Excerpt from Fouling Shot 119 Jan/Feb 1996. By Ron Fortier.
Caseshortening with light and medium cast bulletsloads is something most cast bullet shooters havecome to expect. It happens and I have never beenoverly concerned about it. Over the years I haveheard many reasons for case shortening, rangingfrom the firing pin driving the case forward inthe chamber with sufficient force to set the shoulder back, to the primer backing out of the primer pocket and doing the same thing.
Caseshortening with light and medium cast bulletsloads is something most cast bullet shooters havecome to expect. It happens and I have never beenoverly concerned about it. Over the years I haveheard many reasons for case shortening, rangingfrom the firing pin driving the case forward inthe chamber with sufficient force to set the shoulder back, to the primer backing out of the primer pocket and doing the same thing.
The primer/case shortening theory has always been my favorite. Primer ignition is more powerful than most shooters realize. I once saw an individual, best described as a damn fool, place a primer on an anvil and tap it with a hammer. Tue hammer came back up a lot faster than it went down. During World War I John Garand designed a light machine gun that used primer blow back and a bolt return spring to function the weapon. In the Garand gun, the primer was not supported by the bolt face; a shallow recess in the bolt face allowed the primer to move backwards upon ignition. Movement of the primer backwards, slight as it was, was powerful enough to push the firing pin back with sufficient energy to cause an actuator to open the bolt and extract and eject the empty case. A bolt return spring closed and locked the bolt, chambering a new round in the process.
Garand's first machine gun worked very well; however, sometime between the testing of the first and second gun, the army switched to a slower burning powder, and the second gun did not function reliably. Garand gave up on primer blowback and devoted his efforts to gas operated weapons. The failure of the second machine gun to function reliably with a slow burning powder would indicate that primer flash alone was not powerful enough to function the gun, and that gas from the fast burning powder provided some of the energy needed to drive the primer rearward with sufficient force to operate the action.
Garand's first machine gun worked very well; however, sometime between the testing of the first and second gun, the army switched to a slower burning powder, and the second gun did not function reliably. Garand gave up on primer blowback and devoted his efforts to gas operated weapons. The failure of the second machine gun to function reliably with a slow burning powder would indicate that primer flash alone was not powerful enough to function the gun, and that gas from the fast burning powder provided some of the energy needed to drive the primer rearward with sufficient force to operate the action.
Proponents of the "primer/case shortening" theory believe shortening occurs because the firing pin drives the case forward; then any difference between the length of the chamber ( case head to shoulder) and the case allows the primer to move backwardson ignition. When the primer's rearward movementis arrested by the bolt face, the case is pushedforward by the primer flash with enough energyto shorten the case from the shoulder back. ''Primer/caseshortening" proponents believe there isn'tenough pressure generated with light loads to expandthe case to the dimensions of the chamber.
Recently, letters from CBA members Phil Kaster and Dr. Lawrence McCarty, piqued my interest on the subject of case shortening with light loads. Phil had written that light loads had shortened his cases, but he could bring them back to length by full length resizing them I tried it with a short .30-06 case and it worked. Tue overall length of the case was 2.484" before sizing, and 2.494" afterwards. Tue length of the neck was unchanged.
The .010" increase in case length after fulllength resizing made me question the cause of case shortening; it seemed more likely that the body of the case had expanded away from its longitudinal axis shortening it, than that the shoulder had been set back by force from the primer. Certainly if there was sufficient forward force to set the shoulder of the case back, there should have been sufficient for force to extrude some of the brass into the chamber neck, lengthening the neck of the case.
I decided to investigate possible cures for case shortening, hoping, that in the process, I might gain some insight as to the cause. I had read that reaming flash holes to .0995", and using magnum primers would eliminate case shortening. Selecting four full-length resized Remington .30-06 cases that measured 2.494", I reamed the flash hole of one with a No. 39 drill, gathered up my reloading gear, grabbed my 03A3 and went to the range.
The tests were simple. Selecting the case with the .0995" flash-hole, I neck sized, loaded and fired it ten times using 20 gr. of 5744, WLR primers and SAECO 315 bullets cast from linotype. I did the same thing with the unaltered case, except that I used Remington 7½ magnum primers. The case fired with the magnum primers shortened from 2.494" to 2.490" and the case with the enlarged flashhole went from 2.494" to 2.486". So much for the"enlarging flash holes" and ''magnum primer" theories;although the magnum primers did seem to minimize case shortening.
