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BULLET LENGTH AND  BARREL TWIST RATES

When GSC reccommends a particular bullet for a particular caliber and application, bullet length and twist rate are just some of the considerations. We also look at the probable distance at which the cartridge is likely to be applied, the case capacity of the cartridge, the probable launch speed and so on. It is in our interest to recommend the best possible bullet in our range and to give you a bullet that will outperform any other that you have used.

Your rifle calibre dictates the diameter of the bullet you must use. Similarly, the rate of twist dictates the length of the bullet that must be used. Selecting a bullet by weight, rather than length, makes as much sense as choosing a pair of shoes by weight rather than shoe size. See the International standards of bore and groove diameters and
twist rates.

Barrel Twist Rate and Muzzle Velocity - The Relationship

A bullet needs a particular set of circumstances to fly correctly over the entire trajectory from muzzle to target.  The stabilty factor is influenced by both the twist rate of the barrel and the muzzle velocity.  The most important, by far, is the twist rate of the barrel.  To illustrate this relationship, a practical example is given below. Terminology is important so that, when abbreviations are used, we are all on the same page.  Stabillity can be the gyroscopic stabilty (Sg) or the dynamic stability (Sd).  Usually when 'bullet stability' is discussed, Sg is intended.  Common mistakes are to abbreviate Sg as SG and Sd as SD.  SG is the abbreviation for specific gravity and SD is the abbreviation for sectional density. 

Muzzle Velocity at 1600fps

At a muzzle velocity of 1600fps and a barrel twist rate of 1:10", the stability factor (Sg) is 1.25. 

An  increase in muzzle velocity results in an increase of the Sg and, going from a muzzle velocity of 1600fps to 3600fps, results in an increase of the Sg to 1.34. The chart below shows a muzzle velocity of 3600fps.

 

Muzzle Velocity at 3600fps

A difference in twist rate of half an inch, up or down, will influence the Sg as follows:  At a twist rate of 1:10.5" and at a muzzle velocity of 3600fps, the Sg is  1.215.  At a twist rate of 1:10" the Sg is 1.339. 

So, increasing the twist from 1:10.5" to 1:10", increases the Sg by 0.124 and has a greater effect on the Sg than an increase of muzzle velocity from 1600fps to 3600fps.  It is quite clear that a twist/bullet length mismatch cannot be fixed by adjusting the speed.  A 2000fps increase in speed has less value than a half inch difference in twist rate.


QUESTION:
I have a 25-06 rifle with a twist of 1 turn in 10.5 inches. I do not get good accuracy with your 110 grain HP bullet. It seems to keyhole slightly up to a hundred meters. I am getting good results with 120gr jacketed flat base bullets and 100gr jacketed boat tail bullets. What is the problem?

ANSWER:
The usual twist for a 25-06 is 1:10". Your rifle is a bit on the slow side. We use a gyroscopic stability calculator to calculate what the optimum bullet length should be in a particular rate of twist to stabilize a particular bullet as well as produce the optimum terminal performance. By using the velocity (3000 f/s), length of the 110gr bullet(31.400 mm) and diameter of the bullet (6.525 mm), it indicates that the twist should be 1:9.5" for good performance with our 110gr HP bullet. This bullet is too long for the twist in your rifle. One can always use a shorter bullet than what the twist calls for but never a longer one.

The reason why you are getting a good result with the 120gr flat base and 100gr boat tail is because they are shorter than our 110gr HP bullet. Our bullet is equivalent to a 130gr jacketed lead bullet. Copper is 18% lighter than lead so, a reasonable rule of thumb, when going from jacketed lead to solid copper, is to reduce the weight by 18%, if similar bullet shapes are involved. 

I suggest that you use our 85gr HV bullet. The twist needed to stabilize this bullet is 1:12". The twist in your rifle would be right for good stability in flight and deep penetration when the bullet strikes. The length of our 85gr HV is equivalent in length to a 100gr jacketed lead boat tail bullet.

QUESTION
You recommend tighter twist rates for your HV bullets than what the actual length of the bullets require. We have used the 160gr HV bullet in our 30-06 rifles with a one in twelve twist and both rifles are very accurate, yet you recommend a twist of one in ten with the 160gr HV or the use of the 150gr HV in a one in twelve. Why is this?

ANSWER
The 160gr bullet will deliver stable flight and good accuracy from a one in twelve twist. It is also the preferred bullet for extreme range shooting in a one in twelve twist and .30 calibre as it will "nose over" correctly on the far side of the trajectory. For general hunting applications closer than 500 meters, a different set of requirements must be met. Where stable flight in air is easy to achieve, preventing a bullet from tumbling on impact, requires higher rates of rotation for a given bullet length. A shorter bullet is also in stable flight closer to the muzzle of the rifle, delivering proper terminal ballistics at closer ranges. We therefore advise the use of the 150gr HV in a one in twelve twist 30-06 for general hunting. It gives more reliable terminal performance for all applications closer than 500 meters. The specified twist for a 30-06 should be 1:10" but often rifles are rebarreled with 12" twist barrels or, like Sako, Tikka and Blaser at 1:11", standardise on one twist for all 30 caliber barrels.

QUESTION:
Will you be able to manufacture a 175gr hollow point, boat-tail spitser in 7mm? I have a 7 x 64 and I want the heaviest possible bullet available as I want the velocity to remain below 2700ft/sec.

ANSWER:
We would be happy to make 175gr 7mm monometal bullets as we have been asked for them on numerous occasions. The only snag is we know that they cannot work in standard commercially available 7mm barrels. Any manufacturer who supplies such a bullet is simply responding to an uninformed demand from the market. Here is why: A 7mm 175gr monometal spitser boat tail bullet will be 18% longer than a similar bullet of conventional construction. That means that it will be about 42mm long and will require a twist of 1 in 7.3" to stabilize. Some rifles may shoot reasonable groups with such a bullet but when it strikes, it will tumble instantly, with the resultant unpredictable penetration and massive meat damage a tumbling bullet brings. A 175gr round nose flat base bullet can be made to stabilize, but will not expand, and will be too slow for decent momentum and energy levels as are available from the lighter and faster HV bullets.

QUESTION:
What weight bullet would you recommend for a .300 RUM with a 1-10" twist?

ANSWER:
The correct bullet is the 308 160 HV. It is similar in length to a jacketed lead, 200 grain partition style bullet, but with a better BC. We regularly use the 308 160 HV in 300 Win Mag on game up to eland bulls with excellent results. You would run it up to 3400 from a 24" barrel. 

For more information on matching the twist rate of your rifle to the correct bullet in our range, follow the link below.
Technical Data
 

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GS Custom Bullets, situated in Port Elizabeth on the East Coast of South Africa, manufactures solid copper, turned, monolithic bullets for hunting and sport shooting. These bullets are used by hunters on several continents, hunting from the smallest of antelope to the largest of dangerous game, using the smooth HP bullet, as well as the more popular HV, FN and SP bullets with the patented drive band concept. GSC bullets are configured for the highest possible ballistic coefficients. SP bullets are mainly used for sport shooting. All GS Custom Bullets are coated.