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BULLET LENGTH AND BARREL TWIST RATES
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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.
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" to
stabilize 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 80gr 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 80gr HV is 24 mm. It is the equivalent in length of 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.
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 and it has a BC of 0.515 with a
G5 drag profile.
QUESTION:
I am shooting a 6.5/284 with a 28 inch barrel and a 1:9 twist. What bullet should I use?
ANSWER:
Use the 264 110gr HV. The bullet is similar in length to a jacketed lead 130gr bullet and has a BC of .490 with
a G5 drag profile. You can run it up to 3300 fps from a 24" barrel so your 28" barrel should be good for another 150 fps - 3450 fps.
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 moly coated.