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Relationship Between Twist and MV

 
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Old 10-06-2005, 10:40 AM
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Relationship Between Twist and MV

Is see that larger bullets need a faster twist rate to perform. Is this because, for a given caliber, the larger pills have a lower MV, thus need to be spun faster to obtain the desired spin rate? Using my 243 for example, it shoots 100 gr bullets better than 70 gr. Twist is 1 revolution in 10 inches. At 2900fps this corresponds to 3480rps (revs per second). Taking the 70 gr pill at 3400 fps you get 4080rps. Some bbl makers show you should use a twist of 11 or 12 if you want to shoot a 70 grain bullet. With a twist of 12 at 3400fps you get 3400rps.

So if I want to shoot the 70gr projectiles better out of my 243 is it simply a matter of reducing the load to match the MV of the 100 gr bullets or am I looking at this like a dorky engineer who needs to get out in the woods more often. [img]/ubbthreads/images/graemlins/wink.gif[/img]
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Old 10-06-2005, 12:20 PM
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Re: Relationship Between Twist and MV

One train driver to another.

Consider twist as spin of a gyroscope - you have to have at least enough and it is OK to have more.

An important factor in stabilizing bullets are the moments around the centroid. Long boattails, long ogives and short bearing surfaces need more twist than a heavier bullet with no boattail, regular ogive and long bearing surface. Look at the 95 grain Bergers which need a 1-9 twist but are lighter than the 100 grain Sierra hunting bullets which work well with a 1-10.

Another factor that helps stabilize a bullet is to start it in the lands with no jump.

I do not know which 70 gr you are trying to shoot. If it is the SMK, then I would try seating it out a little further and different powder and primer combinations. I do not think twist and muzzle velocity is the issue. I shoot it (not a 243 Win)at 3700 fps in 1-10 twist into something like 0.5+MOA (3 shots) out to about 500 yds. The 100 gr gameking bullet is grouping about 0.7 for 5 shots at a velocity of maybe 3300-3400fps (don't have my manual handy). A 1-10 twist should stabilize the 70 gr bullet. Of course a good group for me might be considered totally unacceptable by another person.
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Old 10-06-2005, 03:06 PM
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Re: Relationship Between Twist and MV

Bullet stability is dependant on it's design(mostly length), and shooting conditions(mostly twist -distance traveled per turn -NOT RPMs).
Ignore folklore of RPM requirements.
Velocity has little effect, and sways only bullets on the very edge of stable.

IAW, it just ain't that simple.
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Old 10-06-2005, 04:38 PM
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Re: Relationship Between Twist and MV

Well I guess that blows my theories. Most times when something appears too simple to be true it usually is. I'll be doing some testing over the weekend. I haven't really checked to see how far out I can seat the 70gr SMKs. I suspect the jump may be more than optimum. I'll take .5 MOA all day long [img]/ubbthreads/images/graemlins/laugh.gif[/img].
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