am rebarrelling to a 300 wby and can't deciede between a 1-10 twist and a 1-12 twist.. any recomendations? advantages and disadvantages. will the 1-12 give me a little more speed but possibly be less able to stabilize the heavier bullets?
barrel twist
- Thread starter bingo1010
- Start date
Traditionally the older .300 Weatherbys were 12 twist and extremely long throats ... that changed many, many years ago ...
I would recommend the 10 twist and a shorter throat that is more commonly done today. The velocities are high with less powder and better accuracy and you can shoot all the normal bullet weights for hunting. Check with your gunsmith about his reamer.
I would recommend the 10 twist and a shorter throat that is more commonly done today. The velocities are high with less powder and better accuracy and you can shoot all the normal bullet weights for hunting. Check with your gunsmith about his reamer.
bingo1010 said:
Well, a 1 in 14" twist will stabilize a 180 gr matchking at 2600 fps. How heavy a bullet you want to shoot? A slower twist will allow a little more velocity, because of less resistance. I went with a 12" twist when I made my 30-284, and it will shoot half minute with 180 gr gameking at 2700 fps. I would think a 12" in a Weatherby will stabilize 220 gr spitzers.
Copied and pasted from the site of Shilen Rifles, one of the most respected barrel makers:
.308 twist recommendations
- 8" for bullets heavier than 220gr.
- 10" for bullets up to 220gr.
- 12" for bullets up to 170gr.
- 13"* Ratchet rifled 4 groove
- 14"* for bullets up to 168gr.
- 15"* for bullets up to 150gr.
- 17"* for bullets up to 125 gr.
Lilja also reccomends a 10 twist for 150 to 220 grain bullets in a .30 caliber.
It does not hurt to spin a bullet faster within limits...
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.308 twist recommendations
- 8" for bullets heavier than 220gr.
- 10" for bullets up to 220gr.
- 12" for bullets up to 170gr.
- 13"* Ratchet rifled 4 groove
- 14"* for bullets up to 168gr.
- 15"* for bullets up to 150gr.
- 17"* for bullets up to 125 gr.
Lilja also reccomends a 10 twist for 150 to 220 grain bullets in a .30 caliber.
It does not hurt to spin a bullet faster within limits...
.
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I shoot a 1:14 twist in my 308 and the 175 SMK stabilizes just fine, spinning a bullet faster than you have to is detrimental to accuracy. Use the slowest twist that will stabilize your bullet weight for ultimate accuracy.
bigbull
bigbull
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- Somewhere on the Hudson Bay Coast
bigbull said:
Bigbull, I have heard this repeatedly, that fast twists are detrimental to accuracy with light bullets, has your experience supported this idea? The reason I ask is because I have have owned fast twist barrels which shot light bullets very well. I have pretty much concluded that a loss of accuracy due to over stabilization is fiction.
Gyroscopic drift is another matter, but it effects all projectiles leaving rifled barrels to varying degrees. The phenomena is consistent with a particular type of bullet from a particular barrel, and therefore does not pertain to accuracy anymore than does trajectory .
Boomer said:
I have to agree with you. My experience has been the same. With the quality of the bullets and barrels avaiable today I have not seen bad accuracy in rifles with twists faster than what some get by with. Until you get into the extremes, I believe the accuracy loss is theoretical more than physical.
I have built several superbly accurate 300 Weatherbys using 10 twist barrels using 150 grain and 180 grain bullets.
I would measure the length of the slug of preference and use the greenhill formula. 150 diameter squared over length gives twist rate then make it 1" faster twist. Make sure you use real numbers and not the annoying metric of course.
cheers mooncoon
cheers mooncoon
Boomer said:
Yes my experience supports this claim. My 6BR has a 1:14 twist barrel and will outshoot my 1:10 twist 243 with the same 70gr SMK bullets consistently. There is widespread acceptance of this claim, if you take the time to look around you will quickly see that the benchrest shooters all use the slowest twist possible for their rifles and in the case of the 6PPC and 6BR it will usually hover around the 1:13 to 1:14 mark, the same holds true for the various 22 centerfires. They have a choice of using any twist they want but through years of experience it has been discovered that the slowest twist will consistently outshoot the faster twist. Once again it should be said that if you want the most accurate rifle possible use the slowest twist that you can get away with. This claim does not mean that a fast twist barrel will not shoot a light bullet accurately but that it will not shoot it as accurately a slow twist can, the difference might be small but even 1/4" at 100 yds is enough to loose by! Swinging the pendulum the opposite way we see that in choosing the wrong twist the results can be catastrophic! The fast twist barrel is required to stabilise long for caliber projectiles that would otherwise simply start to yaw and eventually tumble in flight and eventually fall to the ground, the long bullets are alot less forgiving of twist choices.
bigbull
mooncoon said:
mooncoon,Your advice is a good rule of thumb for most neophites to the sport, you will not go wrong with this formula in having a stable bullet flight , but not neceserally the most accuracy.