I decided to test the fourth case using WLR primers with 315 SAECO bullets and no powder charge. This test took less time than I thought it would. I had expected the primer to drive the bullet a few inches up the bore, and that I would have to drive it out with a cleaning rod.
For the first firing I belled the mouth of the case. Pointing the rifle muzzle down range I pulled the trigger. Nothing happened, no slight recoil, no noise, nothing. I extracted the case; sure enough there was a healthy firing pin indent, but I could detect no change in the length of the cartridge, or in the seating depth of the bullet. I thought I had a dud primer. I pulled the bullet, deprimed the case and checked the primer (I deprimed the case with a .22 caliber Lee depriming rod to avoid repeating the neck sizing and belling operation). It had ignited. I switched to a Remington magnum primer expecting to at least hear some noise. Again, nothing. There was a mild "pop" when I pulled the bullet. The "pop" apparently resulted from a buildup of primer gas in the case ..
I measured the case after each firing. After the second firing a very slight bit of primer protrusion was noticeable. It took four firings to bring the case to a length of .2487". After ten firings the case measured 2.486"; it had shortened .008". Each time I pulled a bullet there was the "pop". At the end of the test, primer protrusion above the head of the case was .002". I was now convinced that primer blow-back in a loaded round was capable of shortening a case. Pleased with the results of the test, I was happy in my belief for a couple of weeks, then I worked the sighting-in days at the Elk Rapids Sportsman's Club.
I decided to investigate possible cures for case shortening, hoping, that in the process, I might gain some insight as to the cause. I had read that reaming flash holes to .0995", and using magnum primers would eliminate case shortening. Selecting four full-length resized Remington .30-06 cases that measured 2.494", I reamed the flash hole of one with a No. 39 drill, gathered up my reloading gear, grabbed my 03A3 and went to the range.
The tests were simple. Selecting the case with the .0995" flash-hole, I neck sized, loaded and fired it ten times using 20 gr. of 5744, WLR primers and SAECO 315 bullets cast from linotype. I did the same thing with the unaltered case, except that I used Remington 7½ magnum primers. The case fired with the magnum primers shortened from 2.494" to 2.490" and the case with the enlarged flashhole went from 2.494" to 2.486". So much for the"enlarging flash holes" and ''magnum primer" theories;although the magnum primers did seem to minimize case shortening.
NextI tested the "primer/case shortening" theory. Usingthe third case I seated a WLR primer, chamberedthe uncharged round, placed the muzzle againsta soft pine broad and fired the rifle. The reportwasslightly muffled, but still fairly loud. What surprisedmewas the slight bit of recoil that was enoughto push the rifle muzzle away from the board. Irepeated the process nine more times. The length of thecase was unchanged; still 2.494". This test also debunkedthe theory that firing pin fall alone would shortena case.
For the first firing I belled the mouth of the case. Pointing the rifle muzzle down range I pulled the trigger. Nothing happened, no slight recoil, no noise, nothing. I extracted the case; sure enough there was a healthy firing pin indent, but I could detect no change in the length of the cartridge, or in the seating depth of the bullet. I thought I had a dud primer. I pulled the bullet, deprimed the case and checked the primer (I deprimed the case with a .22 caliber Lee depriming rod to avoid repeating the neck sizing and belling operation). It had ignited. I switched to a Remington magnum primer expecting to at least hear some noise. Again, nothing. There was a mild "pop" when I pulled the bullet. The "pop" apparently resulted from a buildup of primer gas in the case ..
I measured the case after each firing. After the second firing a very slight bit of primer protrusion was noticeable. It took four firings to bring the case to a length of .2487". After ten firings the case measured 2.486"; it had shortened .008". Each time I pulled a bullet there was the "pop". At the end of the test, primer protrusion above the head of the case was .002". I was now convinced that primer blow-back in a loaded round was capable of shortening a case. Pleased with the results of the test, I was happy in my belief for a couple of weeks, then I worked the sighting-in days at the Elk Rapids Sportsman's Club.
One of the advantages of working sighting-in days at any club, is the opportunity to scrounge brass from the once-a-year shooters. This year I came home with one hundred-eighty once fired cases. Pleased with my hoard, I started sorting the cases by caliber. Still curious about changes in case length, I picked up my calipers and measured a .3006 case. It was 2.486" long.
