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Twist rates, Engraved angles, Angles of attack and Angles of incidence.

Below are a series of photos and drawings of the engraving on a variety of bullets.

The first four illustrations below are of 7mmRM bullets fired from rifles with a measured 1:9.5" twist rate.  The drawings below the bullets contain the the ratio of height to width of the triangle and the angles of the lines engraved on the bullets.  The angles correspond with the twist rates as measured in the barrels and indicated by the data on the drawings.

The photo and drawing below are of a 404 Jeffery bullet and the photo and drawing corresponds regarding the actual measured rate of twist of the rifle.

The illustrations below are of a 375H&H bullet that tumbled in the target animal.  The twist rate of the barrel, as derived from the drawing, indicates a rate of twist that is very close to 1:14".  The specified twist for a 375H&H is 1:12".  It is not uncommon for a barrel manufacturer to make
a mistake with the twist rate of a barrel.  Our advice is to positively measure the twist of
your rifle barrels to be sure of what you have.

The low gyroscopic stability of the bullet in flight may have been aggravated by the conditions of the particular day. The angled abrasion of the moly coating on the nose of the bullet and the
direction in which the nose of the bullet is bent, supports the theory that the bullet
impacted at an unacceptable angle of attack closing the opening to the hollow
point. The bullet would have tumbled immediately.

It must be noted that high angles of attack (a yawing bullet) will result in tumbling in the target and a bent, unexpanded bullet.  High angles of incidence (a shot that is taken with the impact surface
at an angle to the line of flight) may result in a slightly asymmetric expansion of a GS Custom expanding bullet but we have not yet seen a failure to expand on impact, where a
correctly stabilised bullet was used at appropriate impact speeds.  See the
illustration below for clarity on angles of incidence and attack.

 

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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.