Which is Faster Twist Rate
 

I have been wondering which is faster and which is slower where twist rates are concerned.

GivenTwist Rates: 1:9, 1:8, 1:7 Which is fastest and which is slowest?

I am assuming that 1:7 is faster than 1:9 but I don't know for sure. Have I got it backwards?

You are correct.

1:9 is 1 full turn in 9" of travel, so 1 full turn in 7" of travel (barrel length) is a faster spin. :)

You have it right. Twist rate is just...number of rotations:inches of travel down barrel.

So assuming a 20 inch barrel a 1:7 will spin the bullet almost 3 full revolutions, a 1:9 a little more than 2, a 1:12 a bit less than 2, etc...

I have heard that some ammo works better with a fast or slow twist rate. Any explanation for this?

I believe it was a heavier grained bullet works better with a slower twist rate and a lighter grain works better in a faster twist rate. Have I got this right?

Any explanation on why this is so will be helpful.

No, I got it backwards, heavier bullets work better with a faster twist rate and lighter bullets work better with a slower twist rate. But why is this true.

I found a thread in the Mini 14 forum and a guy said:

1:9 is best for 55 gr
1:8 is best for 62 gr
1:7 is best for 72-77

Just remember, the bigger the object the harder it is to handle, or stabilize. The bigger the bullet, the faster the spin, the more stable it becomes.

Simple physics...a barrel is rifled to spin the bullet in order to stabilize it in flight. A heavier object needs more spin to stabilize hence the faster twist rates, the bullet is spinner faster as it exits the barrel. Lower grain rounds, say 40 gr, typically have thinner copper jacketing that cannot withstand the higher spin rates of a faster barrel. While it is impossible to "overstabilize" a round it is possible for it to disintegrate in flight due to the excessive spin.

In reality a custom rifle will have an optimal barrel length, bullet weight and twist rate, all set to maximize performance at specific distances. Commercial rifles must compromise in order to accomodate the various types of shooting and available bullet weights so you will typically see 1:9 or 1:7 in most off-the-shelf rifles. These rifles will perform well across the entire spectrum of available loads but will have less than optimum performance at either extreme.

BTW, the original AR was designed for a 55gr round with a 1:12, 20" barrel. What was good enough for Mr. Stoner is good enough for me...:)

i totally agree. 1:12 55grn fmj my favorite non meat hunting round. altho 20" 1:12 chrome lined cmp barrels are getting hard to find. right now my sp1 colt has a m16-a1 barrel on it. the original is in the back of the safe

Whiplash, not everything is so cut and dry. I shoot 52gr to 65 grain in my 1:9's, but would not worry about a 75gr which would depend upon the bullet consruction itself.

In point of fact, it's not weight, per se...
 

It is the length of the bullet.

Of course, if one is using the same basic material for the bullet (lead and copper jackets, typically) one cannot produce a heavier bullet without making it longer. (Obviously, it cannot be wider, and lead cannot be compressed in any normal sense of the word.)

I have a .22-250 rifle that will shoot 70 grain RN flat base bullets very well, but will NOT group 69 grain HPBT bullets for love nor money.

Do a search for 'Greenhill Formula' and one may read the entire explanation. Greenhill's work is a bit dated, but it is still the basis for all modern research.

And remember, a lot of overspin is better (for accuracy) than just a little underspin.