Rifling Question
 

I have noticed that most fast shooting .224 caliber rifles like a 22-250 or a 220 swift have a twist rate of about 1 in 14 or 1 in 12. Although I have read that if you want to shoot a heavier bullet like a 75 grain, you need a twist rate of 1 in 8 or faster. Why not use a fast twist rate for the lighter bullets also, or is that a problem? And if not why dont factories originally have the barrel with a twist rate of 1 in 8?

Because those cartridges are primarily varmint cartridges designed to shoot light weight varmint bullets. When you are shooting fast cartridges like the 22-250 and 220 Swift if you spin the bullet so fast that the light weight, fragile varmint bullets can literally come apart in the air.

Basically the longer the bullet is for a specific bullet diameter the faster you will need to spin that bullet in order for it to be stable.

Also the higher the bullets velocity is out of the barrel the slower the twist rate can be and still achieve the required RPM's to stabilize the bullet.

For example regarding the velocity.....
3000 fps with a 1 in 8 twist will result in 270,000 rpm's
4000 fps with a 1 in 8 twist will result in 360,000 rpm's

So assuming 270,000 rpm's will stabilize a specific bullet if you are going to drive that bullet at 4000 fps instead of 3000 fps then the required twist would only be 1 in about 10.5 inches for that bullet.

220Swift your question and Bigbulls answer just about sums up a lot of the debates on why people don't think their rifles won't shoot and are not accurate. First people go out and just buy a rifle. They didn't put any thought into what the intended use was for or what grain of bullet they wanted to shoot through it or the velocity at which they wanted that particular bullet to travel at. It causes a huge dilemma for the rifle owner to figure out. In today's information highway it shouldn't be such a big deal to have the knowledge before you actually purchase a rifle.

About 15 years ago I learned this the hard way. I bought a 22-250 with a 1-14 twist. I went straight to 60gr bullets and with a brand new rifle I was only getting 2 1/4" groups man I was upset. Then I tried some factory 55 gr loads and got some modest 1 1/2" size groups. I was besides myself thinking .223 caliber ought to be dang near holes in holes at 100 yards. Standing in Wal-mart I saw these 45gr Winchesters 4000 fps and I could get 40 rounds for $18.00 so I thought what the heck I'll try them. And they shot 3/4" groups everytime. Then a little light started going off in my head what if this rifle likes smaller grain bullets. So back to the reloading bench I went. And today the same rifle shoots under 1/2" groups every time at 100yds using 52gr bullets. I was upset, had no faith in the rifle even though it was one of my favorite models and I just about sold it. AND THAT ALL BOILED DOWN TO MY LACK OF KNOWLEDGE AT THE TIME. I paid a little for my education but some are paying a lot more than I did for it.

Rates of twists and optimal bullet weights for those twists is a good topic and a great starting place when your thinking about purchasing a new rifle for a particular usage. If your going to shoot 75 gr Amax's out of a .223 you better get the barrel that fits that bullet.

Blackelk> is there any good way to determine an unknown rate of twist, other than trying to count rotations of a land or groove and divide by barrel length? I'm never sure if I followed the same one all the way.

Go to the rifle's manufacturer and tell them your model of rifle and possibly the serial number depending on the year it was made, all depends on how many of those models and changes there are on that particular model. If you know the year you purchased it new that works too. I've personally never tried using a rod and counting the inches per one revolution method. I've always looked it up or contacted the manufacturer if I didn't know.

So lets say i have a 30.06 or a 308 that i want to use for both big game and varmit.What would be a starting point for the varmit load? Lets say both rifles have a 1:10 twist rate.Is it possible to achieve good accuracy out a lighter bullet,say 60g-70g if the rifle is accurate with 180g?

....The Greenhill formula goes like this: 150 divided by the bullet length in CALIBERS equals the required twist rate in CALIBERS.
Let's say you have a .224, 75 gr. bullet 4.5 calibers long (1.008").
150 divided by 4.5= 33.33. So, the rate of twist needs to be one turn in 33.33 CALIBERS. Your caliber is .224, so 33.33 x .224= 1 turn in 7.46". In this example, the bullet MIGHT be stabilized in a 1 in 8" twist, but should definitely stabilize in a 1 in 7" twist. MV does help some in stabilizing bullets fired in a too-slow twist. It's better to have a bullet spinning too fast or "overstabilized" than too slow.

I always love discussions on this subject, because I hear that one of the big advantages of a .30-06 is that it can shoot bullets from 55gr. to 225gr.

I guess they just forgot to mention what kind of groups they get from such a wide range of bullets weights.

Any better explainations ?

You can't go that light, but you may be able to go as low as 110 grains and get decent groups at minute of ground hog accuracy at short-medium ranges. Just don't expect to clover leafs at 300 yards.

Thanks RR, I'll give it a try.

Beyond the Greenhill formula mentioned above to determine optimum rate of twist, is there a calculation to determine optimum RPM for a bullet to stabilize?