I will add a little bit of controversy to the discussion, OK let's have some fun. The 168 SMK has been widely accepted as a standard in 308 comp. shooting and in fact if you look around you will discover that most of the Factory rifles that shoot this bullet use a 1:10 twist! Now when you get into the prescission shooting circles you find that alot of the bellyfloppers shooting 308 shoot everything under the sun except the 1:10
, in fact alot of them shoot slower twist barrels along the lines of 1:11 to 1:14 twist, why would they do that! The answer is simple, for a gain in accuracy! In fact the 168 SMK stabilizes beautifully in the 1:14 twist barrels even though the Greenhill Formula say it shouldn't when you do all the calculationsbigbull
bigbull said:
I agree that with extremes in either direction accuracy is sure to be affected. In the case of using slightly more twist than is actually required you would be hard pressed to determine any accuracy loss... if you could it would be extremely small...it certainly would not be 1/4 minute. In the case of using slightly less twist than is actually required your bullets will tumble and your accuracy will be terrible. The cost of rebarreling is the same regardless of twist, but I don't want to take the chance a twist will be too slow ... the cost of another rebarrel ? I will always err on the side of more twist than less... especially in a hunting rifle which in this case I assumed a .300 Wby would be.
Back in the 70's I built a few heavy barreled .22-250's using 12 twists and they seemed to shoot 51 grain bullets as well as the 14 twists did.
Your accuracy comparison between a 6mmBR and a .243 Win. has more to do with the cartridges than the twists. I don't believe a .243 will ever be as accurate as a 6mm BR ... just as the 6mm PPC has dominated short range benchrest over the 6mm BR.
I think a couple of thoughts come up with regard to relatively fast and slow twists. The first is that a rifle with a fast twist is probably throated for a heavy bullet and a very light bullet may not be able to be seated out far enough to give good accuracy, regardless of whether or not the twist is too fast for it. All of the examples so far have appear to have involved factory rifles or at least rifles chambered for a variety of bullet weights. In saying the twist is too fast, you actually need to chamber and more accurately throat the rifle to suit the specific projectile being tested.
And in an extension of the comment by guntech; I used to own a .218 Bee that was very accurate with 50 gr bullets but boat tails and 52 grain slugs tumbled at 25 yds. My point being that the slow twist was very limited in its usefulness. I only had it because the barrel came with the gun.
cheers mooncoon
And in an extension of the comment by guntech; I used to own a .218 Bee that was very accurate with 50 gr bullets but boat tails and 52 grain slugs tumbled at 25 yds. My point being that the slow twist was very limited in its usefulness. I only had it because the barrel came with the gun.
cheers mooncoon
bingo1010 said:
To answer your question the 1:12 twist will have an advantage in accuracy but a disadvantage in handling heavy bullets. For a hunting rifle you will not have any special problems but if you intend to shoot long range and get into the heavyweight match bullets over 200 grains then you would be at a disadvantage, the 1:10 would be a better choice. The 300 Weatherby was actually built in a 1:12 from the factory.
bigbull
mooncoon said:
The usefullness of any specialised gun is very limited and that is why we build them to suit specific needs. If a versatile gun is to be built then follow the factory twist and you won't go wrong. But that is not what we are talking about. I own a 223 that I tried to shoot 77gr SMK out of and they hit the paper sideways, if they made it to the paper
There is not much out there that hasn't been tried as far as twist is concerned, most record groups come with full descriptions of the guns and equiptment used and never has anyone compromised on the twist for the task at hand, things are fine tuned to the enth degree. Specifying a rifle out and throwing a little more twist in to be on the safe side is nothing more than a compromise, there is no room for that if the ultimate accuracy is being sought! Unless of coarse you are talking about a hunting gun, forget all of what was written above because a hunting gun is a compromise!
bigbull
i have been reading all the responses that are coming in and am following it, one simple question. how does speed play in all this..? i realize that the bullet is still making one rev per 10" or whatever the twist is, but the rpm does vary with the speed of the projectile.
bingo1010 said:
I think you can safely state you should be able to get 3200 feet per second with most 180 bullets out of a 300 Weatherby.
RPM = f/s x 12 x 60 divided by the Twist rate
If it had a 10 twist barrel that would compute to 230,400 RPM.
If it had a 12 twist barrel that would compute to 192,000 RPM.
In order for a 12 twist barrel to obtain 230,400 RPM the velocity would have to be 3840 f/s.
I believe the higher RPM contribute a lot to the hydrostatic shock and killing power of high velocity cartridges.
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guntech said:
I guess i should restate my question, does the speed/ rpm affect the bullet stability in any way.. for example does a slower speed/rpm require a faster twist to stabilize the heavier bullet than say a slower velocity but with a faster twist. like the 22-250 with a slower twist will work with 60 gr bullet but a 223 with the same twist will not, but give the 223 a faster twist then it is fine?
bingo1010 said:
That is correct bingo, the higher speed will compensate for the slower twist rate by increasing it's rpm. That is why some calibers with high velocities can stabilize heavier bullets than their slower brothers when they are both using the same twist rate.
bigbull
