I am fairly new to rifles, I could tell you a lot about shotguns but rifles I am clueless about. But anyway I am building a AR-15 for Coyote hunting. It has a 1-9 twist rate. I hear that different grain bullets wont shot very accuratly because of the twist rate. Can anyone explain this to me? For example Barnes Bullets reccomends that you use a 1-10 or faster twist rate for their 50gr Varmint bullet but they dont say anything for their 36 gr bullet.

I've been told that you need to worry more when shooting heavier vs shooting light.....so if a 1:10 works for 50gr, it should shoot 36gr ok/well, but if you wanted to shoot 75gr, you could run into some problems.

if 1:10 is ideal for 50gr, then over 50gr, you would want a tighter twist....like a 1:8, for lighter bullets, a 1:12 or 1:14 may be ideal.

though if a 1:14 would be ideal for 36gr, and then you used 50gr in it, accuracy may be thrown out, so I think manufacturers tend to use tighter twists, to cover the spectrum better. ie a 1:8 would work better for everything, whereas a 1:14 would only work for the lighter bullets.

Sure other guys will have a more precise answer for you.

If you do decide to use that rifle with the 1:9 and barnes recommends a 1:10 for 50gr, but you want to shoot 36gr, you may consider using a heavier load, or another rifle/barrel with a twist more ideal for lighter loads.

thanks for the info salukipu1 I forgot to mention, that they reccomend a 1-10 or faster twist rate. Sorry that was my fault, i edited my post now. But based on what you told me I think my 1-9 should work. thanks for the info

Actually, it is BULLET LENGTH, not bullet weight, that determines how fast the bullet has to spin to keep flying point-forward. For example, a rifle which has a rifling twisttoo slow to stabilize a 107-grain spitzer boattail bullet might shoot quite well with a stubby round-nose bullet that weighs exactly the same.

IF you are able to measure the LENGTH of the longest bullet you plan to use, you can calculate the spin required to shoot it accurately.

The Greehill Formula lets you calculate this twist requirement:

The formula is T=150 (d/r) for velocities from about 1500 to 2800 fps. (Substitute "180" for the "150" value for muzzle elocities exceeding 2800 fps.) Where "T" is the twist rate; "d" is the bullet diameter. "r" is the bullet length to diameter ratio (length of bullet divided by diameter).

EXAMPLE:

In .308 caliber, it works out fairly easily. Sierra 168 grain Matchking is 0.308 inches in diameter. Bullet length here is1.210 inches, so we have a length to diameter ratio of 3.929 (the bullet is 3.929 calibers long!)

Plugging this value into the formula and using the 150 constant (the proven best velocity for the 168 grain MK is 2550 - 2600 fps), we get T=150 x (.308/3.929); T=150 x 0.078; T=11.76 inches, or required twist isone rotation of the bullet for every 11.76 inchestraveled.

Tc = 150/(1.21/.308) ; Tc = 150/3.9285 = 38.1818 CALIBERS- 38.1818X.308" = 11.759", (or 1 turn in 12")

For MV's over 2800 FPS, the constant changes to 180 instead of 1540.....

IF your rifling twist is TOO SLOW for the bullet, it willkeyhole and shoot wild. If your twist is too fast, the bullet will be "overstabilized" or spunfaster than need be. However, "overstabilization" is not the problem it once was, as bullets are much better these days. It is much better for a bullet to be spun faster than necessary than vice-versa. So for example, a 1/8" twist used with a bullet requiring 1/10" is no big deal.....

I'm pretty certain to be purchasing a .243win with a 1:10 barrel twist, what grain bullets would this shoot best/well?

I'm planning to mainly shoot 90-100gr.

Though occasionally maybe some 75gr....maybe even 55gr, though if neither woudl shoot well, then why bother.

Also if it wont shoot the 90-100gr well, I don't want that either, though I assume itll shoot 90-100 one of those loads well.

http://www.riflebarrels.com/products/caliber_twist_rates.htm

found this link, shows rate twists, by caliber, and bullet weight.....

I too prefer to use a twist that is a little on the fast side, as one that is too slow will keyhole with heavier bullets. But the fast one only has the potential for shooting somewhat bigger groups with the lighter bullets!

I have a 6mm/284 with a Douglas Premium grade 1/10" twist barrel on it. It was built and chambered specifically to shoot the 105-grain Speer spitzer seated even with the base of the neck. (When I built it, that was the longest, heaviest .243" bullet being made.) It sure works well with those bullets, and extremely well with the 100-grain Sierra. I have not tried the 107-grain Sierra HPBT match bullets in it yet, but probably will.

This is five shots @ 100 yards with the 100-grain Sierra......

Elde your post was awesome and informitive as always, but let me give a laymens/readers digest version:

The reason bullet length, not weight, is the reason you need a faster twist is because the diameter (caliber) of a bullet can only get so large. You cannot fit a .244 bullet down a .223 bore. So when the weight goes up, since you can't add diameter, you have to add length. This goes double for bullets such as the barnes TSX that are solid copper. Lead (and in the case of the Coyote bullets tungsten) is heavier than copper. So it takesa greater volume of copper to make 70 grains than it does lead. Asolid copper .223 70gr bullet may be as much as 20-30% longer than itscopper/lead core counterpart of the same weight.

Think about a football and a baseball. No matter how you throw a baseball, its going to be stable, because its round. As long as it has a constant spin and is truely round.The stitches can make it curve or tail, but if it were smooth, it would fly true. A football, however, will travel end over end (like a punt) unless it has spin. So in order to throw a perfect spiral, you have to snap your wrist. If you don't give the ball enough snap.. it willtumble.... exact same thing with bullets. The longer the bullet, the more twist you need to stabilze it.


Salukipv1... I've read you post several times that you cannot for the life of you understand why anyone would want to shoot a .243 when the 6mm is clearly superior ballistically. Twist rate is actually the answer. In the60s when the two were developed, (and they were developed at almost the same time), Remington figured this to be thenext .22-250... flat shooting, long rangevarmint caliber. Winchester envisioned the rifle pulling doubleduty for varmints andmedium game. Remington realeased their offering (initially called the .244 Remington) with a 1:12 twist, geared towards shooting 50-75grain bullets for varmints. Winchester released its .243 with a faster 1:10 twist, that not only shot the lighter 55gr bullets well, but also the 95 and 100gr bullets. As it turned out, the6mm family never really took off as a varmint cartridge likeRemington anticipated... but it sure did take off as a medium game cartridge. The .244 wouldn't stabilize the heavier bullets used by deer hunters and the chambering gained a terrible reputation forkeyholing bullets and just generallydreadful accuracy.Remington saw the error of its ways pretty fast anda few years later re-realsed the cartridge under a new name (the 6mm Remington)with a 1:10 twist barrel. But the die was cast and the damage was done. Subsequently, in answer to your question about which barrel twist would be best for you... 1:10, or quicker if you are going to have a custom barrel made.