the base of the bullet to deviate off the axis of the bore and go into the
rifling at random angles, ie., cockeyed, distorting the bullet and throwing it
out of balance. Remember, some bullets are spinning at over 200, 000 (yep, count
the zeroes) RPM. You know what a little imbalance does to a tire on your car,
and it sure isn't spinning anything like this!
The Effects of
Bullet "Cant" on Accuracy
Vaughn in his book, Rifle Accuracy Facts, studied the effects of bullet "cant"
extensively and verified that how the bullet enters the rifling has a very
dramatic and predictable effect on accuracy. This is excellent reading by a
foremost and nationally recognized ballistics scientist..... not a
"magazine expert" who long ago sold his soul to magazine advertisers and can't
tell the truth for fear of offending an advertiser.
also discusses throat diameter and alignment with the bore and states that
nearly every factory chamber he has studied was deficient in this regard. What I
find most interesting is that I learned this by hands on experience only, but as
a scientist Harold put the hard numbers to the experiments he performed and
validates what I preach about chamber throats. Contact Precision Shooting
Magazine for a copy, or email me.
about the chamber neck helping guide the bullet?
Chamber necks, due to variations in brass thickness, usually have to have
something on the order of at least .003" clearance to be reasonably safe---we're
not talking about turning case necks to match tight necked chambers, another
story. Typical neck clearances are more like .005 to .007," so the chamber neck
and case neck really can't do anything positive. They can hold the base of the
bullet pointed in the general direction, but the forces exerted on the bullet
are far greater than a .012-.015" thick brass case neck can overcome!
Only the steel in the chamber throat can positively align the bullet, yet with
the tolerances you typically find in most all factory barrels and the majority
of custom barrels chambered with off-the-shelf reamers the throat will be
substantially larger than bullet diameter---the same condition as a shot out
specify throat diameter, much of the time you are buying a barrel that is to a
significant degree "shot out" before you fire the first shot through it.
reamers will likely be the worst, older reamers that have the throat section
worn down will be closer to size.
think about this. You read all kinds of stuff about seating depth, overall
cartridge length, seating the bullet into the rifling, or backed off so many
thousandths from the rifling, but tell me this, how many times have you read or
heard anything about THROAT DIAMETER ?????? I don't waste much time reading
magazines any more. Maybe times have changed, but I have seen throat diameter
mentioned only a very few times in all my 52 years, 47 of which reading
co-authored the chapter on chamber throats in the 1995 Precision Shooting
Annual. In that same publication was a chapter about barrel lapping in which the
author MONITORED THE GROWTH OF THE THROAT DIAMETER in the process of fire
lapping his barrel. If your throat is too large to start with and you fire lap
the barrel, you WILL end up with a still larger throat DIAMETER. I want you to
burn DIAMETER into your mind. Give second thoughts to fire-lapping a barrel
without re-chambering it to a longer cartridge to get into fresh rifling with a
I take all
of this into account, when I re-chamber a barrel and use techniques and
tolerances that give the bullet its best opportunity to slip straight down the
tube with the least distortion.
Ok, So what do you have in
any given barrel?
Sit tight. Follow along. I want to guide you into a mindset that will open doors
for you. It will reveal some of the mysteries of the chamber throat.
When you get done with reading this, you will know what you are looking at
when you peer down the breech end of your barrels.
and Encores are great for looking into. With the barrel off the frame you don't
have to look very far into the chamber to see the rifling. And you can readily
roll it around and examine the circumferences of the various cuts at the neck
Go pick up
a barrel. Larger bore sizes are better to start with, easier to see details. You
need a light source, but not too much light. In a well lighted room, looking
through the barrel at a piece of white paper works very well.
Ok. You are looking
down into the chamber, looking right at the ends of the rifling. Between you and
the rifling is a big dark hole. Between the big dark hole and the rifling is the
throat. It is the area the same as the grooves in the barrel, minus the rifling.
The throat is merely the area in the barrel where the rifling are cut away so
the bullet can stick out of the case.
you picked up a .22 Long Rifle, technically .22 LR does not have a throat since
the bullet is the same approx. dia. as the brass case, thus the chamber is cut
the same diameter all the way from the rim to a point past the bullet. This
diameter, by the way, will be anywhere from .001" to .008" larger than bullet
check for these basic elements:
1) Relative length of
2) The geometry of the
3) The DIAMETER of the
throat, relative to the groove diameter of the barrel
4) The alignment of the
throat with the bore, or more correctly the grooves of the barrel.
you can see if the throat looks long compared to the throat in other barrels, or
if it is short. Some are so short as to be virtually non-existent, but rather a
chamfer on the ends of the rifling.
of the throat. Is it a cylinder where the riflings are cut away, or is it a
cone. Most of the more recent factory barrels in .357 Mag, .357 Maximum, and .44
Magnum have a cone approximately .4" long. If you look down the wall of the
chamber you can see where it begins to change from chamber wall to a long taper
that ends on the tops of the rifling. What you want is a cylinder that will
support a significant length of the bullet's shank as it engraves. The only
tapered part of the throat should be just on the ends of the rifling.
what I was saying about the base of the bullet? The case mouth can't hold it in
alignment, and in the .357's for example, the base of the bullet can move about
.012" in any direction off the centerline of the barrel. So what does it do when
it meets the rifling? It has every chance in the world to go in cockeyed.
we're getting down to the more critical part. Look between the ends of the
rifling. If you see a line connecting the ends of the rifling, it means that the
throat portion of the reamer was larger than the groove diameter of the barrel,
and this diameter can be a thousandth or more larger than bullet diameter. For
years the .44 Mag factory barrels had .440" groove diameters! And the eight
groove .35 cal. barrels run as small as .356." So you really can't determine the
actual diameter of the throat, but you can determine a size relationship between
slug the barrel with a dead soft lead slug and mike it, you can get a better
idea of the throat diameter. There are other methods to measure it as well, but
we won't go into that here. I simply want to point out the relationships you are
example you slug a .30 cal. barrel and it mikes a true .308" diameter, then find
a line connecting the ends of the rifling in the throat, think about it, the
throat is larger than .308" in diameter.
The ideal is to have
the correct groove diameter, .308" in our example, and the rifling cut away with
little or no discernable line connecting the ends of the rifling, meaning the
throat is matched to the bullet diameter, .308."
usually will find a line connecting the ends of the rifling. Move the barrel
around so that you can follow the entire circumference of this line connecting
the ends of the rifling. Does it look the same all the way around? Or is it
fainter or non-existent in part of its circumference, and pronounced on the
extent it does not appear uniform all the way around, it is off center from the
grooves. If it is off center, how does the bullet get a straight shot in?
that are more difficult to see the throat: .22 Mag, usually a short abrupt cone
and most of the .22 center-fires where the short abrupt throats are also larger
than groove diameter and hard to see.
have any connections at a hospital, get one of their laproscopes and stick it
down into the throat area. You'll REALLY be able to see what I have described
here. It'll blow you away.
quality of the throat, based on the 4 criteria I listed earlier, can spell the
difference between a barrel that shoots well and one you dump all kinds of money
into trying to make it shoot, then "take it on the chin" when you get rid of it
hoping the next barrel will be a good one.
other factors to look at also, such as the crown, but we are limiting this
discussion to just the throat.
have a problem barrel, what you find in the throat may determine whether you
continue working with it or cut your losses.
yourself some money and grief. Start paying close attention to the throat. While
a borescope helps a lot, you can do a lot for yourself with the "naked eye" now
that you know what you are looking for.