Re: Turning my own Solid Brass bore riders For the 338 Lapua
Drawings would really help here, but I will try to make the distinctions by referencing well known projectiles.
All bore-riders have drive bands (usually one), but not all "banded" bullets are bore-riders.
The typical artillery projectile is a bore-rider. It has a forward, sub-bore diameter quasi-band known as a bourrelet, which fits as snugly as possible to prevent in-bore cant, followed by a supra-groove diameter "drive band" which serves the dual purpose of a gas check, and imparting spin. Many of the recent 50 caliber competition projectiles fall into this category.
GS Custom projectiles are not bore-riders. They "engrave" the full length of the body with a full-bore diameter shaft, and seal with 5-6 supra-groove diameter bands. Strictly speaking, the bands in this application, are neither exclusively "drive" nor "engraving" in function. They primarily seal, and simply aid in transferring spin. GS Custom is responsible, however, for a huge innovation in band design which does have a purely "drive" purpose. Variable width, and spacing lay-out, of the bands allow the use of a gain-twist rifling geometry, which is critical in generating the high spin rates required for 6.5+ caliber projectiles. In this application, either the forward, or rear, bands are side swaged during barrel transit, leaving a single primary band intact as a gas seal. The reason GS Custom does not use this to any great extent yet, is the scarcity of LGT, EGT, or VGT geometry barrels. Gerard's basic "type" is a hybird design, meant to accomodate the greater manufacturing tolerances of factory barrels.
"Full-diameter" bullets have no bands but seal, and spin as the name suggests, by having a supra-groove diameter body. All jacketed bullets, and some solids like the original Lost River, fall into this category (although there is an interesting history on how this eventually became a sub-groove diameter projectile). Radial grooves are found on some varieties presumably to reduce friction. With imagination these structures create "drive bands", but the intent of the designer to patch a basic design flaw is usually transparent. Full-diameter solid projectiles are a bad idea in terms of both friction, and fouling.
My projectiles fall into a pure "engraving-band" class which has it's roots in WWI artillery projectiles produced in Germany. The Paris Gun had two engraving bands, but some smaller calibers had up to six. In this configuration, the projectile body never comes into contact with the bore. Alignment, spin, and sealing all take place within the major, and minor, band diameters. What distinguishes the ZA projectile, is the inclusion of Gerard Schultz's sacrificial band concept, and use of drag minimizing formulas to generate nose and tail profiles.
There is a 6.0 caliber ZA338 hunt projectile which will stabilize from a 1: 10" twist. It will be publicly available in 2-3 months. Contact me if you are interested.