Barrel Twist

[JDT]

I keep reading that faster twist barrels are better at stabilizing longer bullets. What are slower twist barrels better at? Why don't all rifles simply come with super fast twist rates?

 

[wapiti13]

JDT, Good question. You must match the twist to the entended use (bullet used). The reasons are many, but the short answer is speed and stability. They used to use a formula for twist rates that compared caliber, bullet length, & velocity to determine minimum twist. New theories and bullet designs have somewhat made the formula inaccurate, but the concept is still valid. Barrels with a high velocity round don't need as much twist to stabilize a given bullet as a barrel for the same bullet but with a slower velocity. The goal is stabilization,but the faster the twist the slower the barrel will shoot. Also, if you over twist some varmint type bullets can actually come apart in flight since they are spinning too fast for their design. Most factory barrels are designed around the "normal" loads used, but if you start shooting speciality bullets like the vld designs (much longer bullets by design), then you will probably need to rebarrel to a faster twist to stabilize the longer length. Hope this helps clarify a cloudy situation. /ubbthreads/images/graemlins/confused.gif

 

[CPorter]

Yes, the longer the bullet is the more spin it needs to stabilize. There are several reasons why not all guns have real fast twists. Faster spin exaggerates the centrifugal effects on the bullet. The bullet is forced to rotate around the bore axis as it goes down the barrel. When it leaves the barrel it starts to rotate around it's center of mass. It precesses (spirals) around the line of flight until this happens. The faster the bullet spins the longer this takes. Also the are some gyroscopic effects. The faster the spin the more nose up attitude the bullet tends to have to the line of flight(bigger hole in the target). 100 yard target shooters know these effects and choose the minimal twist needed to stabilize thier bullets. Also the faster the twist the more energy is needed to rotate it and this make the bullet slower for a given powder load.

Bullet design comes into play with this also. Thin jacketed bullets, especially those with exposed cores at the base, can't take as much spin without rupturing. These will be listed for a certain speed out of the test gun. The Speer TNT is an example. In .243 it is rated to 3500 FPS out of a 1:10 twist test gun. Faster speeds or rougher bores causes the bullet to blow up at 50-75 yards.

http://www.exteriorballistics.com/ebexplained/index.cfm