights in the 6BR.

[bushwackr]

Thats good to hear. If we go with a 1:8 then that will cover pretty much anything

[bootsmcguire]

Yup. It sure should in the 6BR. Now if you were talking 6mm Rem or 243 Win or even something larger, then 1:8" would probably start over spinning the really light bullets if you sent them full tilt speed.

[bushwackr]

ok well then what if i try sending them down a 1:8 tube in a 6xc. too much???

[bootsmcguire]

Not sure, I have no experience with the 6XC. What kind of speeds for what bullet are you hoping for?

There is a formula for calculating Revolutions Per Minute of the bullet. You factor in velocity and twist and it figure's RPM's. Many of the bullet companies will list a max RPM or a max speed with min twist so you can use the formula to calculate your parameters.

I will see if I can find where I wrote that formula down at later today.

Perhaps Scope Eye will see this thread, I know he has a fair amount of experience at pushing small bullets until they blow up.

[bushwackr]

well i was hoping to see 3100-3300 with moderate loads with 87 v max. I don't want to fry the barrel, I figured that would be plenty fast enoungh. barrel is 1:8 cbi. I don't know if I could load those close to lands or not haven't went down and picked any up yep.

[Wildboarem]

Jbm ballistics has a barrel twist calculator.
I would think an 1/8 twist in a 6xc would be to fast for the light bullets. You didn't say you wanted to shoot 105gr+ bullets so a 1/8.5 or 1/9 should get you into the lighter bullets as well as being to go up to 100-105 class bullets.

[bushwackr]

ya I mostly want to shoot heavier bullets 100 to 107's. it would be fun if I could play with the lighter ones also

[Wildboarem]

I say 1/8" or at least 1/8.5"

[bushwackr]

Thanks all

[jonbearman]

1 in 8 twist will let you shoot everything except super thin jacketed bullets.

[Savage6x284]

Agree with most here.
Twist it fast and you're golden.
If you start blowing up light bullets throw those lousy bullets in the trash and use something good.

Of course, I twist all my tubes fast.

[darkker]

Don't agree with "lousy" bullets, but here are some numbers for you.
First the math:
MV * 720 / twist = RPM
Sierra has the strongest jackets with their blitz bullets having an RPM limit of 320,000.
Nosler is essentially equal. Hornady premium bullets top at 290,000.

Dogtown, Varmint Nightmare, SXSP, TNT's all top out around 210,000-220,000 rpm
Barrel smoothness will play into the ultimate final number before rotational destruction happens.

Here is a very good calc to determine whether or not you are stabile. Just look up the length, and fill in the blanks.
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

[Savage6x284]

I've had the JBM page in my bookmarks bar for many years now and that is a good app. I hope others use your link and become acquainted with the good stuff offered up at the JBM site.

Personally I'd rather not skirt the edges of instability so I twist them quick and push them hard.

If a bullet cannot survive the trip downrange I consign those bullets to my "lousy bullets shelf" and ultimately give them away to someone who has a use for them.

I'll compromise my ability to use the lightly constructed bullets to gain some ability at the top of the weight range. All pushed as fast as I can within the limits of safety and sanity.

Got any three groove barrels with some miles on them? These can even blow up some pretty stout bullets once some firecracking takes hold. Been pretty stuck on 5R since my foray into three groove barrels ended.

[darkker]

To each his own, BUT
Provided they are stable when they leave, Spin decay happens VERY slowly, whereas velocity decay rapidly. So in terms of stability, bullets will gain stability down-range.

Given the OP mentions coyotes for the varmint portion, people typically can't hit varmints past about 300 yards(not without a bench, etc). So in terms of drop with the cartridge mentioned, a few hundred fps is completely irrelevant.

Thin-jacketed bullets may not fit a given application, or preference. Lousy bullets are typically a lack of understanding on the shooters part.