boolit engraving and bore-riders [Archive]

joeb33050

02-13-2010, 08:16 AM

The Second Edition of "CAST BULLETS FOR BEGINNER AND EXPERT" is available at no charge on http://sports.groups.yahoo.com/group/CB-BOOK/ in FILES.

See "UPDATES" for this article with pictures.
2.2 FITTING THE BULLET TO THE RIFLE THROAT
In a rifle, there is a shape cut out of the rear of the barrel to fit the cartridge.
From the end of that space forward there are any number shapes that can be cut-from the cartridge case mouth to the point where the lands and grooves of the barrel are untouched.
Parts of these shapes have names, and these names are not agreed upon.

First we have a 30-30 chamber, with little in the way of a transition:

.
Then the 30/06

(Thanks to Dave Goodrich for these drawings.)

Then a "custom" transition:

(Thanks to Dan Willems for the drawing.)

(Here, “Throat” is taken to mean the area in the chamber from the end of the cartridge case to the point where the lands are full-height.)

First some general notes:
Crimp
Cartridges to be loaded into a tubular-magazine rifle, such as a Winchester 94 or Marlin 336, must have the case necks crimped into the bullets to keep the bullets from telescoping into the cartridge case by the force of the magazine spring.
For all other applications, including loading single shot in tubular-magazine rifles, crimping a rifle cartridge case neck into the bullet commonly decreases accuracy and in my experience, has never increased accuracy. Crimp not if accuracy is your goal.

Overall Length (OAL)
Cartridges to be loaded into a rifle magazine must be of the proper OAL, plus or minus a bit, for the magazine to feed bullets into the chamber properly. Such rifles include the M98 Mauser, Winchester 66, Garand and Remington 760-bolt action, autoloaders, lever actions and pump action rifles.
For applications where the rifle is to be loaded single shot, OAL may be varied to suit the throat of the rifle, with generally an increase in accuracy.

The sized, ready-to-load bullet must fit in an unsized case that has been fired in the gun. If the bullet is too big to be pushed into a fired case by hand, it is too big to be fired safely! There are some who argue with this. I prefer to be safe.

The bullet should not stick in the chamber when the chambered cartridge is extracted. No de-bulleting! I have yet to find accuracy increases with cartridges that de-bullet, and I’ve looked. The mess after de-bulleting, with the powder dribbling throughout the chamber and action, will not cheer you up. No de-bulleting!

The base of the bullet should be in the neck, not down inside the case. If just the gas check is below the neck, there’s generally no problem.

Now, about the bullet, the throat, and accuracy:

The bullet-throat intersection should provide both ALIGNMENT of the bullet with the barrel, and SEALING of the burning powder gas behind the bullet.
I think that alignment is important, that the bullet long axis should coincide with the centerline of the bore of the barrel. I think that if the bullet is “cocked” as it goes up the barrel and out of the muzzle, then accuracy is reduced.
I know that when the bullet does not seal the bore, when the burning powder gas can blow by the bullet, that the bore leads and accuracy disappears.
My definitive lesson was while forming 30-40 Krag cases to make 40-1/78” Sharps cases for a Hepburn target rifle. Rather than properly load the Krag cases with a fast powder, Cream Of Wheat and a wax plug; I cleverly loaded them with 30 caliber bullets and fired them in the Hepburn. The leading that resulted from firing 20 of these was very impressive. Fellow shooters were amazed, snickering was heard. Someone asked if Guinness should be contacted. Lead removal was time consuming and difficult.
Too-small bullets that allow powder gas to leak by cast bullets will help lead the bore.

The cartridge should chamber with a bit of resistance, an indication that there’s a slight interference fit between the bullet and the throat. In the ideal situation there will be both alignment and sealing.

In the ideal situation, the nose of the bore-riding bullet should be engraved by the lands of the rifling.
Here’s a photo of the nose of a 314299 loaded in a 308 Winchester case, and chambered. The picture shows only where one land engraved the nose, however all six lands engraved; I couldn’t get the photo of more than one.

In the ideal situation, the front of the first base band will be shiny and tapered where it mashes into some part of the throat. The forward-most end of the front base band should slightly mash into the ball seat or the tapered free bore or the origin of the rifling. This interference fit is about sealing.

This picture shows the front band mashing into the rear end of that 15 degree 30-30 chamber, at the vertical line, sealing the powder gas behind the bullet.

Here the lands, aligning the bullet in than bore, engrave the nose-bearing bullet. The vertical line again shows the bullet front band sealing area.

The 30-06 with the tapered throat, showing the bullet seal at the vertical line.

Here the 30-06 tapered throat, gas seal at the vertical line and bullet alignment with the lands engraving the bullet nose.

