9.3 twist ?

T

tahoe2

New member
1-12" or 1-14" ? I was just looking at McGowenbarrel website and noticed their 9.3 barrels are 1-12" twist while other notable barrel makers are 1-14" twist. Is their a significant difference?
Bullets in the range of 230 grn to 300 grn are the norm for this caliber, does it really matter in that 70 grn spread ?? :confused:
I have seen 35 Whelen in both twists and was wondering whats best ? I know you can shoot lighter than 230 grns (180, 200, 225) in the Whelen with good results.
 
tahoe2 said:
1-12" or 1-14" ? I was just looking at McGowenbarrel website and noticed their 9.3 barrels are 1-12" twist while other notable barrel makers are 1-14" twist. Is their a significant difference?
Bullets in the range of 230 grn to 300 grn are the norm for this caliber, does it really matter in that 70 grn spread ?? :confused:
I have seen 35 Whelen in both twists and was wondering whats best ? I know you can shoot lighter than 230 grns (180, 200, 225) in the Whelen with good results.
Click to expand...
Depends on what your shooting and the stability of the bullet. Use the Berger twist rate calculator to figure out what twist you should use based upon the bullet. Usually they aren't using a fast enough twist, the big firearms makers. Custom gunsmiths are using fast enough twist most of the time.

I have a 9.3 twist on my .338 Norma Magnum
 
waveslayer,

Imagine trying to figure out which to use with what during the time of the wildcatters, when every conceivable cartridge was being invented. Luckily, a majority logged their results, and that helped to figure the math out to use in the future.
 
Dixie Gunsmithing said:
waveslayer,

Imagine trying to figure out which to use with what during the time of the wildcatters, when every conceivable cartridge was being invented. Luckily, a majority logged their results, and that helped to figure the math out to use in the future.
Click to expand...
+1 with so many wildcat cartridges out there now, it's even crazier.
 
1-12'' would be better for bullets like the 250 grain Barnes TTSX, which is an all copper boat tailed bullet, making it long for it's weight.

A 14'' twist may not stabilize this bullet, especially in colder temps. According to Berger's twist calculator that bullet would have 'Marginal stability' in a 1-14'' twist with an SG under 1.5
With a 1-12'' twist the bullet has 'comfortable stability' with an SG of 1.73 in sub zero temps.
 
Older cartridges like the 35 Whelen and 9.3X62 used the slower twist because they were typically launching round-nosed flat-based bullets, which are shorter than spritzers or spritzer boat-tail bullets. With the prevalence of spritzer bullets these days, it makes sense to go to a slightly faster twist, although it might seem unnecessary.
 
There is not a significant difference between the two twist rates.

If you were using a 9.3x64 Brenneke with bullets over 300 grains, I would choose the tighter twist, but if you are using the 9.3x62 (or 9.3x57) it really doesn't matter, or using the Brenneke with normal weight bullets.

Hope this helps.

Jimro
 
Tahoe2,

You are correct that the weight isn't much different, but using weight to determine twist is an approximation based on the bullets under consideration all having similar shapes and constructions. When they similar in those ways, then adding weight always increases their length. It is actually that length that is most critical to choice of twist. In all but wadcutters, most airgun pellets, and some short Minié ball designs, the center of pressure of a bullet is out in front of its center of gravity. The longer the bullet is, the more distance there is between those two centers, so the longer the lever arm is that air pressure has to overturn the bullet with. It takes faster spin to prevent that from happening.

Bullet weight by itself, if you can add it without increasing length, actually causes you to need less spin. That's because the added mass increases the bullet's gyroscopic stiffness or rigidity. It just is not an effect that increases as rapidly as increasing bullet length increases the overturning moment, and therefore the need for higher RPM.

You can use the free stability estimator on the JBM ballistics site to get a good idea of what you need. It works best for pointed rifle projectiles at supersonic velocities, is good for comparison of other shapes. Unlike the simplified Greenhill formula that is most commonly published, you can enter full atmospheric conditions to see how you might do in subzero conditions if you need that.

The output of that estimator is the gyroscopic stability factor. This is a number that is equal to one when a bullet is right on the edge of tumbling. To get best effective ballistic coefficient and precision on target, you need the stability factor to be a bit higher than that, but not so high that the over-spinning opens groups up. Sierra recommends you choose a twist that gives you a gyroscopic stability factor of 1.3 to 3.0 for hunting accuracy with a rifle, and 1.4 to 1.7 for best match rifle accuracy with a rifle. At transonic and subsonic velocities for shorter ranges (i.e., handgun target ranges), a higher number does better with accuracy because the rpms produced per unit rifling twist are lower and wobble and lateral muzzle jump caused by spinning up to a high stability factor have less influence, so that a high stability factor can be taken advantage of.
 
Haven't visited this in a while due to hunting season, sadly no venison this year. Anyway, I imagine that I would stick with 250, 270 and 286 grn in RN, semi-spitzer, and spitzer type bullets. These will cover anything I will ever hunt, and this project is still in the developmental stage. I thank all that participated, and will post future progress when the build begins. Thanks again !!:cool:
 
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