Above is a photo of the micro-groove twist rate of this 35 Marlin barrel.
First and foremost bullets that do not spin or that do not spin fast enough have no long-range accuracy. Accuracy is a relative term. If you are happy with your bullets accuracy at the distances you shoot, then all is well in the world.
On the other hand, lets say you want to elk hunt out west so you work up a load for your pet .338 Winchester Magnum shooting 250 grain bullets and you can’t get a one inch group out of any load. In fact your best load is 2 inches. Using the 6 inch rule of thumb for a kill zone that would restrict you to a 300 yard shot. True? Yes. But first let us understand why your rifle shoots the 2 inch groups with the 250 grain bullet no matter what you do with powder, bullet seating depth, bullet manufacture etc.
The fundamental question is:
Do you understand what bullet twist rates are doing to the bullets you want to shoot?
I did not for years.
What is the twist rate of your rifles? Don’t know? The truth is most of us don’t know what the twist rate is for the rifle you shoot. For those of us who are experimenters and want to understand what and why, this article is for you!
My reloading manuals talk little about twist rates and resultant effects perhaps because they are trying to sell you bullets and powder.
For years I just accepted the twist rates and tried to reload bullets and use powders to maximize or tighten my groups. In some cases, no matter what I did the groups would not shrink. Enter Sir Alfred George Greenhill.
From Wikipedia; In 1879, Greenhill developed a rule of thumb for calculating the optimal twist rate for lead-core bullets. This shortcut uses the bullet’s length, needing no allowances for weight or nose shape.[3] Greenhill applied this theory to account for the steadiness of flight conferred upon an elongated projectile by rifling.
If I can calculate twist rate for a given bullet then I can match those bullets to my rifles twist rate.Or get another barrel to shoot the bullets I want.
If you are not sure of your rifles twist rate you can measure it with a cleaning rod and the distance it takes to rotate one full turn. You can do this by placing a tight fitting patch and rod in the muzzle end, making sure the rod spins as the patch is pushed is key here. Before pushing, take a marker and place a mark on the rod near the handle. As you push the rod the mark will begin to rotate. When the mark has made one full rotation stop and place a mark on the rod where it enters the barrel. Remove the rod and measure the distance to the patch, say ten inches. This means that your barrels twist rate is 1 in 10 inches and is displayed as 1:10 twist rate.
Armed with this knowledge in hand you can use Sir Alfred George Greenhills formula to determine what twist rate is needed to shoot the bullets YOU want to shoot.
A classic case in point is the shooter that owns an AR-15 and his groups get worse as he goes above 55 grains to either shoot long distance or hunt larger game. Finally the 70 or 80 grain bullet spin so slow as to key hole in the target.
The shooter knows the twist rate is 1:12 and finds out by using the Greenhill Formula what twist rate is needed to shoot an 80 grain bullet for his .223.
where:
- C = 150 (use 180 for muzzle velocities higher than 2,800 f/s)
- D = bullet’s diameter in inches
- L = bullet’s length in inches
- SG = bullet’s specific gravity (10.9 for lead-core bullets, which cancels out the second half of the equation)
To make it easy on ourselves click on the red Greenhill Formula above and enter the data for an 80 grain bullet length, diameter and velocity. Lets enter 1.17 for bullet length, .223 for diameter and 2900 fps for speed and hit enter. Twist rate recommended is 1:8 twist
From the website for the Greenhill formula we enter the data.
A Calculator for Barrel Twist Rate
So that is why there are AR 15 barrels for sale at 1:8 twist.
Now you know! Me too, it was a learning experience. I hope this was helpful to New Hampshire Rifleman readers! Shoot straight and have fun. Ed ©