Hi all

Recently I read Charlie Dell’s “Modern Scheutzen” book and Col. Harrison’s “Cast Bullets” book and have been studying closely what they had to say about dimensions of cast bullets. One take away I got was that the proper fit of the bullet to your chamber/rifling is critical. 

One issue Col. Harrison points that for say many 30-caliber molds that bore riding section often are often under-sized measuring 0.298” or less so the 0.300” lands will not hold the bullet perfectly concentric after the cartridge is loaded in the chamber. This will degrade accuracy performance as the nose should be fully supported by the lands at firing.  He recommends the bore rider should therefore be exactly the land diameter or just slightly over to ensure a concentric bullet upon chambering. 

The issue at hand now is what happens when the throat/initial section of the rifling is quite tapered or worn so that the land diameter here is significantly greater than it is at the muzzle?  What then is the proper bore rider diameter - or is a bore rider bullet design even appropriate?

To illustrate the problem I made some measurements of the throat of my Aydt rifle using combination of a sulfur cast along with pushing lead slugs sized to different diameters up the throat and measuring when they engaged with the rifling. From slugging the bore, nominal land and groove diameter of 0.307 and 0.317”, respectively. 

The plot below shows the results. For example a 0.317” (groove diameter) slug engages the rifling at 2.29” from the breech face, a 0.3125” slug engages at 2.565” and a 0.309” slug engages nearly 8 inches down the 30 inch long barrel. 

I'm looking to have a new bullet mold made with better fitted dimensions to see if that improves accuracy. My current bullet for 8.15x46R cartridge (pictured below) has a bore rider section of 0.307” land diameter and when seated gives an overall length of 2.45”, (case length of 1.83"). This is a stop-ring bullet, loaded in the case - not breech seated. Given the measurements in the plot I realize now that the nose isn’t fully supported by the lands until it's a quarter way down the barrel. So what would be a better diameter to pick for a nose rider? To engage the nose with the lands upon chambering I’d have to make the nose groove diameter, which seems like a bad idea. Is using bore-rider here even worth it or should I try a bullet with groove-diameter bands all the way to the front without a bore-rider section at all?

Thoughts?

-Dave

Wayne,

I've mostly been shooting smokeless (IMR 4227) with 1:20 alloy with a fairly light charge (10-12 gr). I have fired some softer alloy with smokeless as well as some FFg loads with 1:20 to 1:30 alloy but haven't seen too much difference in group sizes between the combinations. Appreciate your thoughts for the smokeless combo.
Thanks,
-Dave