6.5 CM 7:1 twist

[Musicianized]

I just bought the xbolt hunter 6.5 CM and it has a 7:1 twist 22" barrel. I believe maybe to shoot the newer 156gr bergers.

Has there been an issue with this fast of a twist? I could see issues with lighter bullets. Browning just released it's new Hunter long range, still with a 22" barrel, albeit heavy, they are back to a 8:1 twist.

 

[ApacheCoTodd]

Knowing next to nothing about the round - I'm intrigued as to why it is so fast.

Todd.

 

[LoonWulf]

your good with the 156 eols and that 1-7. A 1-8 would be "marginal"

Any questions regarding better bullets (and useful for others) stability you can use this https://bergerbullets.com/twist-rate-calculator/

I've generally found that bullets that are "marginal" shoot just fine. You may be losing BC tho, which negates the value of the longer, and usually slower and more expensive, bullet.

 

[Allen One1]

I'm not sure it hurts you but it does seem a little fast. The Hornady 143 ELD-X is recommending a 1:8 twist. A 1:7 twist on that bullet won't concern me too much but if you are dropping to a bullet weight of 95-120 it would seem to be a little too fast.

 

[Musicianized]

I just don't want to sacrifice accuracy, I Plan on shooting 143's.

But I have a payment down in the gun and it won't be shipping till the end of the week. But I can still cancel it and get something else.

 

[Gtscotty]

If you're getting the 7 twist from their website, I kind of think that's a typo. Some of their heavier long range series rifles have a 7 twist, but the rest of the hunting weight standard rifles show an 8 twist, even the left hand version of the Hunter. If it actually says 7 twist on the barrel, you've got me. I wouldn't be concerned either way, you're wanting to shoot bullets on the heavier end anyway, and X-Bolts tend to be shooters.

 

[LoonWulf]

So I've heard the theory that for best accuracy, and velocity least with flat base bullets, you want to be as slow a twist as possible while still being fully stabilized.

Personally Ive never had the opportunity to test that theory out. The only time twist rate really concerns me is when it's too slow, or I'm shooting bullets too light for the rifles intended purpose.
In a 6.5CM, the 120 class and probably not the lighter bullets either, won't be torn apart, and i doubt your see much change in accuracy....but as always I could be wrong, all of mine have been 1-8s.

 

[<*(((><]

Faster twist will increase pressures earlier, so one wants to have the slowest twist that will properly stabilize the projectiles being used.

 

[Musicianized]

If you're getting the 7 twist from their website, I kind of think that's a typo. Some of their heavier long range series rifles have a 7 twist, but the rest of the hunting weight standard rifles show an 8 twist, even the left hand version of the Hunter. If it actually says 7 twist on the barrel, you've got me. I wouldn't be concerned either way, you're wanting to shoot bullets on the heavier end anyway, and X-Bolts tend to be shooters.

Maybe it is. I hope so because I want to experiment with lighter weights as well.

 

[Gtscotty]

Maybe it is. I hope so because I want to experiment with lighter weights as well.

We're not talking a big difference like 12 twist vs 7 twist, it's just 7 vs 8, and in all likelyhood, it's probably 8. I wouldn't worry about it, but if you are worried, email Browning and tell them that their page says that the X-Bolt Hunter has a 7 twist while it says the left hand version of the same rifle has an 8 twist and you'd like to know what the correct twist rate is. They've been good about answering questions when I've asked in the past.

 

[taliv]

7 is fine. That’s what Id use. 6 wont hurt you.

 

[redbullitt]

For what it's worth I have a 223 barrel that is 1/6.5. It shoots 80s ,77s, 69s, and 70s just fine.

Unless you're planning on very light or cast bullets, either the 8 or 7 twist is fine!

 

[JFrank]

Faster twist will increase pressures earlier, so one wants to have the slowest twist that will properly stabilize the projectiles being used.

I’m not following nor familiar with that study, could you clarify how a faster twist increases pressure faster or produce supporting documents to substantiate.
I’m very interested,
Thank you
J

 

[adcoch1]

I’m not following nor familiar with that study, could you clarify how a faster twist increases pressure faster or produce supporting documents to substantiate.
I’m very interested,
Thank you
J

Pressure increases as powder burns, and the bullet is trying to flow down the barrel in front of it. The faster twist causes some more time to spin the bullet up to speed in that first inch or two of bullet travel, technically more bullet drag in the barrel, and that tiny difference allows more powder to burn before the bullet gets away from the chamber, increasing pressure momentarily.

 

[JFrank]

Humm
Measurable with some type of software I assume.

 

[<*(((><]

I’m not following nor familiar with that study, could you clarify how a faster twist increases pressure faster or produce supporting documents to substantiate.
I’m very interested,
Thank you
J

Thanks for calling it into question, this is something I’ve always understood to be true as it made logical sense to me.

The difference often times will only be problematic when chasing maximum velocities for a projectile, which in turn one is running against maximum pressures. A pressure spike is often what is trying to be avoided. Probably not something for many to worry about.

