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Old January 27, 2020, 11:38 PM   #1
Andy1
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.243 twist rate

I'm looking at buying a Ruger #1 in .243 26" barrel. It comes with a 1:7.7" twist and I'm trying to find out if that's too much spin. I shoot Buffalo Bore 85 grain TSX. Twist calculators call for 1:10" twist. I know I can't go slower, but can I accurately shoot 1:7.7" using this bullet?
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Old January 28, 2020, 03:09 AM   #2
std7mag
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Hmmm.... Interesting...

Typically the lighter the bullet the shorter the bullet/bearing surface, the slower the twist you can get away with.
There is video of 7mm Rem Mag experiencing bullet blow up at 20 yards with a 162gr ELDM.

But being a solid, you needn't worry about that.

What you may experience is called torque. The spin of the bullet traveling down the barrel has an equal opposite reaction. Hence trying to twist the gun while shooting.
Not sure how severe it's gonna be with an 85gr bullet, but a possability.

With that twist rate, you would probably be better off with a heavier bullet.
Seems awfully fast for a 243.

From what i could find, Ruger #1 in 243 listed as 1:10 from the factory.
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Old January 28, 2020, 08:01 AM   #3
Andy1
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This is what the website is showing. I thought it seemed really fast also.

https://ruger.com/products/no1/specSheets/21300.html
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Old January 28, 2020, 08:25 AM   #4
jmr40
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A 1:10 twist used to be standard, but lots of people wanting to shoot high BC bullets now so manufacturers are responding. I'm betting you'll be fine.
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Old January 28, 2020, 09:39 AM   #5
MarkCO
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I shoot 75s from my 1:7.7 twist without any issues. That is as light as I have shot out of that barrel.
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Old January 28, 2020, 11:46 AM   #6
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Andy1,

From an accuracy standpoint, there are four issues with too much spin:
1.) A bullet with any tilt in the bore that offsets its center of mass (CM) from the bore line will have that CM eccentrically orbit the bore line traveling down the barrel. When that eccentrically spinning CM clears the muzzle, it is like letting go of a weight spinning around on a string, throwing the bullet tangent to the side of the bore closest to the CM when it exists. That throw imposes lateral drift away from the intended trajectory path that stays with the bullet all the way to the target (because it is too slow for drag to slow it down appreciably during the time of flight of the bullet). The faster the spin, the higher that drift velocity is, so the more it opens the group by displacing point of impact. Short bearing surface bullets tilt in the bore more easily than long bearing surface bullets, making them more vulnerable to this phenomenon, and therefore more sensitive to excess spin.

2.) An imperfectly made bullet having uneven jacket thickness or a core flaw or a boattail not formed perfectly coaxially with the bearing surface and nose, will have its CM offset from the centerline. For that reason, it will experience lateral drift when it exits the muzzle just as the tilted bullet in the last paragraph did. It will also wobble eccentrically in flight, which results in slight tilting that decreases the ballistic coefficient a little as compared to other bullets in the box that may be perfect. The faster it spins, the faster the drift and the worse the wobble. Avoiding excess spin to avoid these imperfection effects is the primary reason optimal gyroscopic stability factor (what the Miller and McCoy stability calculators provide) is usually taken to be between 1.4 and 1.7. Litz and Miller say 1.5, though it can vary a little with bullet design, stubby short-range flat base custom benchrest bullets being happiest with a slightly lower number than long skinny bullets are. But even for a bullet that is perfectly symmetrical, higher gyroscopic stability factor causes the yaw of repose and spindrift and aerodynamic jump in a crosswind to increase, making sight corrections bigger and ballistic coefficient slightly smaller.)

3.) As bullets get lighter, for a given pressure profile, they go faster. For a given barrel twist rate, they will, therefore, be spinning faster when they exit the bore than a slower and heavier bullet will. A cup-and-core bullet may be spun so fast the centrifugal effect swells the soft lead core outward until it expands and cracks the jacket open along the rifling engraving marks, turning the lead loose to disintegrates and dropping the jacket pieces on the range.

