BoydAllen
Gold $$ Contributor
A blanked primer looks like a round hole was punched in the primer, as if it had been done with a die and punch. It can happen for a variety of reasons, but if your case does not have excessive "headspace" in its chamber, the firing pin spring is not weak, the tip of the firing pin has a proper shape, and protrusion, the clearance between diameter of the firing pin tip, and the hole in the bold face is not too great, you should not have that problem.
Not all bullets have pressure rings. Many do not. The way that you detect one is to measure the bullet's "shank" so that the micrometer (that reads to .0001") does not contact the heel of the bullet or in the case of a BT the transition point between the shank and the BT. That measurement is compared to one taken over either of those areas being careful not to put too much pressure on the mic. when taking the reading, and a pressure ring will show up as the difference between the two measurements. Because I mostly shot Sierra bullets before I got into custom hand made bullets, such as are used for short range benchrest competition, I did not think that any factory bullets had pressure rings, but later, when I checked a variety of factory BTs I found that some brands did in fact have pressure rings.
As to your AI and doughnuts, when you fire form your cases, some of the neck brass is pulled back into the top of the shoulder, so as long as you are careful about setting your FL die for shoulder bump, and your die does not reduce the diameter of your cases excessively during firing , you are unlikely to ever experience the problem. The other factor is shoulder angle, an area where AIs have an an advantage. When a regular '06 is fired, because of its relatively shallow shoulder angle, the combination of the force of the firing pin strike, and the primer explosion's force against the bottom of the primer pocket, force the case a little way into the chamber, and creating a temporary (if the pressure of the load is not too light) increase in the clearance between the case head and the bolt face. As the pressure from the powder's burning comes into play, the body of the case expands and clings to the chamber wall. With further increase in pressure, the case will stretch just above the solid head, until the head comes into contact with the bolt face. One of the advantages of the sharper shoulder angle of your AI is that it prevents most of this from happening, acting as a more effective stop from the case being forced forward during firing. This also makes the case length more stable, so there is less tendency for shoulder brass to be forced into the base of the neck.
I have a 6mmPPC FL die of a type that is no longer in production. It has inserts in the base that allow excellent control of just how much the base of a fired case is sized. Typically, the way that I set it up,it is less than .001. The die does not change the shoulder diameter of fired brass, it can be set to bump shoulders just like an ordinary FL die, and the necks of the cases are sized by an interchangeable bushing. Because I watch shoulder bump so carefully, and the body of fired cases is reduced so little, case growth is very small. On the other hand, for my factory rifles that I use factory dies for, the amount of case body diameter reduction is generally quite a lot, and even though I do not bump shoulders excessively, cases grow in length at a prodigious rate. The brass that I have to keep trimming off of the end of the necks is there because it is force up the case by the sizing process. If a particular brass manufacturer's brass is thicker at the top of the shoulder than a the base of the neck, eventually that thicker brass will be forced, by repeated sizing, up into the neck.
If a chamber has a relatively close fit between the necks of loaded rounds, and its neck, the formation of a doughnut, in combination with the bullet shank being in that area can create an interference fit with the chamber neck. This will cause an increase in peak pressure because it will take more pressure to move the bullet forward when the rifle is fired, and if the load is already on the warm side, an over pressure situation can occur, which may result in some sort of primer failure. I say may because that will depend on the conditions that I described in my discussion of blanked primers.
Years ago, I made a reloading mistake, and overloaded a .30-06 model 03 Springfield to the point where the primer fell out of the case when the bolt opened. The case was neck sized, so that there was no extra clearance head to shoulder, the firing pin had been replaced with one that was the heaviest available, the firing pin tip shape was perfect, as was its fit in the hole in the bolt face, the edges of the hole in the bolt face were not rounded, and the firing pin assembly of Springfields are among the heaviest that I am aware of. The net result was that the primer was not pierced or cratered (not even a little)., On the other hand, the pocket that it fell out of had been expanded way over size. My point in all of this is that if a primer is pierced or blanked that it may be a good idea to do some investigation to find out why, other than simply assuming that the entire reason was the amount of powder in the case.