I measured a hundred and twenty-two .30-06 cases before I stopped; then I started again, measuring the .243's, .257's, .270's, .280's. I measured every case I had scrounged. In every instance the case had shortened upon firing. Of the one hundred and twenty-two .30-06 cases I measured, assuming initial length to be 2.494", one hundred and eleven had shortened between .006" and .014". Several cases were as short as 2.475". Eleven of the cases, nickle plated Federals, had shortened very little, only . 002". In every other cartridge, even the belted magnums, minimum case shortening was at least .006" and in most instances closer to .010"; ten 8x57 Mauser cases had shortened approximately .018". I found only three cases with protruding primers.
Trying to determine where the shortening had occurred, I hit upon the idea of inserting the cases into the case holder of a Wilson Trimmer and measuring case protrusion. The case holder is a steel cylinder with a tapered hole in it; the case is held in by its body taper. By inserting the case into the wrong end of the case holder, any enlargement in the diameter of the body of the case is readily apparent. For example, a once-fired .30-06 Federal case which measured 2.492", protruded from the case holder 1.410". A once-fired .30-06 Remington case measuring 2.483", protruded 1.496". Inserting the cases into the holder as intended, the head of the Federal case protruded .032" more than the Remington case. It was readily apparent from the differences in amount of protrusion of the .30-06 cases that factory chambers, for any given cartridge, come in a variety of shapes and sizes.
I measured a hundred and twenty-two .30-06 cases before I stopped; then I started again, measuring the .243's, .257's, .270's, .280's. I measured every case I had scrounged. In every instance the case had shortened upon firing. Of the one hundred and twenty-two .30-06 cases I measured, assuming initial length to be 2.494", one hundred and eleven had shortened between .006" and .014". Several cases were as short as 2.475". Eleven of the cases, nickle plated Federals, had shortened very little, only . 002". In every other cartridge, even the belted magnums, minimum case shortening was at least .006" and in most instances closer to .010"; ten 8x57 Mauser cases had shortened approximately .018". I found only three cases with protruding primers.
Trying to determine where the shortening had occurred, I hit upon the idea of inserting the cases into the case holder of a Wilson Trimmer and measuring case protrusion. The case holder is a steel cylinder with a tapered hole in it; the case is held in by its body taper. By inserting the case into the wrong end of the case holder, any enlargement in the diameter of the body of the case is readily apparent. For example, a once-fired .30-06 Federal case which measured 2.492", protruded from the case holder 1.410". A once-fired .30-06 Remington case measuring 2.483", protruded 1.496". Inserting the cases into the holder as intended, the head of the Federal case protruded .032" more than the Remington case. It was readily apparent from the differences in amount of protrusion of the .30-06 cases that factory chambers, for any given cartridge, come in a variety of shapes and sizes.
I tried full length resizing a number of the .30-06 cases to see how much I could increase their length; unlike the cases used for my light cast bullet loads, the best I could do was about .005". The fired Federal cases that measured 2.492" increased to 2.496" finally, case lengthening.
As a result of the tests and measuring, I now believe that the case shortening gremlin is a fat chamber. When a cartridge is fired, whether it is with light loads, heavy loads, or even with just a primer and bullet, the brass is forced out against the chamber walls causing the body of the case to expand. Expansion of the brass, at right angles, away from the longitudinal axis of the case, causes that axis to shorten. The degree of shortening is dependent on case and chamber size. For those who still believe that the force of the primer flash is enough to push the case forward and set the shoulder back, why did the seating depth of the bullet in the case loaded with just the primer and bullet remain the same? Surely if there was enough forward force to set back the shoulder of the case, there should have been enough force to drive the bullet at least partially forward in the case neck.
How did the belief that light loads shorten case length start? Maybe it's because most cast bullet shooters only necks size their cases. Since we don't work our brass enough to cause it to flow towards the neck of the case, we are more aware of case shortening than the shooters using full power loads, who full length resize their cases as a matter of routine and get longer case necks as a result.
How did the belief that light loads shorten case length start? Maybe it's because most cast bullet shooters only necks size their cases. Since we don't work our brass enough to cause it to flow towards the neck of the case, we are more aware of case shortening than the shooters using full power loads, who full length resize their cases as a matter of routine and get longer case necks as a result.
After all the years I believed only light and medium loads shortened cases, I still find it difficult to accept the results of my test. I just got up from the computer and measured some once-fired .30-06 National Match cases I had stashed away. You guessed it short, 2.482".