4.) As bullets get lighter, for a given pressure profile, they go faster because they are accelerated more rapidly. That acceleration is both longitudinal (down the barrel) and angular (rotational acceleration). For two barrels having the same lengths and bore and chamber dimensions, but with one of them having a faster rifling pitch, that faster twist barrel will produce more angular acceleration. If the twist is too fast, angular acceleration can be so great the bond between the jacket and core can't overcome the core's inertia, so the core slips inside the jacket. Because rifling indents the inside of the jacket, the core gets squeezed narrower as it slips and becomes loose inside the jacket. This is called core stripping. It happens at angular acceleration too slow to cause the core to swell to disintegrate the bullet, but too fast for jacket and core to stick together. When such a bullet exits the muzzle, the core is spinning more slowly than the jacket, and when the two equilibrate their spin rates, the result is slower spin than intended, which may not stabilize the bullet, and the jacket is loose and rattling against the core. As you might expect, accuracy is terrible when core stripping occurs.
If you are using solids with no soft lead core to expand, you don't have to worry about 3.) or 4.). Manufacturing by bullet specialty companies has become more precise over time so the mass asymmetry issue in 2.) is a smaller problem than it was thirty years ago, especially with match bullets. You can still get Service Rifle match accuracy out of 168-grain MatchKings, for example, even with gyroscopic stability numbers between 2 and 3. Most surplus military bullets and some cheap brands are not going to do well at high spin rates, but they won't be great even at an optimal 1.5, anyway.

The only thing you want to watch out for with your solids is because of 1.), a bullet being spun too fast will benefit from taking extra care to load it straight. That means minimizing cartridge runout (search for any of the several existing threads on the board that addresses this issue). Investing in a runout gauge will let you segregate the most accurate commercial rounds out of a box. You can even bend them straight by drilling a bullet-size hole in a piece of plastic (like a cutting board) or hardwood and putting the bullet in and pushing the case to the side until the gauge tells you the runout is within a thousandth of an inch or so.
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Old January 28, 2020, 02:02 PM   #7
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My .243, not a Ruger No.1, is 1 in 9.125". That'd the standard before the 1 in 10. It likes heavy bullets. Most commercial .243's, even Varmint rifles, are rifled for deer weight bullets, 85 grains and up vs varmint bullets.
The part that's most important is to use the right bullet for the game. As in no light varmint bullets(most of those under 85 grains) for deer sized game. And there ain't nothing like varmint hunting with your deer load as practice for deer season.
The 1 in 7.7 is a Talo Distributor Special. A TSX is a solid copper bullet. Don't think that matters though.
There are custom barrels, like Lilja's, that start at 1 in 7.
Shilen says their 1 in 7 or 8 barrels are for 100 grain plus bullets. 1 in 10 for up to 95 grains.
Twist calculators don't always tell you anything that's accurate.
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Old January 28, 2020, 02:30 PM   #8
Jim Watson
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I wonder about the practical effects in a hunting rifle, especially a No 1 which can be tricky anyhow.
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Old January 28, 2020, 02:47 PM   #9
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I have 243 and 243AI and I do reload.

Your not reload, going to shoot Buffalo Bore 85 grain TSX ammo. I would give them a call see what they have to say.

I think Buds is sell new standard #1 243.
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Old January 28, 2020, 07:00 PM   #10
MarkCO
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Quote:
Originally Posted by Jim Watson View Post
I wonder about the practical effects in a hunting rifle, especially a No 1 which can be tricky anyhow.
I don't ever hunt with anything over 105 grains in .243, and my 1:7, 7.7, 9 and 10 twists all shoot them the same out to about 500, where the 1:10 loses stability before it gets to 600. I'd go with 1:8 if I had a choice of twist in a hunting .243. 115s in a 9 or 10 twist .243 are not going to sing well past about 300 to 400.

I have 55 grain varmint bullets that run at 3700 fps out of my 1:7, never popped a single one. Sometimes theory is just a lot of words.
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