If a primer is pierced in some manner, in the area of the firing pin strike, the tip of the firing pin can be cut by the hot gases that escape, and that cut can cause normal pressure loads to have primer failures. Also pieces of blanked primers can end up inside of the bolt, and interfere with ignition.
Not all bullets have pressure rings. Many do not. The way that you detect one is to measure the bullet's "shank" so that the micrometer (that reads to .0001") does not contact the heel of the bullet or in the case of a BT the transition point between the shank and the BT. That measurement is compared to one taken over either of those areas being careful not to put too much pressure on the mic. when taking the reading, and a pressure ring will show up as the difference between the two measurements. Because I mostly shot Sierra bullets before I got into custom hand made bullets, such as are used for short range benchrest competition, I did not think that any factory bullets had pressure rings, but later, when I checked a variety of factory BTs I found that some brands did in fact have pressure rings.
As to your AI and doughnuts, when you fire form your cases, some of the neck brass is pulled back into the top of the shoulder, so as long as you are careful about setting your FL die for shoulder bump, and your die does not reduce the diameter of your cases excessively during firing , you are unlikely to ever experience the problem. The other factor is shoulder angle, an area where AIs have an an advantage. When a regular '06 is fired, because of its relatively shallow shoulder angle, the combination of the force of the firing pin strike, and the primer explosion's force against the bottom of the primer pocket, force the case a little way into the chamber, and creating a temporary (if the pressure of the load is not too light) increase in the clearance between the case head and the bolt face. As the pressure from the powder's burning comes into play, the body of the case expands and clings to the chamber wall. With further increase in pressure, the case will stretch just above the solid head, until the head comes into contact with the bolt face. One of the advantages of the sharper shoulder angle of your AI is that it prevents most of this from happening, acting as a more effective stop from the case being forced forward during firing. This also makes the case length more stable, so there is less tendency for shoulder brass to be forced into the base of the neck.
I have a 6mmPPC FL die of a type that is no longer in production. It has inserts in the base that allow excellent control of just how much the base of a fired case is sized. Typically, the way that I set it up,it is less than .001. The die does not change the shoulder diameter of fired brass, it can be set to bump shoulders just like an ordinary FL die, and the necks of the cases are sized by an interchangeable bushing. Because I watch shoulder bump so carefully, and the body of fired cases is reduced so little, case growth is very small. On the other hand, for my factory rifles that I use factory dies for, the amount of case body diameter reduction is generally quite a lot, and even though I do not bump shoulders excessively, cases grow in length at a prodigious rate. The brass that I have to keep trimming off of the end of the necks is there because it is force up the case by the sizing process. If a particular brass manufacturer's brass is thicker at the top of the shoulder than a the base of the neck, eventually that thicker brass will be forced, by repeated sizing, up into the neck.
If a chamber has a relatively close fit between the necks of loaded rounds, and its neck, the formation of a doughnut, in combination with the bullet shank being in that area can create an interference fit with the chamber neck. This will cause an increase in peak pressure because it will take more pressure to move the bullet forward when the rifle is fired, and if the load is already on the warm side, an over pressure situation can occur, which may result in some sort of primer failure. I say may because that will depend on the conditions that I described in my discussion of blanked primers.
Years ago, I made a reloading mistake, and overloaded a .30-06 model 03 Springfield to the point where the primer fell out of the case when the bolt opened. The case was neck sized, so that there was no extra clearance head to shoulder, the firing pin had been replaced with one that was the heaviest available, the firing pin tip shape was perfect, as was its fit in the hole in the bolt face, the edges of the hole in the bolt face were not rounded, and the firing pin assembly of Springfields are among the heaviest that I am aware of. The net result was that the primer was not pierced or cratered (not even a little)., On the other hand, the pocket that it fell out of had been expanded way over size. My point in all of this is that if a primer is pierced or blanked that it may be a good idea to do some investigation to find out why, other than simply assuming that the entire reason was the amount of powder in the case.
If a primer is pierced in some manner, in the area of the firing pin strike, the tip of the firing pin can be cut by the hot gases that escape, and that cut can cause normal pressure loads to have primer failures. Also pieces of blanked primers can end up inside of the bolt, and interfere with ignition.
