does twist rate affect accuracy

[mxreloader]

is it true that if you shoot to heavey\to light bullets out of a gun with a certian twist it wont be accruate? also how is one supposed to mesaure the twist on a barrel:barf:

[BillCA]

Twist rate certainly does affect accuracy.

Other factors include the expected velocity of the bullet and its weight. If you consider most rifles range from 1-7 to 1-12 twist while most handguns tend to be slower twists (1-14 to 1-18).

I'm sure someone smarter than me will come along with some kind of formula to describe it.

[Unclenick]

It's actually bullet length more than weight that matters, but yes, twist rates are chosen to be optimal with certain bullets. For example, .222 Remington rifles are chambered in 14" pitch barrels (one turn every 14"). Most 50 grain jacket and core bullets and cast lead bullets are in the right length range to do best with that twist. But if you try to shoot a 69 grian match bullet in that twist it will become unstable, tumble and whiz off like a curve ball and often won't even stay on paper, depending on the range. The military originally used 12" twists for bullets that diameter, then switched to 9" to better stabilize them up in the 62 grain range. Most match shooters running 69-80 grain bullets have 7" or 8" twist barrels.

If you run the short bullets in the fast twists, they go so much faster down the tube that the number of RPM's the fast twist gives them can be too high for the bullet structure to take it without flying apart. If that does not happen, the faster spin rate can sometimes cause core stripping, where the lead core slips inside the jacket during rotational acceleration in the barrel, and when those bullets exit their mass is no longer symmetrically distributed so they wobble in flight and the groups open up. Even if that doesn't happen, spinning faster than necessary exaggerates any minor imperfection in manufactured mass symmetry of the bullet, making it wobble more in flight than the correct twist rate would.

The other thing that happens when a bullet spins too fast (is "overstabilized") is its flight trajectory winds up with higher yaw off the line of trajectory. This increases drag, effectively lowering the ballistic coefficient and decreasing maximum range and increasing wind drift. This typically matters only at longer ranges. Though the bullet may still shoot well at, say 100 yard, sometimes at 200 yards and almost always at 300 yards and beyond, the difference starts to tell.

You can determine the twist rate you need from the length tables and the stability calculator at the JBM ballistics site.

You can measure twist by putting a tight dry patch on your cleaning rod, putting a pencil mark on the rod, then seeing how far down the barrel you have to push the rod to make that mark rotate one turn. That distance will be your rifling pitch. Obviously you have to push the rod with a light grip that lets it turn.

[Dave R]

I am not the expert here, but I'll share what I read on the internet, and some personal experience.

In the world of ARs and .223 varmit rifles, you can get twists ranging from 1:12 to 1:7. The 1:12 twists will stabilize bullet weights up to 55gr, maybe 62gr. It really shines with the 40-45gr. bullets. I have an NEF with 1:12 that is brilliant with 40gr. V-max's.

The 1:9's will stabilize anything up to maybe 72 gr, and for the really heavy bullets over 72gr, you need the 1:7 twist.

OTOH, the 1:7 twist may not perform well with the lighter bullets, because it may spin them fast enough to rip 'em apart in flight. Especially at higher velocities (which is why you're shooting the light bullets anyway, right?) That has happened to me, shooting 45 gr. bullets through a 1:7 rifle. Interesting reading those targets...

In other calibers, the same principle applies, but I'm not as clear on bullet weights. On a .308, I believe you'll want a 1:10 to stabilize the 175gr and heavier bullets, and 1:12 will do fine up to 168 gr?

[mxreloader]

that explaines why when i shoot 70gr bullets its not as accurate as when i shoot 40 grain bullets then?

[zippy13]

For a specific bullet and velocity, in the extreme: too little twist won't be sufficient to stabilize the bullet so it may tumble, and too much twist can create enough force to spin the jacket off of the core. The twist rate of your barrel is based on the gun makers best guess of what velocity you'll be shooting.

