Bullet weight vs length / barrel twist

[trickytune]

Ok I see different formulas for calculating barrel twist and have a question. My 1:12 .223 groups well with short 63g bullets but not as well with longer 55g bullets. Now going off bergers website and using their calculator if I shorten the bullet stability increases. So for example if I have a 75g amax thats way too long for stability then remove the tips the lengths will be shorter but weight almost the same. These should group better? Im thinking about testing some Custom comp or Sierra match king bullets that are progressively machined shorter to see how it affects the groups. Anyone done any testing with this?

 

[J E Custom]

Normally bullet weight dictates barrel twist because of velocity and BCs of the bullet used.

The heavier the bullet the slower the velocity and the faster the twist rate normally.

Trimming the meplat may improve the ballistics of the bullets (More uniform) but it should not effect the twist rate.

The object of proper twist rate is to stabilize the bullet (You can over rotate the bullet just as easy as under rotating it by picking the wrong twist.

The most common twist rate for a 175 grain+ bullet in a .223 is a 1 in 7. and a 1 in 8 for 50 to 70 grain bullets a 1 in 9 or 1 in 10 are normally used with bullets weighting less than 55 grains. The fast 22s (220 swift and the 22 250 will normally have a 1 in 12 to 1 in 14 twist because of there velocity.

To get good groups you may have to go to 45 or 50 grain bullets.

J E CUSTOM

 

[trickytune]

Its more out of interest. The stability calculator gives a higher number the shorter it is. And with a hollow point bullet almost no weight would be removed when shortening. I am just trying to understand how length without weight can affect stability.

 

[MudRunner2005]

Lighter bullets are normally shorter. And shorter bullets are generally shorter in the bearing surface (where the jacket meets the rifling). The longer the bearing surface, the faster twist you will need to stabilize it. The shorter the bearing surface, the slower the twist will be required. Which is why it is probably telling you that a shorter bullet in the same weight will increase in stability...Because it is probably taking the measurement differences out of the bearing surface. Sometimes, bullets like Bergers, that have a long ogive design (VLD, Hybrid, etc...) still have a normal length bearing surface allowing you to still use them in proper twist barrels for that calber of bullet.

 

[tim_w]

This is very rudimentary as many factors of the bullet such as its shape also effect the stability but at its core........ The most overriding point to stability factor and or needed twist rate within a given caliber (bullet diameter) is the length of the bullet.Well length and density but most have it effect backwards on the later. Just because a bullet weighs more does not mean it will be moving at a slower velocity. That is a factor of the cartridge powder load barrel length. In fact bullet weight or more correctly density actually increases the stability factor for a given length.

Go to one of the bullet stability calculators and enter a vel, bullet length, and twist rate that will not stabilize the bullet. Now start upping the weight of the bullet and watch the stability factor go up and eventually the calculator will say its stabilized. This is why solid copper bullets of the same length as lead core even with the same being the same end up needing more spin to stabilize. Now for a given cartridge usually the lighter bullet will be pushed faster if loaded to and thus makes up the typical amounts but still weight or density decreases needed spin.

 

[Beng]

Just forget all the bs I wrote before.
Here is the Miller Stability Formula we use to estimate Stability.

SG = 30 * m / t² * d³ * l * (1 + l²)

t : rifling twist in calibers
d : caliber of the bullet in inches
l : length of the bullet in calibers

For further information read pages 135f of "Applied Ballistics for Long Range Shooting" by Brian Litz, or pages 36f of "Modern Exterior Ballistics" by Robert L. McCoy.

 

[Mikecr]

The plastic tip does not really add length w/resp to stability.
You can subtract the tip at JBMs: JBM - Calculations - Stability

But I would not do this on real bullets. Only to find a better stability number for what I'm shooting.

 

[tim_w]

The very best thing IMO you can do is use the calculation provided thru real world testing such that is done by Bryan Litz. He had given some of the most valuable data for the long range shooting community thru all his work and ballistics as well as the rest of the shooters of other disciplines.

This is also a good thread on bullet stability and the effect of plastic tips etc.

http://www.longrangehunting.com/for...mula-plastic-tipped-bullets-83456/index2.html

 

[trickytune]

Pretty sure Brian shot down those adjusted formulas recently. Berger website does not allow for a length change due to open tip design as other formulas do. I hope to do a test soon.

 

[Mikecr]

Let us know what you determine.
It's a tricky thing to change a bullet and then account for the multiple attributes affected.

For instance; Removing or changing the tip will affect drag/BC. Stability follows drag. So while the bullet may be shorter, increasing drag in making it that way could be counter productive.