There is much talk these days about twist rate. For decades nobody much wondered about how fast their bullets spun but now we have a choice in some cases and it can lead to a great deal of confusion. We have a pretty good handle on why bullets should spin but what difference does it make how fast they spin?
A good metaphor for understanding twist rate is to imagine throwing a baseball and a football. Now, even a guy like me who is completely devoid of athletic talent, can manage to toss a baseball in a reasonably straight line. A football, on the other hand, normally becomes a floppy, uncontrolled mess when I throw it, endangering nephews and pets alike. What’s the difference? Since the baseball is round it requires very little spin to keep it moving in a straight line. The football, however, is oblong, so to stabilize it in flight a high rate of spin must be imparted.
Now let’s take a look at two bullets. First, let’s consider a full metal jacket (FMJ) 230gr .45 caliber bullet for the good old 45-auto that is traveling at 850 feet per second (fps). This bullet was our handgun service round for a long time and is commonly (and very fittingly for our purposes) known as ball ammo. A 230gr FMJ bullet is almost round and, as such, is usually fired through a barrel with a 1:16 twist. This means that the bullet makes one full rotation after traveling 16 inches. This is a rather leisurely twist rate by firearms standards, but is more than adequate given the shape of the projectile.
Next, we’ll consider another FMJ. This one is a 300gr .338 caliber bullet that we wish to fire from a 338 Lapua somewhere in the neighborhood of 3000 fps. This bullet is long and sleek. Between its beautiful tip and slimming boat tail, it slips effortlessly through the air, allowing it to preserve precious velocity. It also looks a lot like a football, so the Lapua makes use of a fast 1:9 twist (1 full rotation for every 9 inches of travel) to keep it flying straight.
Why waste time with a longer, pointer bullet? Generally, sleeker designs result in less drag, and less drag means higher downrange velocity. All bullets, regardless of weight, will fall towards the ground at the same rate over time. Faster bullets drop less because they are able to cover more distance in a given amount of time, and decreasing the drag on a bullet means that it will be going faster for longer. So, less drag directly translates to less drop.
Some engineers put a lot of thought into this, so why ask a consumer who likely doesn’t have a physics degree to choose? Most of it has to do with gun companies not knowing how you plan to use your rifle.
Somewhere along the line, it was noticed that the 223 Remington works a lot better on larger game like Coues deer or burglars if a heavier bullet is used. Traditionally a 223 Remington uses a 1:12 twist and shoots gophers like nobody’s business.
These days, bullet weights for the 223 are creeping from 45grs up to 80grs. This change in bullet shape requires a faster (generally 1:9) twist to stabilize them. The .338 bullet discussed above didn’t even exist when I was a kid. It is part of the new very low drag (VLD) breed becoming so popular these days. .338s used to end at 250grs, but times have changed. Too many shooters’ VLDs are just too good to pass up and, if twist must be tailored to make them work then so be it.
The most important factor in choosing a twist rate is to decide what you intend to use the gun for. Standard twists work well for standard bullet weights. Oddities, either heavier or lighter, will require a different twist that may or may not work well with standard weights. Make a decision as to the intended use first and then pick the twist that fits your needs.