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Concentricity .. how important?
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<blockquote data-quote="Clark" data-source="post: 1449554" data-attributes="member: 6600"><p>from "The American Rifleman" ~ 1960, and the NRA says it is ok to quote.</p><p>This is how my program scanned the old printed book.</p><p></p><p>"Gauging Bullet Tilt"</p><p></p><p>THE MOST PRECISE AMMUNITION</p><p>FROM A LOT CAN BE SELECTED</p><p>WITH A BULLET ALIGNMENT GAUGE.</p><p></p><p>By A. A. ABBATIELLO</p><p></p><p>Other factors being normal, bullet</p><p>tilt with respect to the case center-</p><p>line affects group size. If the barrel</p><p>length and twist are known, it has been</p><p>found possible to predict the direction</p><p>from the group center in which the tilted</p><p>bullet will strike. If the amount of tilt</p><p>is known, the distance from the group</p><p>center can be predicted.</p><p>Significant score improvement has</p><p>been noted by those who have tried</p><p>such gauged ammunition.</p><p>In cal. .30 long-range shooting, the</p><p>best match-grade ammunition will group</p><p>in one to 2 minutes of angle under test</p><p>conditions. Part of this spread is due</p><p>to the bullet tilt with respect to the</p><p>case centerline, imposed by the bullet-</p><p>seating tool. This tilt displaces the bul-</p><p>let's center of gravity slightly to one</p><p>side; in bullets such as the cal. .30 Ml,</p><p>the amount is about 1/8 the displace-</p><p>ment of the bullet point. It enlarges</p><p>groups by amounts up to one minute.</p><p>These deviations become proportion-</p><p>ately less as the tilt is reduced. Tilts</p><p>over .O04" do not seem to increase the</p><p>dispersion of the group beyond the ex-</p><p>pected one minute. Perhaps this is</p><p>because a well-fitting chamber has a</p><p>tendency to straighten any rounds</p><p>which are excessively tilted. Other ex-</p><p>planations are possible.</p><p>The gauge consists of a V-block</p><p>which permits rotating the round about</p><p>the bullet point and 2 tangent spots</p><p>near the case head. A dial indicator</p><p>which reads in tenths of thousandths of</p><p>an inch (.0OO1") bears on the bullet</p><p>near the case neck. Half the total indica-</p><p>tor reading is used as the displacement</p><p>for determining the classes into which</p><p>the rounds are separated. The high point</p><p>is also marked at this time for orienta-</p><p>tion of the round in the rifle chamber.</p><p>Rounds with .0O2" tilt or less can</p><p>be considered good enough for long-</p><p>range use, while those with .O03" and</p><p>.OO4" tilt are best used only at short</p><p>ranges. In general, it was concluded</p><p>from target results that each .0Ol" of</p><p>tilt will increase the group spread about</p><p>1/4 minute of angle, up to a maximum</p><p>of .OO4" as mentioned above.</p><p>Under test conditions, it was found</p><p>that when the rounds were chambered</p><p>with the high point always in the same</p><p>orientation, the groups were smaller</p><p>than when it was randomly oriented.</p><p>Gauging and orienting the rounds can</p><p>produce the smallest groups of which</p><p>that ammunition is capable.</p><p>These ammunition refinements are</p><p>becoming important, particularly in</p><p>long-range matches.</p><p>The essentials of the tilted bullet were</p><p>discussed in detail no less than 50 years</p><p>ago by Dr. F. W. Mann in his book</p><p>"The Bullets In Flight". He pointed out that</p><p>the balance of the bullet and the spiral</p><p>path of the center of gravity are of</p><p>high importance in accuracy.</p><p></p><p>Following a discussion between</p><p>George L. Jacobsen of Frankford Arse-</p><p>al and the writer at the 1959 National</p><p>matches, a trial of the effect of neck</p><p>concentricity was carried out by Jacob-</p><p>sen. He described his results in ".30-'O6</p><p>Cartridge Cases And Accuracy", which</p><p>appeared in THE AMERICAN RIFLEMAN,</p><p>January 1960, page 20.