rockydoc
Member
Cartridge head space as I understand it is measured from the face of the cartridge head (face of the bolt) to that part of the cartridge which stops the cartridge from moving any farther into the chamber(the datum). In the case of the .308 family of cartridges the datum is that point on the shoulder which is .400" in diameter.
This is a definition, not an explanation and can be confusing. Isn't head space effectively just the space between the bolt face and cartridge head? If there is too little head space the bolt will not close on the cartridge? In other words the cartridge is too long from the head to the datum. If there is too much cartridge head space the cartridge is too short.
The more dangerous of those two situations it would seem to me, would be too much cartridge head space, when the cartridge is too short. If there is so much space the firing pin could not strike the primer there is not so much danger as would be the case where the firing pin drives the case forward until the shoulder stops it then the primer ignites the powder charge. The pressure expands the case, the neck obturates and anchors the case to the chamber wall while the head is not touching the bolt face (too much head space) allowing the head to separate from the case releasing up to 60,000psi of expanding gases. That could, as they say "cause equipment damage and bodily harm".
Now to the subject of "shoulder bumping". I am not sure what that means. I think it means that if a cartridge shoulder has moved forward and making the cartridge too long (too little head space) you would move the shoulder back in a full length sizing die.
I have .260 Remington cases (new Nosler) which I have never full length sized. I have neck sized only with a Lee Collet Neck Sizer. They have been fired and reloaded three or four times.
The numbers for my cases measured with the Hornady Head Space Gage with the .400" datum:
New unfired and unsized= 1.611"
Fired and reloaded 3-4 times neck sized only= 1.618"
SAAMI chamber specs:
Minimum = 1.630"
Maximum = 1.640"
SAAMI cartridge case specs:
1.6340"~.0070" (Does this mean the allowable amount of space between case head and bolt face is .007"?
If that last line means that a range of 1.6340 to 1.6270 (1.6340-.0070), then my brass is shorter than any of these dimensions. Does this mean that my head space is too great? I have not had any signs of trouble. I would think that if my head space was too great I would have some elongation of cases, wouldn't I? The controlled round feed of my Kimber rifle should hold the case to the face of the bolt, allowing the firing pin to strike the primer, but if the excess space is between the case shoulder and chamber shoulder that would allow cases to elongate. But they haven't.
There is more to this than I can grasp. Or am I making things more complicating than they need be?
Rockydoc
NRA Life member since 1956, Benefactor
This is a definition, not an explanation and can be confusing. Isn't head space effectively just the space between the bolt face and cartridge head? If there is too little head space the bolt will not close on the cartridge? In other words the cartridge is too long from the head to the datum. If there is too much cartridge head space the cartridge is too short.
The more dangerous of those two situations it would seem to me, would be too much cartridge head space, when the cartridge is too short. If there is so much space the firing pin could not strike the primer there is not so much danger as would be the case where the firing pin drives the case forward until the shoulder stops it then the primer ignites the powder charge. The pressure expands the case, the neck obturates and anchors the case to the chamber wall while the head is not touching the bolt face (too much head space) allowing the head to separate from the case releasing up to 60,000psi of expanding gases. That could, as they say "cause equipment damage and bodily harm".
Now to the subject of "shoulder bumping". I am not sure what that means. I think it means that if a cartridge shoulder has moved forward and making the cartridge too long (too little head space) you would move the shoulder back in a full length sizing die.
I have .260 Remington cases (new Nosler) which I have never full length sized. I have neck sized only with a Lee Collet Neck Sizer. They have been fired and reloaded three or four times.
The numbers for my cases measured with the Hornady Head Space Gage with the .400" datum:
New unfired and unsized= 1.611"
Fired and reloaded 3-4 times neck sized only= 1.618"
SAAMI chamber specs:
Minimum = 1.630"
Maximum = 1.640"
SAAMI cartridge case specs:
1.6340"~.0070" (Does this mean the allowable amount of space between case head and bolt face is .007"?
If that last line means that a range of 1.6340 to 1.6270 (1.6340-.0070), then my brass is shorter than any of these dimensions. Does this mean that my head space is too great? I have not had any signs of trouble. I would think that if my head space was too great I would have some elongation of cases, wouldn't I? The controlled round feed of my Kimber rifle should hold the case to the face of the bolt, allowing the firing pin to strike the primer, but if the excess space is between the case shoulder and chamber shoulder that would allow cases to elongate. But they haven't.
There is more to this than I can grasp. Or am I making things more complicating than they need be?
Rockydoc
NRA Life member since 1956, Benefactor