Below is a article by German Salazar a avid reloader and national competitive shooter. His now shut down website is where I first read the term "The cartridge should fit the chamber like a rat turd in a violin case" and Mr. Salizar was quoting the late Jim Hull of the Sierra ballistic test lab.
In 1976 I bought a military 1943 Remington 03-A3 30-06 that had seen better days, but it made the perfect knock around "rain gun". The problem I had was new cases fired for the first time shot very good groups. But my reloaded cases would shoot five inch or larger groups at 100 yards and this drove me nuts. I took my fired cases to work for a machinist and benchrest shooter to look at and found out they were warped and banana shaped. And this was caused by the rifles large diameter chamber and Remington 30-06 cases that did not have uniform case wall thickness.
The cure for this old military rifle was to stop neck sizing and start full length resizing with loads well below maximum. And full length resizing would reduce the case diameter and reduce the bullet guiding effect the case body had with the bullet in the throat.
The vast majority of competitive shooters are now full length resizing and one of the reasons is its hard to make a perfect case with uniform case wall thickness. Mr. Salazar sets his case limits at .003 variation in case wall thickness for competition. Bottom line, I have seen some of my Remington cases with over .009 case wall variations and these type cases "WILL" warp when fired. So in my opinion the average reloader with a off the shelf factory rifle is better off full length resizing with Remchester cases. And even with Lapua cases Mr. Salazar had cases with over .003 case wall thickness variations.
Thanks to the Wayback Machine I was able to find Mr. Salazar's old website link below. (And why the case should fit the chamber like a rat turd in a violin case)
Reloading: Audette Case Checking Method
Checking Case Wall Concentricity
by Germán A. Salazar
https://web.archive.org/web/20150320075816/http://riflemansjournal.########.com/2009/07/reloading-audette-neco-case-checker.html
NECO CONCENTRICITY, WALL THICKNESS AND RUNOUT GAUGE
http://www.neconos.com/details.htm
Also referred to as "The Case Gauge," this item is designed to measure:
1) The curved "banana" shape of the cartridge case;
2) The relative wall thickness variation of a cartridge case;
3) The cartridge case head out-of-squareness;
4) Individual Bullets - out-of-round "egg shape" and/or curved "banana" shape (excepting very small bullets);
5) The seated bullet and cartridge runout of loaded rounds. The accuracy of any firearm is determined -- and limited -- by the quality of the ammunition shot in it. The effect of imperfections in ammunition is cumulative; each flaw adds to the influence of all others. Precision shooters spend much time and effort "uniforming" cartridge cases, using advanced techniques to eliminate variation. Yet until recently, one of the most important of these variations has not been susceptible to detection by any device readily available to marksmen.
Normal manufacturing tolerances cause brass cartridge cases to vary in wall thickness around the circumference of their bodies. Under the stress of firing, a case with such variation stretches more readily along its thin side, transferring more pressure to the bolt face at that point and introducing an unbalanced force which contributes to bolt whip and vibration of the barreled action in its bedding. This whip and vibration varies from one shot to the next as cartridges are fired with their thin sides randomly oriented at different angles, causing reduced accuracy. The problem is made even worse if the brass is too hard or springy to completely fireform to the shape of the chamber, in which event the greater stretching of the case's thin side will cause it to develop a curve along the length of its body. These "banana" cases cannot hold a bullet aligned with and centered in the bore, undercutting the effectiveness of the handloader's careful case preparation.
In 1976 I bought a military 1943 Remington 03-A3 30-06 that had seen better days, but it made the perfect knock around "rain gun". The problem I had was new cases fired for the first time shot very good groups. But my reloaded cases would shoot five inch or larger groups at 100 yards and this drove me nuts. I took my fired cases to work for a machinist and benchrest shooter to look at and found out they were warped and banana shaped. And this was caused by the rifles large diameter chamber and Remington 30-06 cases that did not have uniform case wall thickness.
The cure for this old military rifle was to stop neck sizing and start full length resizing with loads well below maximum. And full length resizing would reduce the case diameter and reduce the bullet guiding effect the case body had with the bullet in the throat.
The vast majority of competitive shooters are now full length resizing and one of the reasons is its hard to make a perfect case with uniform case wall thickness. Mr. Salazar sets his case limits at .003 variation in case wall thickness for competition. Bottom line, I have seen some of my Remington cases with over .009 case wall variations and these type cases "WILL" warp when fired. So in my opinion the average reloader with a off the shelf factory rifle is better off full length resizing with Remchester cases. And even with Lapua cases Mr. Salazar had cases with over .003 case wall thickness variations.
Thanks to the Wayback Machine I was able to find Mr. Salazar's old website link below. (And why the case should fit the chamber like a rat turd in a violin case)
Reloading: Audette Case Checking Method
Checking Case Wall Concentricity
by Germán A. Salazar
https://web.archive.org/web/20150320075816/http://riflemansjournal.########.com/2009/07/reloading-audette-neco-case-checker.html
NECO CONCENTRICITY, WALL THICKNESS AND RUNOUT GAUGE
http://www.neconos.com/details.htm
Also referred to as "The Case Gauge," this item is designed to measure:
1) The curved "banana" shape of the cartridge case;
2) The relative wall thickness variation of a cartridge case;
3) The cartridge case head out-of-squareness;
4) Individual Bullets - out-of-round "egg shape" and/or curved "banana" shape (excepting very small bullets);
5) The seated bullet and cartridge runout of loaded rounds. The accuracy of any firearm is determined -- and limited -- by the quality of the ammunition shot in it. The effect of imperfections in ammunition is cumulative; each flaw adds to the influence of all others. Precision shooters spend much time and effort "uniforming" cartridge cases, using advanced techniques to eliminate variation. Yet until recently, one of the most important of these variations has not been susceptible to detection by any device readily available to marksmen.
Normal manufacturing tolerances cause brass cartridge cases to vary in wall thickness around the circumference of their bodies. Under the stress of firing, a case with such variation stretches more readily along its thin side, transferring more pressure to the bolt face at that point and introducing an unbalanced force which contributes to bolt whip and vibration of the barreled action in its bedding. This whip and vibration varies from one shot to the next as cartridges are fired with their thin sides randomly oriented at different angles, causing reduced accuracy. The problem is made even worse if the brass is too hard or springy to completely fireform to the shape of the chamber, in which event the greater stretching of the case's thin side will cause it to develop a curve along the length of its body. These "banana" cases cannot hold a bullet aligned with and centered in the bore, undercutting the effectiveness of the handloader's careful case preparation.
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