Barrel Twist Rates
There have been some questions about this subject lately and I have responded to the best of my ability specifically for each of them.
There is a post back on page three asking about the 450 Marlin and another on page one for the 458 WM twist.
In those two posts are formulas called the Greenhill Formula. Well, I make up my formulas as I go along because I understand math and what we are calculation here so the formulas are not exactly alike.
One is: Twist=150 X D/r (the length of the bullet in calibers)
150 is a constant.
D=diameter of the bullet.
The other is: Twist=150 X D squared/L (length of the bullet in inches)
These are the same thing and will give the same results with just simple check book balancing math.
A .458" diameter bullet is 1.374" long (500 grains) the twist will be 22.9" with either formula. I just recommended using the constant value of 100 for better impact stability and penetration.
That same bullet would be calculated at; 15.26" or 15.26" depending on which formula you choose.
I will add to this for those who care, a way to find the maximum bullet length that can be stabilized in a particular barrel.
Maximum Bullet Length (inches)=D x D x 150/Twist Rate
This way we can go backwards to find what bullet is optimum in our rifle.
Based upon my ancient experience with the 244 Remington (1:14 w/ 100 grian psitzers) and the early 250-3000 and 257 Weatherbys (also 1:14 but w/ 115 grain and heavier spitzers), I'm always more concerned with twists too slow than too fast. It's great to have the formula, especially the Murphy Version, to help take the guesswork out when choosing twist for a custom job.
I have never used a formula for figuring twist rate. So far I have just taken into account the known common rates in factory rifles, and relied on my limited experience. A few years ago I began to suspect that things could be improved by not just going with the standard one-size-fits-all method, which has generally meant choosing the fastest rate commonly used in a given caliber. That's what the factory and most custom rifle makers do, I suppose figuring it is the safest method for customer satisfaction, and it also means less inventory.
Since I mostly use custom barrels anyway, and plan to use one particular bullet weight in a given rifle, my first experiment with twist rate was with the 338 WM. I had one with a Shilen barrel, 10" twist, and it would shoot 250 gr and heavier great, but accuracy fell off with 225, and especially with 200 gr bullets. I sold that rifle, why I can't remember, and later I bought a stainless Winchester 338, which would not shoot accurately with anything. I messed with it for a couple of years off and on, thinking the action may be good for something.
Realizing that the 338 was initially offered with 300 gr bullets, I figured the standard factory 10" twist must be what it takes to stabilize that heavy bullet. I also figured that if the heaviest bullet I wanted is 250 gr., then I could use a 12” twist in 338. It was my money, so I didn’t listen to anyone, and just did it – bought a Lilja SS barrel in 12” twist, chambered in .338 WM. The results were immediate, and as expected. This custom Winchester is one of my most accurate rifles. I have shot many three and four shot groups with that rifle where the bullet holes were touching or very close to each other – at 100 yards plus a little – off the bench and sand bags of course. I mostly use 225 gr bullets, but it shoots accurately with all the 200, 210, 225 and 250 gr bullets that I have tried. I have made my all-time longest shot on game with this rifle – about 300 yards. That deer went down, and just kicked a little. I could hear the bullet hit, whop.
I also later tried a 12” twist in a .308, and got similar results. Over time, I had several rifles in .308 or 06, standard 10” twist, and they all worked with 180 gr, but I never got any of them to shoot as well with 150 or 165 gr bullets. I tried a lot, because I read so many times of other’s success. The 12” twist gave immediate and satisfactory results with 150 & 165 gr bullets.
Recently, I had a .308 barrel made in 11” twist, and it is accurate with 165, 175, 180, and 185 gr bullets. I haven’t tried it with 150 gr bullets yet.
I have a 280 in 10” twist, and its favorite is 140gr. Using 150 gr and heavier and accuracy falls off. I have another 280 with 9” twist, and it likes 154 & 160 grs. It doesn’t shoot the 140 gr. as accurately. I’m still working with the 175 gr. in this one. Surprisingly, I have yet to get the same accuracy with any 150 gr 7mm bullet in either rifle. My next 7mm project will be with a 9.5 twist, and the first bullets I’ll try will be 150 gr.
Like Murphy, I usually develop one load for a particular rifle. So, my point is that I think the twist rate is important. I don’t think one-size-fits-all. Granted, there are some cartridges for which twist rate is rather standardized, like the 375 H&H and the 458 WM. All my 9.3 rifles have a 12” twist which seems to be good for any weight I use in that caliber – 250 gr thru 300 gr. But, in the 338, I am confident that the 12” twist is appropriate if the heaviest bullet to be used is 250 gr., and it’s ideal for 225 and 210 gr. I’m also sure that the 12” twist is ideal in .308 for 165gr and lighter bullets, and the 11” twist is perfect for 180-185 gr. 308 bullets.
Now, you see what I mean by actually trying the different twist rates at the range, rather than trying to predict the outcome with a formula. It is working for me, but I don't get radical either. I'm also willing to go with whatever the rifle will shoot well, rather than forcing it.
I also agree with Murphy about the .458WM or the 450M, using 350 gr. to 450 gr. bullets at about 2000 fps, and maybe cast bullets. I think for my money, I’ll get the 20” or possibly 22” twist for the 450M I’m planning. I’m not sure yet, but I’m leaning toward the 20”. Certainly not the 14”, which makes the 458WM so special with 500 – 510 gr bullets. The 458 WM has that special niche covered, but that’s not the rifle I’m looking for. Again, I believe that I won't go wrong and waste my money because there are plenty of examples of the 20" twist rate working very well in the 45-70, etc.
Anyway, that’s my view, others may disagree, ok.