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This is a Paul Mathews designed bullet. It is of "bore rider" design, which means the front driving bands are smaller diameter than the rear bands. This allows the bullet to be seated further out of the case to make more room for powder.It is a great BPCR design and carries a lot of lube. On a good day, I've gone ten shots without fouling out with this bullet. Most days, I only make it to eight shots before fouling takes its toll on accuracy. That's still very good with no wiping or blow-tubing.Unfortunately, not every Lyman mold in this design yields completely cylindrical bullets. I have two molds of this design and one of them makes bullets that are more elliptical than cylindrical. Thus, only four stars. Wonderful bullet design, less than impressive mold.
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BUMPING THE TAPERED BULLET
By Paul A. Matthews
The concept behind the design of Lyman’s No.
457677 and 410678 tapered bullets is two-fold:
first, we wanted to get as much of the bullet as
possible into the bore of the barrel before firing
without the extra operation of breech-seating the
bullet, and second, we wanted to get as much of
the bullet as possible engraved by the rifling at
the instant of firing, again without the extra
operation of breech-seating.
The first objective is accomplished by having the
diameter of the first two driving bands just under
that of the bore diameter of the barrel across the
tops of the lands, and the diameter of the third
driving band small enough so that it will enter
the tapered leade of the rifling and contact the
rifling there when the fourth driving band is in
contact with the root of the rifling just ahead of
the chamber. With the bullet thus positioned, the
second objective is reached when the bullet
“bumps up” or obturates and moves forward the
width of one driving band at the instant of firing.
At that time we have four driving bands engaged
with the rifling instead of one driving band as is
the case with a non-tapered bullet.
In combination, these two factors help promote
superior accuracy with the tapered bullet. The
first two driving bands being within the bore of
the barrel help to properly align the bullet before
firing. And the simultaneous engagement of four
driving bands with the rifling helps prevent
torsional slippage of the bullet during its initial
forward movement.
For all of this to take place as planned, these
bullets must bump up or obturate at the instant
of firing. This means that you should be selective
in your choice of alloy. You should avoid any
alloy containing antimony and preferably use a
straight lead-tin alloy of 30 to 1 or even softer.
This is important. If you look carefully at the
bullet, you will note that it has a very short nose,
thus little weight up front to assist in the
obturation process. So to make certain your
tapered bullet bumps up at the instant of firing,
use a soft lead-tin alloy.
Because internal barrel dimensions, including
chamber, throat, leade, bore and groove
diameters vary from rifle to rifle, the as-cast
diameters of the front driving bands are a
compromise. For example, the bore diameter
across the tops of the lands on my two Snoverbuilt
45-70s measure very close to 0.4495 inch.
You will find many other barrels to be 0.450
inch and you will find some that will go as large
as 0.4515 inch. To design a bullet or to make a
bullet mould that will cover this wide a range of
dimensions necessitates a compromise on front
driving band diameters.
Not only are the internal barrel dimensions
different from rifle to rifle, you will also find that
the as-cast diameters of the three front driving
bands are usually a tad smaller than the design
specifications. This is because of machining
tolerances used in the making of the bullet mould
regardless of whether it is a Lyman production
mould cut with a cherry or a custom lathe-bored
mould. No mould maker can be expected to
produce a bullet mould without having some
tolerance, some leeway, on the dimensions. If
they are within half a thousandth of the specified
dimension, they have done a good job.
But to get the ultimate accuracy from a tapered
bullet, you want that bullet to fit your rifle barrel
as closely as possible at the instant of firing. That
means that in my Snover barrels I want the two
front driving bands on bullet No. 457677 to
measure 0.4495 inch, and the third driving band
to measure 0.453 inch. The last three driving
bands will go through a 0.4575 lubri-sizing die.
But how do you get driving bands to measure
0.4495 and 0.453 inch when they come out of
the mould measuring somewhat less than that?
How do you get that perfect fit with the bore
dimension of your barrel? And is it worth the
hassle?
