Gunsmithing Tight twist rate fade?

[jagerMR]

Hey everyone, looking at rebarreling my 300wm. I wanted to pose the question and thought the smithing section was proper, but, what is the idea/reasoning behing the fast twist rates for modern barrels? Is it the projectile manufacturing that has evolved to a point that issues of ja ket separation isnt as problematic or are the quicker twist for projo specific applications? Just curious...

Currently running 1-10, but was contemplating 1-8. mainly 190gr

 

[JTJACKSON]

We have a lot of heavy for caliber, longer bullets these days that require a faster twist to stabilize. For 190s the 1-10 would be plenty sufficient I would think. As a matter of fact, a lot of the .30 caliber options out there will shoot fine with a 1-10. There are MANY other options for a 300 win mag these days, too.

 

[gnochi]

Hey everyone, looking at rebarreling my 300wm. I wanted to pose the question and thought the smithing section was proper, but, what is the idea/reasoning behing the fast twist rates for modern barrels? Is it the projectile manufacturing that has evolved to a point that issues of ja ket separation isnt as problematic or are the quicker twist for projo specific applications? Just curious...

Currently running 1-10, but was contemplating 1-8. mainly 190gr

Higher BC bullets tend to be longer; a longer bullet requires a faster twist to have the same degree of stability. In addition, there’s some data to suggest that a faster twist measurably increases BC.

That said a 1-9 will stabilize just about any jacketed 308-caliber bullet on the market right now.

It’s also worth pointing out that jacket separation isn’t a thing with monolithics, and as caliber goes up the BC to speed ratio starts to favor monolithics more and more. You’ll notice that almost everyone in KO2M is using lathe-turned solids, usually Cutting Edge Lazers.

 

[W54/XM-388]

I would not go with a 1:8 twist unless you plan to primarily shoot the monolithic bullets.

 

[jagerMR]

I tend to agree on both points. But that being, is 1-10 to fast for a 110gr or 125gr projectile? Is there really a point thats to fast to spin a projectile up to, or just the combo of high velocity and to much twist. Again, I understand having a quicker twist for heavy projos, maybe sub applications, Remibgton did that AAC 700 chambered in 308 with a 1-8. I would assume for more 190-220 usage, but a 168 smk spins up ok fron what ive seen?
So if 190-208gr max is where youd like to be load wise, is there any draw backs to a faster than 1-10 in a magnum load?

 

[gnochi]

I tend to agree on both points. But that being, is 1-10 to fast for a 110gr or 125gr projectile? Is there really a point thats to fast to spin a projectile up to, or just the combo of high velocity and to much twist. Again, I understand having a quicker twist for heavy projos, maybe sub applications, Remibgton did that AAC 700 chambered in 308 with a 1-8. I would assume for more 190-220 usage, but a 168 smk spins up ok fron what ive seen?
So if 190-208gr max is where youd like to be load wise, is there any draw backs to a faster than 1-10 in a magnum load?

Drawbacks of faster twist include higher spin drift (which is calculable and correctable), and higher dispersion if bullet center of mass and center of twist aren’t perfectly aligned. In general, you want the slowest twist that will stabilize the longest bullet you plan to use, without overspinning the lightest bullet you plan to use.

If you’re planning on loading both 110gr varmint loads and 198 Flatline ELR loads, I’d seriously think about building a second gun.

 

[Ledzep]

In a 30 cal magnum, 1:10 will get you to 230gr. 1:9 will work for the 250gr A-tip. 1:8.5 or 1:8 will completely ensure the 250 A-tip is stable, even in the winter on the coast of Alaska.

1:8 is a little faster than is necessary unless you're shooting solids or really focusing on ELR where Magnus moment can be countered to an extent by faster spin rates. Faster spin rates can negatively impact dispersion, which usually shows itself as being a "picky" barrel.

Take, MV * 12 / (twist rate) * 60 to get rpm. For example a 1:8 twist at 3300 fps:

3300 * 12 / 8 * 60 = 297,000 rpm.

So that's what a 150gr class bullet would experience in an 8 twist 300 wm. 300,000 rpm is about the cutoff most bullet makers suggest where you're liable to start blowing bullets apart. In .30 cal it's not very common, but when you get into .22 cal thru 7mm magnums you can start getting twist rates and MV's that get well over 300k rpm. A 22 Creedmoor, for example with a 6.5 twist shooting 3400 fps is in the 375,000 rpm vicinity. Most bullets are pretty "happily stable" at around 200,000-250,000rpm.

Blowing bullets apart is a combo effect of both the bullet and the barrel. Barrels with rough throats, tight spots, tight bores, tight grooves, and for whatever reason, square-cut grooves of even number (4 or 6) seem to have tendencies towards blowing up bullets. 5R rifled barrels for whatever reason tend to be much less common in the "I blow up bullets" category. Bullets with thin jackets tend to be the worst about letting go, but not necessarily always the case. I've seen barrels that blow up one lot# but not the next, and I've seen barrels that blew up Bergers, Hornadys, and Sierras no matter the state of cleanliness of the bore. Once a barrel starts blowing them up (throat/bore condition deteriorates), the trend is that it usually just keeps getting more and more frequent until it's basically a 50/50 if the bullet will make it 25yd.

I know "more twist is better" has been the trend as of the last several years, but there are limitations. Typically seen in heavy-for caliber bullets that require faster twists, and cartridges that provide north of 3000fps MV. 6.5mm and under are the typical offenders.

 

[jagerMR]

Thank you!
Ive played with the berger twist calculator and i leaves a bit to guessing.

 

[Ledzep]

Using the Berger (or others') calculator for SG, a 1.0 technically should be stable. How things play out in the real world can vary as you approach that 1.0 number. I personally am A-okay with anything over 1.2. Below that, between 1.0 and 1.2 I'll do more research and may start exploring my options.

If I can avoid it, I don't go much over 1.7. Not the end of the world if I do, but I darn sure don't flirt with anything over 300k rpm. Anyway, that's just me. Lots of guys aim for 1.5 on the bottom end and that's fine, too.