I don’t have any scientific studies but an understanding of physics would provide insight that twisting a bullet faster (all other things being equal) will yield more friction thus more back pressure yielding increased chamber pressure , how much is the million dollar question. But there have been lots of barrel manufacturers that play with number of lands, twist rate, gain twist, throating, to yield lower pressures to push a bullet down the barrel so to be able to increase powder charge thus gaining velocity within safe pressures.

But maybe it’s insignificant, but I’ve always followed that rule, as far as matching a twist to a bullet, and not over twisting.

 

[JFrank]

To further the conversation.

I have zero idea or knowledge of how these things are calculated when designing Bullets or twist rate vs pressure on and on but I have seen Bullets keyhole I’n targets also video’s of Bullets being over spun and. blow up in mid air.
I do feel it’s hard to over stabilize a bullet so I kinda lean that direction.

 

[taliv]

depends on the bullet. you can definitely over spin them and blow them up. super thin jackets and fast speeds. some popular bullets were designed to shoot around 3300 and below. shoot them and 3800-4000 and kablooie

but generally, you're right.

the niche solution to that has always been gain twist rifling.

 

[Musicianized]

I would think too much barrel twist on a light bullet at high velocity might shave the jacket off and blow it though the barrel instead of spinning it in the rifling. Or at least damage the bullet, causing tumbling etc.

 

[JFrank]

Long bearing surface and a slow twist can get a Bullet tumbling pretty quickly as well.

 

[Varminterror]

As a rule of thumb, a guy needs to be mindful of their loads any time they’re getting 300-350k+ RPM’s. Some bullets may tear apart even below 300,000 RPM’s, but many more will fail over 350,000. Any time I get over 325,000-330,000, I’m making phone calls to manufacturers or checking forums to be sure someone else has spun that bullet that fast before I buy them.

So working backwards, 300k RPM in a 1:7” twist is 2916fps. Anything under 2900fps should be relatively safe. However, 350k RPM in a 1:7” is 3400fps. A quick review of load data for 6.5 Creed reveals a 95 grain bullet should slip right up under 3,400 fps in a 24” barrel, guessing somewhere around 3,300-3,325 top end in a 22”. So a 95 might blow up in a 22” 1:7” 6.5 creed, but wouldn’t in a 1:8” twist. I’d venture you can’t push most 100 or 120’s fast enough with a creedmoor case - 3100 or less under 325,000rpm.

So for me - I’d be calling manufacturers and asking other shooters before I bought any bullet weight in that 1:7 which data says could break over 3100fps, which is really only stuff lighter than 100grn.

 

[JFrank]

As a rule of thumb, a guy needs to be mindful of their loads any time they’re getting 300-350k+ RPM’s. Some bullets may tear apart even below 300,000 RPM’s, but many more will fail over 350,000. Any time I get over 325,000-330,000, I’m making phone calls to manufacturers or checking forums to be sure someone else has spun that bullet that fast before I buy them.

So working backwards, 300k RPM in a 1:7” twist is 2916fps. Anything under 2900fps should be relatively safe. However, 350k RPM in a 1:7” is 3400fps. A quick review of load data for 6.5 Creed reveals a 95 grain bullet should slip right up under 3,400 fps in a 24” barrel, guessing somewhere around 3,300-3,325 top end in a 22”. So a 95 might blow up in a 22” 1:7” 6.5 creed, but wouldn’t in a 1:8” twist. I’d venture you can’t push most 100 or 120’s fast enough with a creedmoor case - 3100 or less under 325,000rpm.

So for me - I’d be calling manufacturers and asking other shooters before I bought any bullet weight in that 1:7 which data says could break over 3100fps, which is really only stuff lighter than 100grn.

I have know idea if this is factual or fiction, but it sounds good so I’ll take it as written and call it a night....
Shoot Small y’all
J

 

[taliv]

i think that is usually caliber dependent.

 

[Varminterror]

i think that is usually caliber dependent.

Moreso bullet type/model/construction - and it isn’t always intuitive.

Some little varminting 20’s and 22’s won’t even make it over 275,000 rpm - like Speer 22cal TNT’s which are documented to need to stay below 240,000, I’ve seen 75 HPBT Hornady’s come apart at 298,000, but also driven 50 vmax’s to 340,000 rpm without a hitch. No idea why a vmax holds together so much better.

It’s probably fair to say smaller calibers are used for smaller game, so the bullets tend to be lighter construction, and bigger caliber bullets can be more heavily constructed and still expand due to their own mass, so there tends to be a correlation between RPM tolerance and caliber, but there are curves in that particular road.

 

[DocRock]

The Swedish Mauser, designed to shoot round nose 160s, had a 1:200mm twist ratw, about 1:7.5 I find that it shoots rather conventional 140 grs bullets very well. 1:7 will serve you very well with the Berger 156 VLDs and any 140s you care to shoot. Enjoy.