</p><p></p><p>SEATING TOOL A FACTOR</p><p></p><p>The effects which Jacobsen found,</p><p>though small, are essentially in agree-</p><p>ment with the work reported here.</p><p>However, he did not separate the effects</p><p>of neck eccentricity and the bullet cen-</p><p>ter-of-gravity location with respect to</p><p>the bore. The angular direction of the</p><p>bullet seating tool is a controlling factor</p><p>in the initial position given to the bullet,</p><p>rather than merely case neck eccen-</p><p>tricity. Case necks can be centered or</p><p>eccentric, and the bullet can be inclined</p><p>in completely random directions. The</p><p>tilted bullet is believed to be the main</p><p>cause for center—of-gravity side shift.</p><p>The cal. .30 boattail bullet of 173 grs.</p><p>weight was selected for these tests be-</p><p>cause it is in common use and is of</p><p>sufficiently high quality for use in the</p><p>National Matches.</p><p>Using the gauge shown, 42 ammuni-</p><p>tion lots were sampled and the high</p><p>point was marked on each round gauged.</p><p>These rounds were grouped in steps of</p><p>.OO1" bullet tilt, and the data tabu-</p><p>lated. The results gave a bell—shaped</p><p>curve for 829 rounds of match ammu-</p><p>nition, peaking at about .0O2" (see</p><p>illustration). Measurements on Service</p><p>ball ammunition produced a curve of</p><p>similar shape, but peaking at about</p><p>.0025" tilt.</p><p>This graphically illustrates that even</p><p>match-grade ammunition has appreci-</p><p>able variations. There is a large spread</p><p>among particular lots and boxes. In</p><p>general, 10% to 20% of each lot, de-</p><p>pending on ammunition quality, falls</p><p>into .0O3", .0O4" or even up to .O10"</p><p>tilt. Run-of-the-mill ammunition can</p><p>thereby enlarge groups to about twice</p><p>the size which the same ammunition</p><p>can show when it is gauged before firing.</p><p>Since the tilt angle of the bullet is</p><p>so small (about 1/4 °) it is difficult to</p><p>perceive visually. The gauge, however,</p><p>makes the sorting a fast, routine step.</p><p>A mathematical solution of this prob-</p><p>lem was also tried (see box) and is in</p><p>good agreement with the results ob-</p><p>tained. It is gratifying to find the math-</p><p>ematical solution and the experimental</p><p>results in agreement.</p><p></p><p></p><p></p><p>MATHEMATICAL SOLUTION</p><p></p><p>A laterally displaced center of</p><p>gravity moves through the rifle bore</p><p>in a helical (screw) path. The pitch</p><p>of this helix is the pitch of rifling,</p><p>and its radius is the lateral displace-</p><p>ment of the center of gravity. On</p><p>leaving the muzzle, the center of</p><p>gravity continues in the direction it</p><p>had at that point. For example, if it</p><p>leaves at top of the bore and rifling</p><p>is to the right, the departure will be</p><p>to the right. The bullet travels ap-</p><p>proximately 2l.5" in a 24" barrel,</p><p>making 2.15 turns in the 10" twist</p><p>of rifling. The number of turns</p><p>shows the orientation on emergence</p><p>compared with that in the chamber</p><p>before firing. The angle of emer-</p><p>gence is that angle whose tangent is</p><p>2 pi times the lateral displacement</p><p>divided by the rifling pitch. For</p><p>.004" point displacement and I0"</p><p>rifling pitch, the tangent is 1/8(2·pi)</p><p>(.004)/l0 and the corresponding</p><p>angle is 1.1 minutes.</p><p>The displacement on target from</p><p>this cause is proportional to the</p><p>range and can be obtained without</p><p>noting the angle. For example, ,004"</p><p>point displacement gives in l0"</p><p>rifling pitch, so far as this mecha-</p><p>nism goes, a target displacement at</p><p>100 yds. (3600") indicated by the</p><p>proportion .00l· pi /10=X/3600, from</p><p>which x =1.1".</p></blockquote><p></p>
[QUOTE="Clark, post: 1449554, member: 6600"] from "The American Rifleman" ~ 1960, and the NRA says it is ok to quote. This is how my program scanned the old printed book. "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". [/QUOTE]
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