Drive Band Bullets and Grooved Bullets

A rifle barrel has to conform to several specifications.  The obvious ones are: the bore diameter (A) from land to land and the groove diameter from groove to groove (B).  A specification that is often forgotten is the cross-sectional bore area. This is the groove diameter (B) from which the area of the lands (D) is subtracted.  Left below is a four groove barrel.  It has the same cross-sectional bore area as the six groove barrel on the right because, on the six groove barrel (E), the width of a land is less than a land of the four groove barrel but, all six together, are the same as the four of the four groove barrel.

GROOVED BULLETS

Grooved bullets are manufactured to conform to the groove diameter (B) of the barrel.  Grooves are cut in the shaft of the bullet to reduce the volume of bullet material that must be displaced when the bullet is engraved.  With a grooved bullet, the displacement of bullet material starts on the ogive of the bullet.  Part of the curve of the bullet leading up to the shaft is engraved, because it becomes a larger diameter than the bore (A).  Then the shaft of the bullet is engraved up to the first groove and from groove to groove.  Finally the section of shaft joining the base/boattail of the bullet is engraved. Where the diameter of the bullet becomes less than the diameter of the bore (A), displacement of material stops.

Engraving the section of ogive as welll as the full length of the shaft, averages around 3500psi. Resistance to displacement of material is reduced, but only after the ogive and shaft has been engraved up to the first groove.

Grooves are not cut to any precise depth. The grooves of a bullet with deep grooves, play no role in the centralising of the bullet in the rifling. Centralising is achieved by contact of the bullet ogive and shaft in the grooves of the rifling. This limits the width and benefit of grooves that can be cut in the bullet. Wide grooves in the bullet shaft will reduce the mechanical centralising of the bullet in the bore and reduce accuracy. Note that the grooved bullets above show that the barrel was not in contact with the bullet diameter. Most monometallic bullets are manufactured under size and allows gas to leak past the bullet until it exits from the muzzle. 

DRIVE BAND BULLETS

GS Custom HV, FN, HG and SP bullets are manufactured so that the shaft fits the bore diameter of the barrel (A).  A series of driving bands are machined to fit the groove diameter (B) of the barrel. The bullet shaft rides on the lands of the rifling (green sections) and the ogive and boattail are not engraved. Although the shaft touches the barrel, material is not displaced from the shaft of the bullet.  The thin drive bands on the shaft simply deform (red sections) into the spaces behind them and the resistance to engraving is the lightest of all bullet designs.

Acceleration through the freebore is greatly improved, resulting in the pressure peaking at the furthest point down the bore of all current designs. The ratio between muzzle velocity and pressure cannot be matched by any other design. HV, FN, HG and SP bullets can be loaded safely in to a cartridge overall length that places the bullet in full contact with the rifling, without a significant increase in pressure, should it be required.  Note that this is not our standard recommendation but we mention it because presure spikes are not a factor with GSC drive band bullets. 

Alignment of the bullet in the barrel is good because the shaft runs in contact with the lands of the barrel.  The drive bands on the bullet fill the grooves of the barrel completely, sealing it from gas leakage and further aiding centering of the bullet in the barrel.  The volume of bullet material that has to be displaced, is one third to one quarter of that of a grooved bullet or a smooth shanked bullet.  This displacement is also lighter because the space provided is greater than the volume of the displaced material. 

This reduction in start pressure coupled to the fact that the barrel seals immediately against gas leaking past the bullet, allows an increase in speed without increasing pressure, a reduction in gas erosion of the leade, a cooler running barrel, a barrel that is easier to clean and less friction between the barrel wall and the bullet.

DEFINITIONS

Drive Band Bullet: A bullet where the shaft of the bullet conforms to the land diameter of the barrel and should be in contact with the faces of the lands.  The drive band diameter of the bullet conforms to the groove diameter of the barrel and should seal the barrel against gas blowing past the bullet.  The total width of the drive bands must be less than 50% of the length of the engraved section of the bullet where bullet material is displaced. Displaced material volume is less than 50% of the volume of material that would be displaced, if the bullet had no drive bands.

Grooved Bullet: A bullet where the shaft diameter of the bullet is in relationship to the groove diameter of the barrel.  The shaft of the bullet may be under size and may not seal the barrel against gas blowing past the bullet.  The depth to which grooves are cut into the shaft of the bullet, do not conform to the land diameter of the barrel and are deeper than the land standoff of the barrel is high.  The total length where bullet material is displaced, is more than 50% of the length of the bullet shaft that is in contact with the barrel.