To get an idea about twist rates, check out a selection of AR-15 barrels. The twist rates vary significantly, but recommendations for what bullet weight (and corresponding velocity) goes best with which rate are pretty much standard.

As far a determining the rate of twist, I've always taken the gun maker's specs as gospel. Perhaps there's a Doubting Thomas out there who has any easy way to verify twist rates.

[Prof Young]

Some time ago I started reloading my own 223 with a heavier bullet. 68 gr as opposed to the 55 gr I'd been shooting. My accuracy went out the door. In fact my bullets were tumbling. That's when I learned about twist rate. My rem seven had a one in 12 rate. The heavier bullets needed a one in ten or better to fly right. I sold the rem seven bought a savage rifle with one in nine twist and am a happy guy.

Live well, be safe
Prof Young

[mxreloader]

should the gun say what twist rate is has?

[DiscoRacing]

nope
what caliber you talking about

[ScottRiqui]

should the gun say what twist rate is has?

Depends on the gun. My AR has the twist rate stamped on the barrel.

[Dave R]

You can figger it yourself. Put a tight patch on a cleaning rod. Put a "flag" of tape on the rod so you can see when its made one revolution. Push it down the bore, and stop when the rod has made one revolution. Measure the distance. There ya go.

[Jim243]

MX

that explaines why when i shoot 70gr bullets its not as accurate as when i shoot 40 grain bullets then?

That is correct. Depending on the twist rate of your rifle.

It is what Unclenick said, but to put it in plain language if you take a 40 grain 223 (which is .224 inches in diameter) and a 70 grain 223 (also .224 inches in diameter) how do you get the extra weight into the bullet, the answer is to make it longer since no matter what it has to be .224 inches wide.

That extra weight (length) now pushes the center of balance of the bullet forward and makes it more unstable so it needs more spin to stablize. Just as you would throw a fast ball FASTER with more spin (to go straighter) on it than a curve ball which requires a different type of spin and less speed to grab the wind.

Lighter/shorter bullets require less spin (say 1 revolution per 12 inches 1:12) than a heavier/longer bullet (say 1 revolution per 7 inches 1:7). And since not all bullets are made the same (quality) the lighter/shorter bullet has a tendancy to pull the jacket apart from the core of the bullet when it is excessly spun and will fragment before it hits the target. Today some of the better made lighter/shorter bullets have the jacket bonded to the core and can take the extra spin, but of course they are more expensive to purchase.

For a quick take on the twist of YOUR rifles use the cleaning rod method as mentioned in the above posts.

Good luck and stay safe.
Jim

[mxreloader]

Disco
I'm talking about a 22-250...if i can't get it to shoot right,ill sell it to you cheap lol

[DiscoRacing]

how cheap?? ... i had the same trouble with mine. i used 69 grain bullets, 62 grain, 55 grain, 45 grain, and 40grain.

[DiscoRacing]

stop down here... I have some bullets you can try.

[Unclenick]

I want to emphasize again that bullet length is key. Length is the lever arm by which air pressure overturns the bullet. You will sometimes find, for example, that you can stabilize a heavier round nose bullet than a pointed one, just because it is shorter. Or, a weight you've done fine with, shooting normal jacket and core bullets, will be unstable if you go to a solid copper bullet that same weight because the less dense copper bullet is longer.

So, it's a balancing act. Unfortunately it means that for all serious accuracy work you need to decide in advance what kind of bullets you want to shoot. Explosively fast, light and short flat base varmint bullets will send you to longer twist pitch, while heavy VLD long range match bullets will send you to shorter pitch. You need to know what you are buying the gun for? Alas, one size won't fit all.

[Farmland]

Does it matter if the gun is a hand gun?

[farmerboy]

Thanks JIM243, very good information there!

[Unclenick]

Farmland,

The same rules of stability apply. A bullet too long won't shoot out of some handgun barrel twist rates without tumbling.