(Thanks to Wayne Burnett for these drawings.)

Bullets with bands sized to different dimensions may/will produce different maximum overall cartridge lengths. Sizing a bullet sizes the bands. Seat the unsized bullet way out, and some band is going to hit something in most rifles. The front edge of the band will hit the chamfer in a 30-30 style chamber. In a chamber with a ball seat, the front edge may hit the rear end of that ball seat. If not, the front edge may hit the taper in the leade. The front edge of the base band of a sized bullet is going to be stopped by some feature, else there will be no seal and the bore will lead. (Generally the first or top band hits first, but see below where the 31141 has the third band hit a feature).

The cartridges shown below with 31141 and 314299 bullets are the result of a lot of fiddling. The rifle used was a 308 Winchester M98 Mauser action rifle built by Creighton Audette. Cartridges were loaded to different OALs, and tried in the rifle. When the cartridge would chamber without undue force-but with some resistance, that was the OAL recorded. The object was to maximize the OAL for a given bullet and diameter.

Here is a photo of 308 Winchester cartridges loaded with 31141 “Top Dot” bullets. (The double cavity mold has a prick punch dent high-Top Dot, and low-Bottom Dot on the noses in the cavities. These dents yield identifying dots on the bullet noses. I know which cavity the bullet came from.)
The left cartridge has a 31141 sized .312” and a maximum easily-chambered OAL of 2.665”. Note that the gas check is below the neck.
The right cartridge has a 31141 sized .3095” and OAL of 2.760”. The red line points to the place on the third band where the bullet is scraped, where the band ran into the ball seat or leade or some feature.
There are no marks of the lands on either bullet, there’s no bore-riding going on here.

Here are 308 Winchester cartridges loaded with 314299 “Dot” bullets. (DC mould has a dent on one nose, no dent on the other. Hence, “Dot” and “No Dot” bullets.
The left cartridge has a 314299 sized .3095” and OAL of 2.885”.
The right cartridge has a 314299 sized .312” and OAL of 2.815”. The red line points to the place on the top band where the bullet is scraped, where that land ran into some feature.
Both bullets have very faint marks made by the lands. One has 2 land marks, 180 degrees apart. The other has 3 land marks, on one side of the nose. Again, these land marks are just the slightest polishing of lines on the nose-very faint.

The cartridges shown below are loaded with the same bullets as above, and chambered in a Savage Striker pistol in 308 Winchester. Cartridges were loaded to different OALs, and tried in the pistol. When the cartridge would chamber without undue force-but with some resistance, that was the OAL recorded. The object was to maximize the OAL for a given bullet and diameter.

The left cartridge has a 31141 Top Dot bullet sized to .312”. This sizing doesn’t touch the bands except here and there, the bullet bands vary in diameter from ~. 3125” to .309”. There are slight marks, on the top band, of all six lands. These marks go about half way down the band. The third band shows marks from scraping by some feature such as the beginning of the free bore.
The right cartridge has a 31141 Top Dot sized to .3095”. Again, land marks are clear on the first band and the third band shows the same marks from scraping by some feature such as the beginning of the free bore.
OAL for both cartridges is 2.765”

The left cartridge has a 314299 Dot bullet sized to .312”. Engraved land marks, all six lands, are clear on the nose. The first band is scraped about half way by some feature such as the beginning of the free bore. OAL is 2.821”
The right cartridge has a 314299 Dot bullet sized to .3095. All six land marks are clearly engraved on the nose. The first band is sort of polished, not scraped-again by the beginning of the free bore. OAL is 2.825”

Bullet Sized Gun OAL
31141 .3095” Audette 2.760”
31141 .312” Audette 2.660”
31141 .3095” Striker 2.765”
31141 .312” Striker 2.765”
314299 .3095” Audette 2.885”
314299 .312” Audette 2.815”
314299 .3095” Striker 2.825”
314299 .312” Striker 2.821”
Sizing the bullet in a lubrisizer makes the nose bump up larger. We’re talking about 1 to 2 thousandths of an inch. Almost every time I’ve checked, after seating gas checks and sizing the bands, there has been an increase in nose diameter of .001” to .002”. Bigger as-cast to sized differences make the nose bump up more than when there are smaller differences. Size a .314” to .308” and the nose will bump up more than when sizing from .314” to .312”. Softer bullet noses bump up more than harder bullet noses.
This increase in nose diameter is caused by the force of the top punch pressing down on the nose of the bullet, seating the gas check and forcing the bands through the lubricating die.
Sizing and/or lubricating the bullets in a machine where the bullet is pushed straight through the die with force on the base will not bump up the nose.
Not sizing the bullet also eliminates this bumping up.