Importance of loaded round concentricity at ogive?

[WinJim1863]

I have been measuring concentricity with the Hornady setup. In their instructions they say up to .002 to .003 is acceptable. That seems to be a lot. My question is what are others who measure seeing and how important do you feel that measurement is? If important do you have comparative shooting results that show a difference at extended ranges? ( I am shooting to 600 yards now with hope to go to 1,000)

Concentricity sounds important to me. I can imagine the tremendous force applied to the bullet and the affect on a soft copper jacket as it is forced off center into the rifling. there has to be some deformation and out of balance imparted.

For my last two matches Ive measured 70-80 rounds each time. My average out of round was .0009 on the last loading and .0008 on the one before that. That is better than the Hornady numbers but is it good relative to what experienced reloaders can do? (obviously I'm new at this).

I assume that case neck thickness variation (and out of round relative to case body) is most of the problem. I have measured neck thickness (ball mic, every 90 degrees) and everything I'v measured has variation (Hornady several firings- .0006, Hornady once fired- .008, Starline new .0008 and Norma new- .0014). unsized case neck runout goes between .0012 and .0025 depending on sample (loaded bullet runout is less which I don't understand unless seating die alters case neck??)

Would turning case necks significantly improve bullet concentricity?

For background the round is 6.5 Creedmoor. Full length sized with Redding neck bushed die (.289) (I am also starting to compare with Forster custom full length cut to .290; Sinclair .263 mandrel expander, Redding seater die. Experimenting with annealing on every reload.

Will appreciate any help/advice, thanks, JimB

 

[Black max]

8 or 9 tenths is more than good! Turning case necks has more to do with neck tension than neck concentricity, especially if your using any kind of neck expander. If you've proven your load shoots and has TIR of .0008 do yourself a favor and never check them again.

 

[WinJim1863]

Black Max,
Thanks, "never check them again"; Yes, I sometimes look for rabbit holes to jump down but I do enjoy the experimentation and sometimes I learn something.

 

[mdmp5]

I don’t measure concentricity because there isn’t anything I can do to fix it if I did have a problem. Measured it once about 8 years ago and never since. Ammo shoots fine so I just leave it alone

 

[308pirate]

I've never measured loaded bullet concentricity, and I can make ammo that easily shoots sub .5.

 

[Kopfjager1]

http://www.snipershide.com/shooting/snipers-hide-reloading/39404-concentricity.html
from "The American Rifleman" sometime between 1950 and 1968 when they put it in the compilation book that I scanned.

"Gauging Bullet Tilt"

THE MOST PRECISE AMMUNITION
FROM A LOT CAN BE SELECTED
WITH A BULLET ALIGNMENT GAUGE.

By A. A. ABBATIELLO

Other factors being normal, bullet
tilt with respect to the case center-
line affects group size. If the barrel
length and twist are known, it has been
found possible to predict the direction
from the group center in which the tilted
bullet will strike. If the amount of tilt
is known, the distance from the group
center can be predicted.
Significant score improvement has
been noted by those who have tried
such gauged ammunition.
In cal. .30 long-range shooting, the
best match-grade ammunition will group
in one to 2 minutes of angle under test
conditions. Part of this spread is due
to the bullet tilt with respect to the
case centerline, imposed by the bullet-
seating tool. This tilt displaces the bul-
let’s center of gravity slightly to one
side; in bullets such as the cal. .30 Ml,
the amount is about 1/8 the displace-
ment of the bullet point. It enlarges
groups by amounts up to one minute.
These deviations become proportion-
ately less as the tilt is reduced. Tilts
over .O04" do not seem to increase the
dispersion of the group beyond the ex-
pected one minute. Perhaps this is
because a well-fitting chamber has a
tendency to straighten any rounds
which are excessively tilted. Other ex-
planations are possible.
The gauge consists of a V-block
which permits rotating the round about
the bullet point and 2 tangent spots
near the case head. A dial indicator
which reads in tenths of thousandths of
an inch (.0OO1") bears on the bullet
near the case neck. Half the total indica-
tor reading is used as the displacement
for determining the classes into which
the rounds are separated. The high point
is also marked at this time for orienta-
tion of the round in the rifle chamber.
Rounds with .0O2" tilt or less can
be considered good enough for long-
range use, while those with .O03" and
.OO4" tilt are best used only at short
ranges. In general, it was concluded
from target results that each .0Ol" of
tilt will increase the group spread about
1/4 minute of angle, up to a maximum
of .OO4" as mentioned above.
Under test conditions, it was found
that when the rounds were chambered
with the high point always in the same
orientation, the groups were smaller
than when it was randomly oriented.
Gauging and orienting the rounds can
produce the smallest groups of which
that ammunition is capable.
These ammunition refinements are
becoming important, particularly in
long-range matches.
The essentials of the tilted bullet were
discussed in detail no less than 50 years
ago by Dr. F. W. Mann in his book
"The Bullets In Flight". He pointed out that
the balance of the bullet and the spiral
path of the center of gravity are of
high importance in accuracy.

Following a discussion between
George L. Jacobsen of Frankford Arse-
al and the writer at the 1959 National
matches, a trial of the effect of neck
concentricity was carried out by Jacob-
sen. He described his results in ".30-’O6
Cartridge Cases And Accuracy", which
appeared in THE AMERICAN RIFLEMAN,
January 1960, page 20.

