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Rifling Twist Rates & Bullets

We all understand that a rifle barrel has spiral grooves (rifling) in order to impart spin to bullets. This stabilizes them so they fly true, like a well-thrown football. But what’s all this talk about twist rates like 10-inch or 1-in 14? What’s that all about and how important is it?

Twist rate numbers reference how many inches of barrel are used to make one complete, 360 turn of the rifling lands/grooves. This is important because the longer the bullet, the faster the twist must be to stabilize it. So, a barrel with 1 rifling turn in 12 inches would stabilize a longer bullet than one with 1 turn in 20 inches. How long of a bullet? That depends on the caliber (diameter) of the bullet and its velocity.

The standard 10- or 12-inch twist barrel of a .30-06 will stabilize all these 308 caliber bullets, which run from 100- to 165-grains, but a 220-grain one would probably require an 8-inch twist barrel to stabilize.

Fortunately for us, engineers have figured out optimum twist rates for standard cartridges using typical bullet weights. Manufacturers build barrels with appropriate twist rates to match based on industry standards.

Select manufacturers may change these slightly, but rarely radically. Some cartridges, such as the .30-06, accommodate an amazing number of bullet weights/lengths. With other cartridges, a smaller variety of bullet weights/lengths are stabilized. For instance, most .30-06 barrels have a 10-inch or 12-inch twist. These are adequate to stabilize bullets as light (short) as 100 grains and as heavy (long) as 200 grains, sometimes 220 grains. A safer upper end (long bullet) is 200 grains.

Length is the big factor here rather than weight. A 220-grain, lead flat-nosed bullet might stabilize in a 10-inch twist .30-06, while a 220-grain boat-tail spire point would be too long to stabilize.

In the .22-250 Rem., the 12-inch twist rate usually stabilizes lead-core bullets from 40 grains to 55 grains, sometimes 60 grains.

Same caliber bullets come in a wide variety of weights/lengths. A single barrel twist rate can’t stabilize them all. The longer the bullet, the faster the twist needs to be to stabilize it.

Bullets made of materials with less specific gravity than lead (less mass per given area,) such as copper or gilding metal, need to be longer for a given weight. But if too long, they won’t stabilize in traditional twist barrels. This is why the new non-toxic varmint bullets from Nosler, Barnes and Hornady are so light. In .22 caliber, a 40-grain Nosler BT Lead-Free bullet is almost as long as the lead-core 50-grain Ballistic Tip. Nosler specifies that the 40-gr. BT Lead-Free requires a 1-12-inch twist barrel. Barnes specifies faster barrel twist rates for many of its longer bullets.

So why bother with the heavier bullets? Why not just stick with shorter, lighter bullets that stabilize in slower twist barrels? Because short bullets have lower ballistic coefficients than long bullets. Lighter weight reduces ballistic coefficient, too. The upshot is that a light, short bullet that stabilizes in standard twist barrels suffers decreased ballistic performance. It drops faster than longer, heavier bullets and loses more energy at all ranges.

The cure for this is indeed additional bullet length and the weight that comes with it, but such bullets require faster rifling twists. And those are slowly becoming more common. I suspect, and hope, that this will accelerate because there are so many long bullets coming on line.

The new VLD (Very Low Drag) bullets from Berger and others are extremely long, sleek, boat-tail designs that deliver optimum long range performance.(Sold right here at The Sportsman’s Guide http://www.sportsmansguide.com/net/cb/cb.aspx?a=892641

). But when you try shooting them in traditional twist barrels, they don’t stabilize. This is why you rarely, if ever, find factory loaded ammunition with extreme VLD bullets. VLD bullets are instead sold as components to handloaders with warnings printed on the box about necessary twist rates.

Berger makes many VLD bullets, which are extremely long and ballistically efficient, but if they get too long, faster-than-traditional barrel twists are needed to stabilize them. Note the twist recommendations on the boxes.

For the most part, you don’t have to worry about twist rates if you shoot factory loads and factory standard rifles. Both are designed and built to work well together. But if you get into non-toxic ammunition or VLD bullets, you could begin seeing your accuracy suffer.

If your groups begin opening, suspect poor stabilization. If bullet holes in targets begin elongating, your bullets aren’t stabilizing. They’re wobbling and hitting the target slightly tilted on their sides. Horribly unstable bullets will keyhole (enter sideways). Higher velocity sometimes cures this, but that risks dangerous pressures. A safer, better solution is to buy a fast twist, aftermarket barrel. A gunsmith should be able to do the job for between $350 and $700, barrel included.

Shop Sportsman’s Guide for a great selection of Rifles!

Shop The Sportsman’s Guide great selection of Rifle Ammunition!

 

 

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4 Responses to “Rifling Twist Rates & Bullets”

  1. Mike Mullen

    Thanks Ron, I feel better informed about twist rates now!

    Thank You for the tips on rifle twist rates! I feel better informed to make choices in the future.

    Reply
  2. Dave

    I have a Colt 223 military AR 15
    I have been told it has a 1 in 7 twist ?? And it would shoot it best pattern with a upper 60’s grain bullet ??
    Before I start target shooting with it could you direct me on what cartridge would be the best for accuracy ? If all the above is true then I assume 55 grain is not recommended ?
    Thank You for your time

    Reply
  3. Rob Menagh

    Interesting… This could be the reason my Arisaka does not like the 140 grn cast lead bullets that I make. I’ll have to look in to shortening them…

    Reply
  4. glenn chaney

    Reading your 30-06 brought back recollection of a Remington 742 Carbine in 30-06 I acquired back in the 80’s when I don’t recall any conversations about twist rates, we were all carrying wooden furniture hunting rifles. I loved that carbine, it would snatch right out of the FJ-40 Landcruiser and never bump anything. Took it to the Sheriff’s range and she was tumbling bullets. I got rid of a very nice gun, likely due to that green and yellow ammo they slupped in Florida.
    I got a taste of bullet blowup in Bama on bigger deer with my Rem 700 .270. Assumed I needed to go back to the 30-06, but wait…Along came Nostler Partitioned Federal Premium and life was suddenly good. It is the bullet. Transfer of energy is the bottom line to terminal ballistics.
    Now, getting old, those old hunters hurt if you shoot for fun, so I finally joined the black rifle crowd.
    5.56 1X9 at first, then a 1×7, then a 300 AAC, then some 7.62 x39’s at 1×10, and now building the 6.5 Grendel for the needed 500 plus yards for the piggies. Only my recent exposure to building, and needs for bigger bullets for varmiting forced me to become educated.
    Excellent wake-up call to remind folks that it’s “All about the base” I mean the bullet.

    Reply