Re: magnum primers
Posted: Thu Sep 03, 2015 7:31 pm
The OP's description sounds like 'cratering' around the firing pin indentation which is extremely common with the 6.5X47L and similar high-pressure numbers that use SR primers. It's a sign of excessive pressure being applied to the primer cup metal in relation to (1) the cup thickness and brass alloy strength and (2) the firing pin fit and clearance in the bolt-face and can also be linked to a weak bolt mainspring.
So .... cratering which when it gets bad enough turns into 'primer blanking' when the cup fails around the indentation and a small plug detaches and is blown back into bolt body can be the result of:
excessive pressures. This would normally be accompanied by severe flattening and other pressure symptoms such as hard initial bolt lift / case extraction, marked case-heads especially swaging into the ejector button recess, short case life ... etc, etc
the bolt firing pin aperture to firing pin fit issue. Many factory rifle actions produce cratered / blanked primers with perfectly acceptable pressure loads. Custom target actions from BAT, Stolle, RPA and others use a smaller diameter pin than Winchester 70s and Remy 700s and suchlike and have a minimal clearance fit so don't suffer this effect even at max pressures. (For some reason, SR primer cartridges are more badly affected than LR primer specced numbers.)
Then there is the primer make / type issue as it affects cup strength. Apart from the usual variations in brass alloys in use between makers, there are three variations on the SR primer theme. Whilst LR size primers all use the same nominal thickness cups (0.028") irrespective of whether standard / BR / Magnum, SR primers are specced as
standard SR: 0.020"
Magnum / BR: 0.025"
and just to be different Federal SR match (F205M) are 0.0225" splitting the difference.
Remington advises that its #6 1/2 standard model SR primer should only be used in low-pressure cartridges such the .22 Hornet, Bee and mild .222 Rem loads with the #7 1/2 Magnum or 7 1/2BR used for modern high-pressure cartridges such as .223 Rem in normal loadings. The other makers just don't say anything, but people know that some crater and blank very easily
For the 6.5X47L, I'd always advise a magnum or BR primer for its extra strength. Moreover with charge weights in the high 30s gn mark getting on for 40gn in some cases, the SR primer is close to its thermal capabilities. This applies even more so to the .308 Win Lapua Palma case where 45-49gn charges are at the SR primer's limits and can be adversely affected by cold weather. (Why Lapua says Palma brass should not be used for loading 'hunting ammunition' with the North American winter season deerhunter in mind.) The extra heat and energy of the magnum / BR models is potentially essential in these cartridges.
Halodin's pic of the whole primer cup backed out of the pocket is a headspace issue which can be exacerbated by a light load. There may be nothing wrong with the rifle chamber dimensions per se, or it's at the max CIP or SAAMI tolerance, as it can be the case dimensions as measured by case-head face to headspace datum line point on the shoulder that are over-short. In any event, when the primer backs out like this, it's evidence of an over-large gap between the case shoulder and the matching point of the chamber. When the rifle is fired, the firing pin pushes the cartridge forward until it contacts the chamber in the shoulder area at which point there is a small gap between the case-head and bolt-face. Unless it is a milspec round with a staked or ringed in primer, the primer is unsupported and its detonation will cause it to move backwards until it contacts the bolt and is stopped and it will also flatten to a greater or lesser extent, the amount depending on the internal pressures generated by the main charge burning and the cup's tensile strength. As pressure builds up in the case, it fireforms to the chamber, the case-head is pushed back into hard contact with the bolt and this reseats the primer fully. Unless the headspace mismatch is very great and/or it is a low pressure load that is, in which conditions the primer will remain partly proud of the pocket.
So .... cratering which when it gets bad enough turns into 'primer blanking' when the cup fails around the indentation and a small plug detaches and is blown back into bolt body can be the result of:
excessive pressures. This would normally be accompanied by severe flattening and other pressure symptoms such as hard initial bolt lift / case extraction, marked case-heads especially swaging into the ejector button recess, short case life ... etc, etc
the bolt firing pin aperture to firing pin fit issue. Many factory rifle actions produce cratered / blanked primers with perfectly acceptable pressure loads. Custom target actions from BAT, Stolle, RPA and others use a smaller diameter pin than Winchester 70s and Remy 700s and suchlike and have a minimal clearance fit so don't suffer this effect even at max pressures. (For some reason, SR primer cartridges are more badly affected than LR primer specced numbers.)
Then there is the primer make / type issue as it affects cup strength. Apart from the usual variations in brass alloys in use between makers, there are three variations on the SR primer theme. Whilst LR size primers all use the same nominal thickness cups (0.028") irrespective of whether standard / BR / Magnum, SR primers are specced as
standard SR: 0.020"
Magnum / BR: 0.025"
and just to be different Federal SR match (F205M) are 0.0225" splitting the difference.
Remington advises that its #6 1/2 standard model SR primer should only be used in low-pressure cartridges such the .22 Hornet, Bee and mild .222 Rem loads with the #7 1/2 Magnum or 7 1/2BR used for modern high-pressure cartridges such as .223 Rem in normal loadings. The other makers just don't say anything, but people know that some crater and blank very easily
For the 6.5X47L, I'd always advise a magnum or BR primer for its extra strength. Moreover with charge weights in the high 30s gn mark getting on for 40gn in some cases, the SR primer is close to its thermal capabilities. This applies even more so to the .308 Win Lapua Palma case where 45-49gn charges are at the SR primer's limits and can be adversely affected by cold weather. (Why Lapua says Palma brass should not be used for loading 'hunting ammunition' with the North American winter season deerhunter in mind.) The extra heat and energy of the magnum / BR models is potentially essential in these cartridges.
Halodin's pic of the whole primer cup backed out of the pocket is a headspace issue which can be exacerbated by a light load. There may be nothing wrong with the rifle chamber dimensions per se, or it's at the max CIP or SAAMI tolerance, as it can be the case dimensions as measured by case-head face to headspace datum line point on the shoulder that are over-short. In any event, when the primer backs out like this, it's evidence of an over-large gap between the case shoulder and the matching point of the chamber. When the rifle is fired, the firing pin pushes the cartridge forward until it contacts the chamber in the shoulder area at which point there is a small gap between the case-head and bolt-face. Unless it is a milspec round with a staked or ringed in primer, the primer is unsupported and its detonation will cause it to move backwards until it contacts the bolt and is stopped and it will also flatten to a greater or lesser extent, the amount depending on the internal pressures generated by the main charge burning and the cup's tensile strength. As pressure builds up in the case, it fireforms to the chamber, the case-head is pushed back into hard contact with the bolt and this reseats the primer fully. Unless the headspace mismatch is very great and/or it is a low pressure load that is, in which conditions the primer will remain partly proud of the pocket.