1:8 Twist

More
14 years 7 months ago #4090 by huntenthusiast
1:8 Twist was created by huntenthusiast
In a 1:8 twist barrel, what is the range in bullet weight can you reasonably run? Are there any charts available online for twist and bullet weights for the .308? Thank you in advance. Jim

Please Log in or Create an account to join the conversation.

More
14 years 7 months ago #4092 by The Duck of Death
Replied by The Duck of Death on topic Re:1:8 Twist
That's a pretty quick spin for a .308 Winchester. I

Please Log in or Create an account to join the conversation.

  • a Guest
  • Visitor
  • Visitor
14 years 7 months ago - 14 years 6 months ago #4095 by a Guest
Replied by a Guest on topic Re:1:8 Twist
Last edit: 14 years 6 months ago by a Guest.

Please Log in or Create an account to join the conversation.

  • a Guest
  • Visitor
  • Visitor
14 years 7 months ago - 14 years 6 months ago #4096 by a Guest
Replied by a Guest on topic Re:1:8 Twist
Last edit: 14 years 6 months ago by a Guest.

Please Log in or Create an account to join the conversation.

More
14 years 7 months ago #4098 by huntenthusiast
Replied by huntenthusiast on topic Re:1:8 Twist
Does that mean since the 1" bullet is most likely heavier than the 1/2" one, it needs a faster twist rate for stability?

Please Log in or Create an account to join the conversation.

  • a Guest
  • Visitor
  • Visitor
14 years 7 months ago - 14 years 6 months ago #4099 by a Guest
Replied by a Guest on topic Re:1:8 Twist
Last edit: 14 years 6 months ago by a Guest.

Please Log in or Create an account to join the conversation.

More
14 years 7 months ago - 14 years 7 months ago #4102 by The Duck of Death
Replied by The Duck of Death on topic Re:1:8 Twist
Here's a place to start. Gunsmiths generally use the Greenhill Formula to determine the optimal twist for a given bullet. The formula is T=150(d/r) for velocities from about 1500 to 2800 fps. Substitute 180 for the 150 value for velocities exceeding 2800 fps. "T" is the twist rate. "d" is the bullet diameter. "r" is the bullet length to diameter ratio (length of bullet divided by diameter). In .308, it works out fairly well. Sierra 168 grain Matchking is 0.308 inches in diameter. Bullet length is about 1.210 inches so we have a length to diameter ratio of 3.929. 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 11 3/4 inch twist
Last edit: 14 years 7 months ago by The Duck of Death.

Please Log in or Create an account to join the conversation.

More
14 years 7 months ago #4108 by silver78
Replied by silver78 on topic Re:1:8 Twist
I not a rocket scientist on ths topic, but from the little I know a 1 in 8 twist is not needed for most commercially available 308 loads. So unless you reload it's not needed.

I believe there are some disadvantages to fast twist rates too. Perhaps MSH can fill us in on the downside of fast twist rates.

Please Log in or Create an account to join the conversation.

More
14 years 7 months ago #4110 by The Duck of Death
Replied by The Duck of Death on topic Re:1:8 Twist
The spin imparted on the bullet by the rifling stabilizes the bullet like a gyroscope. The bullet will keep its orientation, to a point, as long as it is spinning fast enough. The bullet is traveling perfectly true to its axis as it leaves the barrel but then begins to fall. It accelerates downward. As it falls it now is no longer traveling true to its axis. The bullet would need to point down a little bit more as it increases the downward velocity. If the bullet orientation could fallow this arced movement, that would be perfectly stabilized. If the bullet is over stabilized it will continue to point in the direction it left the barrel and pointing up relative to its direction of travel. This adds pressure to the bottom of the bullet and will cause it to move left or right like a curve ball, and will also decrease its ballistic potential.

There is a lot more to it than just bullet length, weight and shape. The balance of all these parameters with the air pressure on it, the velocity it is traveling, center of balance/mass point,air pressure, gravity, alignment of the moon and stars... :woohoo:

Please Log in or Create an account to join the conversation.

  • a Guest
  • Visitor
  • Visitor
14 years 7 months ago - 14 years 6 months ago #4111 by a Guest
Replied by a Guest on topic Re:1:8 Twist
Last edit: 14 years 6 months ago by a Guest.

Please Log in or Create an account to join the conversation.