To answer the last question first, if you are
serious about your shooting and want to squeeze
the last bit of accuracy from your rifle, it is
definitely worth the extra effort to “pre-bump”
the front three driving bands prior to lubricating
the bullet. This pre-bumping process is nothing
new. Members of the Cast Bullet Association
were doing it twenty or thirty years ago to get a
precise bore-riding fit on the nose of their
bullets. It worked to good advantage then in
smokeless powder rifles and it will work today in
black powder rifles. The big difference is that we
are interested in bumping up the three front
driving bands on a tapered bullet where the CBA
members were bumping up the nose diameter of
the bullet.
This bumping up process is done in your lubrisizer.
And I have to say right now that the lubrisizer
should be one of the newer models having a
steel handle, and not one of the older ones with a
cast iron handle. Although little pressure is
needed to increase the diameter of the driving
bands by a thousandth or thousandth and a half
of an inch, a cast iron handle is not made for this
kind of work.
To start with, put a sizing die in your lubri-sizer
that is just small enough to clean up the main
driving bands on your bullet. That is, if your
bullet casts 0.459 over the rear driving bands,
install a 0.459 or 0.458 inch sizing die. What you
are interested in here is to hold the bullet
perfectly centered in the die with no wiggle
room. I personally use a 0.458 inch die.
Next, install a nose punch or top punch that is a
precise fit with the nose of the bullet being
bumped up. This nose punch must fit the nose of
the bullet precisely in order to prevent any
deformation during the bumping process. Set a
bullet on top of the die and push it into the die as
deeply as possible while still leaving the three
front driving bands in the upper entrance part of
the die. Adjust the bottom stop on the lubri-sizer
to maintain this position.
With the nose punch in full contact with the nose
of the bullet, exert a brief, firm downward
pressure against the nose of the bullet. Do not
“bump” the nose of the bullet or give any quick
thrust against the handle of the lubri-sizer.
Merely exert a steady even downward pressure
against the nose of the bullet. Eject the bullet
from the die and measure the diameter of the two
front driving bands.
You will have to process only four or five bullets
to determine how little pressure is required to
increase the diameter of the front driving bands.
And about the easiest way to determine when
you have the correct diameter is to test the fit of
the front driving bands in the muzzle of the rifle
barrel. They should be a smooth even push with
no side-to-side wobble and, in my opinion, no
visible engraving marks of the rifling.
Remember, you will be chambering cartridges
loaded with this bullet in a barrel fouled with
black powder.
Once you have determined the proper diameter
of the front driving bands for your rifle, you will
want to rig some kind of mechanical stop on the
lubri-sizer that will limit the travel of the nose
punch and allow you to bump every bullet
quickly and easily and exactly alike. Because of
the differences in lubri-sizers, I can only tell you
how this was accomplished on my Saeco.
With a pre-bumped bullet of the correct diameter
in the sizing die, and with the nose punch in full
contact with the nose of the bullet, I measured
the distance between the top of the die holding
boss and the bottom of the nose punch ram. This
came out to be 0.600 inch. Since the top flange
on the die was a shade less than ¾ inch in
diameter, I purchased a copper sweat coupling
for ¾ inch copper pipe. From this, I had a sleeve
made that was just 0.600 inch long and would
just nicely encircle the die flange and set
between the die boss and the nose punch ram,
thus limiting the downward travel of the ram to a
precise point. Minor adjustment of the bullet
position within the die could then be made with
the bottom stop on the lubri-sizer. Doing it this
way, as long as my bullets are all of a precise
length, they will bump up a precise amount each
time. I mention bullet length because quite often
bullets cast at the beginning of a casting session
or following an interruption during the casting
session are one or two thousandths of an inch
shorter than normal and will not bump up the
same, if at all.
There you have it. That is the way I am working
with Lyman No. 457677 tapered bullet at the
present time. And despite the little extra time and
effort involved, the results on the target make it
worthwhile. This does not mean that your
tapered bullets have to be bumped up to deliver
superb performance.
By all means, first try your bullets just as they come from the mould and, depending upon the internal dimensions of your rifle barrel, they may perform beyond your expectations. If, after two or three range trials you are not satisfied with their performance, then
try the bumping up process. It might be the little
extra that puts you over the top!
Reprinted by permission of The Single Shot
Exchange, 67 North Congress St., York, SC
29745
Excellent information