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Old June 10, 2010, 02:23 PM   #1
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Is there an optimum bullet length:diameter ratio for accuracy?

I am finally doing a very systematic approach to each rifle I have in the quest to find one or two very accurate loads for each one. One thing I have noticed so far is that I am having more trouble locating a great accuracy load for a 170 gr .323 bullet for my 325 WSM than any other cartridge/bullet combination. Maybe I just haven't hit on the right powder yet. But it occurred to me that compared to the round that has been the easiest so far (a .270 WSM with a 130 gr bullet), that particular bullet is more "stubby" than any other. I measured some of the bullet lengths that I have been working up loads for so far. Then I divided the total length of the bullet itself by the diameter to get a ratio. Perhaps it's just coincidence so far, but those bullets that have a ratio of say 4.4 or 4.5:1 of length versus diameter seem to be pretty accuraate no matter what powder or charge I use. Whereas the previously mentioned bullet has a ratio of 3.1:1 of length to diameter and it's tough getting accuracy out of that one.

Is this a general rule, or just a function of my limited experience at loading different rifle calibers?
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Old June 10, 2010, 02:42 PM   #2
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I don't think you can boil it down to one figure of merit and say that is what you need. I would just accept it that a 170 grain bullet was not a good fit for that barrel and try something else. I think that would be more rewarding than changing powders.
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Old June 10, 2010, 02:54 PM   #3
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At 100 yards, just about any well-constructed bullet will be accurate.

At about 300-400 yards, you start seeing the longer bullets with higher "ballistic coefficients" being more consistent in downrange accuracy.

Your ratio calculation has a lot in common with a ballistic coefficient.

And, as a rule, heavier bullets have higher ballistic coefficients and tend to be more accurate... as long as your rifle barrel has a rate of twist that sufficiently stabilizes that weight of bullet.

Take the .223 cartridge for example. A 1:12 twist typical varminting bolt action rifle will stabilize up to a 55gr bullet. Step up to 62gr and the bullet starts to tumble. But, a 1:9 twist will work fine with 62 and 69 gr bullets, but starts to lose the ability to stabilize the 75-80gr match bullets. So, 1:7 is popular with folks shooting the heavy match bullets.

But if you shot that 55gr bullet, even in a 1:7 twist barrel, at targets 300+ yards away, you'd have a lot of group dispersion because the bullet just doesn't ride the wind consistently or overcome air resistance consistently.

I don't own a .323 bore, but I would expect the .323 and .338 bores to do best with 200-250gr bullets.
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Old June 10, 2010, 08:28 PM   #4
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The problem in your theory is that you do not take velocity into the factors. And it is a big one.
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Old June 11, 2010, 12:10 AM   #5
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I would definitely step up from 170 grains.

Even in 8x57JS, the 195+ grain bullets seem to do better than lighter stuff. For the WSM, I would be looking at the 200-225gr range.

If it were me, I would be trying some of the following:
200gr Accubond (longer than most 220gr SPs)
200gr Partition
200gr Triple Shock (ridiculously long)
220gr Woodleigh Protected Point
220gr Swift A-Frame
220gr Sierra Game King

There are more 200-250gr bullets out there, but they're not as main-stream; and either pricey, fairly stubby, or hard to find.
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Old June 12, 2010, 01:53 AM   #6
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Depending on the bullet you are using, you are pretty close on the calculation.

It will really depend on the center of gravity of the bullet you are using. A simple test is to just drop the bullet from an out streached arm point down and see how it lands. If it lands nose first it's center of gravity is toward the front of the bullet and will need less stabilization. If it lands tail first, the center of gravity is towards the rear of the bullet and will need to be pushed faster and can use a faster twist rate than one that is nose heavy.

As the bullet travels through the air the center of gravity is pushed faster than the lighter areas of the bullet, so the best bullet is one that the center of gravity is at mid point of the bullet.

A bullet with the center of gravity shifted far to the rear will require much higher spin rate to maintain nose-forward attitude compared to a bullet with the CG shifted further forward. Most bullets have their center of gravity somewhat aft of the linear midpoint. If dropped from a height with the nose pointed down, they would land base first. A nose-heavy bullet, on the other hand, would land nose first. In flight it would require less spin to maintain nose-forward attitude. With the CG moved too far forward, the bullet would require little or no spin to strike nose first, but would follow the launch attitude over its trajectory path (that is, the nose would continue to point in the direction and angle at which it left the barrel, so that as the trajectory arc became larger with greater distances, the bullet would tend to fly at an angle to the direction its nose pointed, and would begin to tumble).
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