1:7 vs 1:8

[idunham]

Hey all,

I'm getting ready to order my barrel from Apache Barrels. Good price, high quality barrel it seems.

I want a barrel that could push 115s if i choose to shoot them. However, I also want flexibility.

If i get a barrel with a twist of 1:7, am I limiting myself to any other bullets? Will other weights work fine, as well?

If i go 1:8, i will not be able to shoot 115s at all.

So, is there a con to going with a 1:7?

[short round]

What are you having it chambered for?

[darkker]

+1,
Without the chamber we can't say anything.
Next is "115gr bullets" , this isn't very helpful by itself. Bullet weight doesn't mean much, bullet length is what is critical. Stabilisation is about length.
Unless you are talking 5,500+ fps, velocity doesn't harm bullets. RPM is what will destroy them. Most match bullets RPM limit is @ 290,000 - 320,000 RPM. Which bullet is dependant, as well your specific barrel.

[LoneWolf]

I would guess 6mm of some sort. 1:7 is my suggestion guessing you would be looking towards 115DTACS or 115VLDs. I don't know too many other 115gr bullets in the 6mm realm. H100V worked real well in my 243 with the DTACS though. I did print a 1.438" group at 415yds with that combination.

[idunham]

Oh man...I really though I put that on my post...


243 WIN is what I'm having it chambered in.

[LoneWolf]

Here's some more specs for your notes:

27" 1:7 twist XCaliber
243 Win spun by AGW

Load
hornady 243 brass
CCI 200 primer
40.0grs of H100V
115DTAC

I was getting 2950fps with that load.

[LoneWolf]

I've since switched to an 8 twist and 105 class bullets. The AMAX has been good to me at 3050-3100fps from a 26" barrel.

[darkker]

Oh man...I really though I put that on my post...


243 WIN is what I'm having it chambered in.

Right.... But still haven't said WHICH bullet.... I'll assume you don't care about Barnes 115gr round nose.
Berger's - 1.365"
DTAC - 1.290"
Precision Ballistics - 1.330"

A muzzle velocity of 2500fps
7-twist - stability factor 1.526. Very good
8-twist - 1.168, marginal. EXCEPT DTAC, stability good 1.379

So bump speed to 3k
8-twist - stability factor 1.241, marginal.
Now remember that velocity decay happens rapidly, whereas rotational does not. So the 8 Twist doesn't automatically mean unstable.

Now your flexibility math. MV * 720 / Twist = RPM
So let's assume super smooth barrel gives optimal results.
Nominal RPM limit on modern match/Varmint bullets:
290,000 * 8 / 720 = 3,222 fps max
290,000 *7 / 720 = 2,819 fps max

So. YES, you can stabilize A 115gr with an 8-Twist, just use the correct bullet. No, you don't get to shoot 4,000 fps Varmint bullets. Just depends upon what you want.

If you buy a Ferrari, you don't complain there is no room for groceries:cool:

[idunham]

Right.... But still haven't said WHICH bullet.... I'll assume you don't care about Barnes 115gr round nose.
Berger's - 1.365"
DTAC - 1.290"
Precision Ballistics - 1.330"

A muzzle velocity of 2500fps
7-twist - stability factor 1.526. Very good
8-twist - 1.168, marginal. EXCEPT DTAC, stability good 1.379

So bump speed to 3k
8-twist - stability factor 1.241, marginal.
Now remember that velocity decay happens rapidly, whereas rotational does not. So the 8 Twist doesn't automatically mean unstable.

Now your flexibility math. MV * 720 / Twist = RPM
So let's assume super smooth barrel gives optimal results.
Nominal RPM limit on modern match/Varmint bullets:
290,000 * 8 / 720 = 3,222 fps max
290,000 *7 / 720 = 2,819 fps max

So. YES, you can stabilize A 115gr with an 8-Twist, just use the correct bullet. No, you don't get to shoot 4,000 fps Varmint bullets. Just depends upon what you want.

