What is a “bore rider” bullet?

Typically they are solids. The driving band (on the aft portion of the bullet) will be at groove diameter and the forward 'bore-rider' portion, between the driving band and the ogive, will be .0001-.0002 under bore diameter. The bore rider portion of the bullet rides on top of the lands while the driving band is engraved by the lands and seals the bore.

Cutting Edge Lazers are a good example.
 
There have been many cup and core "bore rider" bullets. The original Winchester design of the 264WM used a bore rider bullet to reduce pressure and keep velocity up. It didn't match the advertised velocity, but was faster than a full shanked bullet.
The bullet has a section near the base at full bullet true diameter, then the rest of the shank is at bore diameter generally.

Cheers.
 
There have been many cup and core "bore rider" bullets. The original Winchester design of the 264WM used a bore rider bullet to reduce pressure and keep velocity up. It didn't match the advertised velocity, but was faster than a full shanked bullet.
The bullet has a section near the base at full bullet true diameter, then the rest of the shank is at bore diameter generally.

Cheers.
The bullet has a section near the base at full bullet true diameter, then the rest of the shank is at bore diameter generally.!!!!!!!!!!!!!!!!!!!
Good post!
 
Here's another picture of one.
Screenshot_20220510-170928_Message+.jpg
 
Here's another example of bore rider bullets!
Well, projectiles may be a better term. Just a bigger version of the above 20 mm.
The first is a 16"-50 caliber 2700 lb AP projectile shown with one of it's two powder canisters (590 lbs). The rotating band was removed to "de-militerize" it.
the next photo is of two 16"-50 1900 lb HE, and one 2700 lb AP. At one time I had over 30 of these.
20210926_184437.jpg
20210921_135126.jpg
 
Am I correct in thinking that bearing surface area is one of the factors in determining ideal barrel twist for a given projectile? And if that is correct, then what is the ideal twist for a "bore-rider" where only a short section of the projectile is engaged in the rifling?
 
Am I correct in thinking that bearing surface area is one of the factors in determining ideal barrel twist for a given projectile? And if that is correct, then what is the ideal twist for a "bore-rider" where only a short section of the projectile is engaged in the rifling?
No, the projectile length to diameter is the primary factor. The bore rider design reduces friction, especially on longer projectiles. They are often used in conjunction with gain twist rifling to help get heavier projectiles turning easier. Like in the 20 mm Vulcan with the projectile posted above, the rifling starts pretty much straight at the beginning and ends up twisted like a cork screw to spin it fast enough to stabilize. The bore rider also gets the projectile aligned in the bore prior to firing, unlike standard bullets.
 

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