SEATING TOOL A FACTOR

The effects which Jacobsen found,
though small, are essentially in agree-
ment with the work reported here.
However, he did not separate the effects
of neck eccentricity and the bullet cen-
ter-of-gravity location with respect to
the bore. The angular direction of the
bullet seating tool is a controlling factor
in the initial position given to the bullet,
rather than merely case neck eccen-
tricity. Case necks can be centered or
eccentric, and the bullet can be inclined
in completely random directions. The
tilted bullet is believed to be the main
cause for center—of-gravity side shift.
The cal. .30 boattail bullet of 173 grs.
weight was selected for these tests be-
cause it is in common use and is of
sufficiently high quality for use in the
National Matches.
Using the gauge shown, 42 ammuni-
tion lots were sampled and the high
point was marked on each round gauged.
These rounds were grouped in steps of
.OO1" bullet tilt, and the data tabu-
lated. The results gave a bell—shaped
curve for 829 rounds of match ammu-
nition, peaking at about .0O2" (see
illustration). Measurements on Service
ball ammunition produced a curve of
similar shape, but peaking at about
.0025" tilt.
This graphically illustrates that even
match-grade ammunition has appreci-
able variations. There is a large spread
among particular lots and boxes. In
general, 10% to 20% of each lot, de-
pending on ammunition quality, falls
into .0O3", .0O4" or even up to .O10"
tilt. Run-of-the-mill ammunition can
thereby enlarge groups to about twice
the size which the same ammunition
can show when it is gauged before firing.
Since the tilt angle of the bullet is
so small (about 1/4 °) it is difficult to
perceive visually. The gauge, however,
makes the sorting a fast, routine step.
A mathematical solution of this prob-
lem was also tried (see box) and is in
good agreement with the results ob-
tained. It is gratifying to find the math-
ematical solution and the experimental
results in agreement.



MATHEMATICAL SOLUTION

A laterally displaced center of
gravity moves through the rifle bore
in a helical (screw) path. The pitch
of this helix is the pitch of rifling,
and its radius is the lateral displace-
ment of the center of gravity. On
leaving the muzzle, the center of
gravity continues in the direction it
had at that point. For example, if it
leaves at top of the bore and rifling
is to the right, the departure will be
to the right. The bullet travels ap-
proximately 2l.5" in a 24" barrel,
making 2.15 turns in the 10" twist
of rifling. The number of turns
shows the orientation on emergence
compared with that in the chamber
before firing. The angle of emer-
gence is that angle whose tangent is
2 pi times the lateral displacement
divided by the rifling pitch. For
.004" point displacement and I0"
rifling pitch, the tangent is 1/8(2·pi)
(.004)/l0 and the corresponding
angle is 1.1 minutes.
The displacement on target from
this cause is proportional to the
range and can be obtained without
noting the angle. For example, ,004"
point displacement gives in l0"
rifling pitch, so far as this mecha-
nism goes, a target displacement at
100 yds. (3600") indicated by the
proportion .00l· pi /10=X/3600, from
which x =1.1".


The chamber throat will bend the neck straight by pushing on the bullet.

That happened to be 2.2 moa increase in group size calculated, and then verified in a SAMMI chambers 30-06 in the above work by the army, NRA and AA.

If you have some 6mmPPC chamber that is so tight that a spec of dust prevents easy chambering, then a crooked neck will have much less effect on accuracy. That non factory tight chamber will bend the cartridge almost perfectly straight.

In math terms, we have a linear relationship between accuracy and runout, until the runout is so large that it will not fit in the chamber that bent.

As the run out increases beyond what the chamber will straighten out, there is no change in accuracy, as at the time of firing all ammo has the same run out.... the most runout that fits the chamber.

The part of the chamber that affects this is the throat.

In a 30-06 the throat specification and ammo specification per the SAAMI drawings are so lose that the fit is unknown.

This is part of the reason that many rifles shoot better with the bullet jammed into the lands... at least one end of the bullet will be concentric.

 

[Black max]

What is up with the white text? Makes it extremely hard to read...

 

[B-P-UU]

I have been measuring concentricity with the Hornady setup. In their instructions they say up to .002 to .003 is acceptable. That seems to be a lot. My question is what are others who measure seeing and how important do you feel that measurement is? If important do you have comparative shooting results that show a difference at extended ranges? ( I am shooting to 600 yards now with hope to go to 1,000)

Concentricity sounds important to me. I can imagine the tremendous force applied to the bullet and the affect on a soft copper jacket as it is forced off center into the rifling. there has to be some deformation and out of balance imparted.

For my last two matches Ive measured 70-80 rounds each time. My average out of round was .0009 on the last loading and .0008 on the one before that. That is better than the Hornady numbers but is it good relative to what experienced reloaders can do? (obviously I'm new at this).

I assume that case neck thickness variation (and out of round relative to case body) is most of the problem. I have measured neck thickness (ball mic, every 90 degrees) and everything I'v measured has variation (Hornady several firings- .0006, Hornady once fired- .008, Starline new .0008 and Norma new- .0014). unsized case neck runout goes between .0012 and .0025 depending on sample (loaded bullet runout is less which I don't understand unless seating die alters case neck??)

Would turning case necks significantly improve bullet concentricity?

For background the round is 6.5 Creedmoor. Full length sized with Redding neck bushed die (.289) (I am also starting to compare with Forster custom full length cut to .290; Sinclair .263 mandrel expander, Redding seater die. Experimenting with annealing on every reload.

Will appreciate any help/advice, thanks, JimB

What exactly are you using to accurately measure in the tenths of thousands of an inch?

 

[Threadcutter308]

What exactly are you using to accurately measure in the tenths of thousands of an inch?

I measure concentricity just as you have described with a NECO concentricity gage. There is nothing unique or wiz-bang about the NECO, compared to other gages. I find variations between about .0002" and .0015". I have tried to straighten the "bad" ones in the past, but I am of the impression it just made things worse. I still check loaded rounds for runout, but it's more of a matter of curiousity now.

 

[Kopfjager1]

What is up with the white text? Makes it extremely hard to read...

Are you using the standard theme? It was black when I quoted it which made it difficult for me to read.