If you buy a Ferrari, you don't complain there is no room for groceries:cool:

So, If i got a 1:8 twist, I should be able to shoot everything up to 115gr (In most cases, assuming i don't use Bergers round nose?).

I'm trying to figure out if I should go with a 1:8 twist, will i regret not going with a 1:7 twist.

[darkker]

.... What???!!
Berger doesn't make a 115 RN... I said BARNES as a joke.... You aren't getting this, so I'll explain simply again.

You need to stop talking about bullet weights, as a thing.
Look at my previous post again. The same weight-class, same match style bullets, BIG difference in length.

Twist rate is for stability.
Stability is for a LENGTH, weight doesn't matter.
Weight is for choosing a date, or worrying about when they tone for a medical assist...

Here:
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

Length look-ups are on the left margin.


Again,
These fast twists limit how fast you can push bullets.
So bullet dependant, and depending upon how smooth your barrel is, here are the speed limits before the RPM destroys them.

8-twist - @ 3,200
7-twist - @ 2,800

[WinnieTheBoom]

My RPR is a 1:7.7 twist.

I've had good luck with the 115gr DTACs, and the 115gr VLDs have showed some promise as well, but I haven't experimented as much with the latter. Both definitely prefer the faster twist barrel.

[Lynn]

I have a very large pile of barrels in 8 twist from 6BR and 6Dasher all the way to 6mm-06 and can push a 115 over 3300 fps.
The earlier Bergers blew up from heat allowing the core to melt and the thin jacket not holding up. The sloution was thicker jackets on the target bullets usually fired from very long barrels where frictional heat becomes an issue.
In a 6mm-06 with a typical 30 inch barrel 57 grains of H1000 and using Lapua 30-06 brass gives me 3433 fps using the 103-108 grain bullets and best accuracy.
If I step up to the heavier bullets like the DTAC or Berger I can achieve a little over 3300 fps before I see pressure but best accuracy is at 3200 fps and giving the 103-108 bullets a ballistics edge.
I wouldn't go with a straight 7 twist but as previously posted a 7.7 or 7.8 twist if you absolutely have to shoot the 115's.
Rotational forces have very little to do with bullet blow ups as they rarely exceed 20,000 psi.

[idunham]

.... What???!!
Berger doesn't make a 115 RN... I said BARNES as a joke.... You aren't getting this, so I'll explain simply again.

You need to stop talking about bullet weights, as a thing.
Look at my previous post again. The same weight-class, same match style bullets, BIG difference in length.

Twist rate is for stability.
Stability is for a LENGTH, weight doesn't matter.
Weight is for choosing a date, or worrying about when they tone for a medical assist...

Here:
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

Length look-ups are on the left margin.


Again,
These fast twists limit how fast you can push bullets.
So bullet dependant, and depending upon how smooth your barrel is, here are the speed limits before the RPM destroys them.

8-twist - @ 3,200
7-twist - @ 2,800

Ok, right on, that makes sense to me.

[darkker]

I have a very large pile of barrels in 8 twist from 6BR and 6Dasher all the way to 6mm-06 and can push a 115 over 3300 fps.
The earlier Bergers blew up from heat allowing the core to melt and the thin jacket not holding up. The sloution was thicker jackets on the target bullets usually fired from very long barrels where frictional heat becomes an issue.
Rotational forces have very little to do with bullet blow ups as they rarely exceed 20,000 psi.

You have the right end result, but the wrong info.
Until you exceed around 5,500 fps velocity, air friction doesn't catastrophically damage bullets. What makes they pop, is Centripidal force over coming centrifugal force. A thicker jacket will hold things together at higher rotational speeds, true. Back to specific bullet discussions and not generalities. I called all the majors a while back and asked for RPM limits on bullets.
These numbers are HIGHLY dependant upon barrel smoothness. Not all jackets can take the same stresses. Cooking bullets in a hot chamber doesn't melt the core, but damages the integrity of the jacket.
Hornady - SXSP - 240,000 rpm. A, V, SP = 290,000 rpm
Nosler/Sierra - 320,000 rpm. I can't say the older thin jacketed name, but it was 240-260,000rpm
The math again, if you want it: MV * 720 / Twist = RPM

This is a slippery-slope in that, while higher RPM can gain a slight edge in BC; it also exacerbates ANY bit of concentricity issues. That is where the notion of "over-stabilized" comes from.
115 DTAC's will happily stabilize in an 8-twist.

[Lynn]

I don't think I ever mentioned air friction at all.
The problem Berger Bullets were suffering was do to the thin jackets not being able to withstand the acceleration forces which run over 100,000 G,s and not rotational forces.
In laymens terms thin jackets don't like long rough barrels and twist rate doesn't play much of a role in bullet failures.

The whole 4-5 pages can be seen on Benchrest Central with Eric Stecker reporting the MIT results. It is in the F-Class forum if my memory is any good.

[Lynn]

I figured most wouldn't be able to look up the actual results as posted by Bergers Eric Stecker so here you go.
http://benchrest.com/archive/index.php/t-49336.html

[darkker]

Yes Lynn, re-read what it all says.
I agree he talks about a melted core, secondary failure is high RPM. We aren't talking factory twist rates, so RPM becomes a very significant concern.
And he reiterates near the end of his post from 7:01pm. That the loads in question were generating 254,000 RPM, which as he said "was well within acceptable limits". This also corresponds with what I have said, and been told by the bullet makers.

As HBC explains just below that post, in fact jacket damage is what caused the failure, not a melty core. Damaged jackets under high rotation stress cause a yard sale.

As Eric says, there are many things that play into it. In high RPM, high volume shooting causes the light jacket, AKA VLD hunting line, to be an un-appropriate choice.

[Lynn]

I have read it many many times as I am the Lynn Dragoman Henry the bullet God Child's refers to in his post.
A typical 30 caliber 200 grain bullet at 3000 fps will have around 127,000 G,s during it acceleration down a 30 inch barrel but less than 30,000 pounds of rotational force.
If rotational force was tearing the bullets apart they would also be collapsing from the much higher forces associated with there acceleration.
And that was what happened to the original 338 OTM Hybrids. The BC was originally over 0.900 but nobody could get there claimed BC number to match at longer distances. When Bryan Litz looked into it the ogives were collapsing.
Now we have either 0.818 or 0.822 as the bc instead of 0.950 like the original version.

[darkker]

Lynn,
If RPM is not the destroyer, them why don't 22-250's Swifts, WSSM's ALL just use 9-twists? The answer is because the RPM will destroy them. You only use right twists to stabilize long bullets, which are fired slower thusly don't have the same RPM.

As to your 338 example.
Litz is constantly changing BC numbers on his bullets. Now because I'm not familiar with the issue in question, I'll play along. Assume the ogive was collapsing. This is a completely side issue to RPM. If in the thread link, the jacket alone was being destroyed by G's; then Berger would not have ever kept the original jacket. Which is what the hunting line is.

[Lynn]

The hunting line is what most of the Long-range Benchrest shooters use in there 600/1000 yard chamberings because the jackets are more concentric.
If you are a Competitive Shooter you only switch to the target bullets if you are experiencing failures.
Friction is what caused the failures not rotational forces.
Bullet length all else the same determines twist rate not velocity.
If your gyroscopic stability is 1.5 it doesn't matter what velocity your shooting because a 1.5 GS means the bullet is stable.

The only time twist rate comes into play is guys with a GS of 1.2 shooting 1000 yards and maximum loads in a very long barreled 308.
They are already on the edge and small variances can mean big trouble quickly.
The guys winning Benchrest matches at 1000 yards with the 6 Dasher are using 8.3-8.7 twist barrels and 103-108 grain bullets. When asked about there twist most just say 8 twist to avoid confusion.