Back pressure
- Thread starter Blench99
- Start date
Any gas cutting on the bolt face?
HonestJohn
Well-Known Member
Plastic bolt shroud ?
Wouldn’t blame the ammo
Wouldn’t blame the ammo
Perhaps the shroud just came off, not an unknown thing, maybe an imagined primer issue.?
Any debris or soot in the mag well?
HonestJohn
Well-Known Member
Definitely have a good smith inspect it for stress fractures that could cause a real accident before using again
Heym SR20
Well-Known Member
Factory Ammo? Rifle cleaned with oil left in chamber? Sooty streaks on case? Primer pearced?
Potential lack of obturation and sealing of the chamber thus allowing gases to blow back along the bolt race way.
Yup, Best guess on the picture of bolt is gas / soot markings.
HonestJohn
Well-Known Member
Pitting happens over time. I’m guessing he reloads this rifle, you stated Norma bullets, not ammo? Or, previous owner reloaded for it?
We are looking at a blown primer, the face pitting is minor and another matter, although there are signs of escaping gas and cutting indeed
What caused the blown primer is what requires investigation. If factory ammo indeed, it is highly likely to be something ‘else’. I don’t see the striating marks typically seen on cases from oil-in-chamber, but if he did leave oil on the case for xyz reason, it is possible. He would have had a stiff bolt as well to say the least,
The primer is not cratered or flattened, pin just went right through! Check firing pin protrusion as well, esp after gas blow back.
Here’s one from another post -
.I've had primers blow on a Tikka, and what usually happens is that a bit of the primer anvil gets blown in through the firing pin hole which can cause the pin to jam so you just get a click if you try to fire another round.
Then if the pin frees itself when the bolt lugs are disengaged as you unload, you could have a pipe bomb going off next to your face.............and
I’d really have a smith take the bolt apart, check it, measure protrusion, etc.
We are looking at a blown primer, the face pitting is minor and another matter, although there are signs of escaping gas and cutting indeed
What caused the blown primer is what requires investigation. If factory ammo indeed, it is highly likely to be something ‘else’. I don’t see the striating marks typically seen on cases from oil-in-chamber, but if he did leave oil on the case for xyz reason, it is possible. He would have had a stiff bolt as well to say the least,
The primer is not cratered or flattened, pin just went right through! Check firing pin protrusion as well, esp after gas blow back.
Here’s one from another post -
.I've had primers blow on a Tikka, and what usually happens is that a bit of the primer anvil gets blown in through the firing pin hole which can cause the pin to jam so you just get a click if you try to fire another round.
Then if the pin frees itself when the bolt lugs are disengaged as you unload, you could have a pipe bomb going off next to your face.............and
I’d really have a smith take the bolt apart, check it, measure protrusion, etc.
HonestJohn
Well-Known Member
As noted above, doesn’t look like it, but check for oil in chamber. Happened to me, but it left clear marks on the cases that I’m not seeing.
It is highly unlikely Norma f’d up IMHO. The gas cutting also indicates a possible further issue, worth having headspace checked.
I would not just keep going with the rest of the box, a bolt shroud coming off from gas flying through the bolt and raceway should send warning flags waving
It is highly unlikely Norma f’d up IMHO. The gas cutting also indicates a possible further issue, worth having headspace checked.
I would not just keep going with the rest of the box, a bolt shroud coming off from gas flying through the bolt and raceway should send warning flags waving
Ratwhiskers
Well-Known Member
Take the rifle to a competent gunsmith (to be on the safe side), and have him drop a headspace gauge into the chamber, then have him measure a live round from the ammunition remaining, l should expect a discrepancy of 5-7thou' short in the live round.
Factory ammunition is produced "Within Spec' - according to either SAAMI or CIP, depending upon who made it, and as such needs to be able to chamber in each and every rifle it is to be used in.
Here lays the potential problem... The indicator is there on the case situated on the left.
A cartridge case is made thicker towards to rear, or case head, this has several attributes. lt allows to case mouth and shoulder to expand first, so sealing the chamber at the throat to retard gas leakage, then as the pressure builds the expansion progresses down the case until it reaches the case head/bolt face, where it finally stops.
This is all initiated by the firing pin pushing against the primer, which forces the whole case forward until it makes contact against the front/shoulder of the chamber, and here's where the 5-7thou' comes into significant play.
As the firing pin pushes the case forward, and the primer ignites and sets about burning the powder column, back pressure is already building. This can result in the primer acting as an independent piston to push back against the firing pin (into that 5-7 thou' gap between the case head and the bolt face), and begin to move backwards out of it's aperture in the case head, and now that the firing pin energy is spent, pushing the firing pin back into the bolt face. This you can see on the un-pierced primer (case head on left) which shows primer cup flow into the firing pin aperture (sometimes the whole primer will look as if it has been riveted flat into it's aperture) .
This can sometimes cause the edges of the primer cup to shear and a small section enter the firing pin aperture (sometimes causing a firing pin jam, or not) and in so causing gas flow back into the bolt chamber, and away through whatever system has been put in place by the manufacturer to deal with such an event taking place.
Conclusions....
Weak primer cup, firing pin aperture to large, too much headspace on the live round/rifle chamber, all of the afore mentioned, we only know that it does happen from time to time, in some rifles exhibiting curtain characteristics.
The bolt held, and the shroud popped off, as it's meant to do in such circumstances having deflected much of the pressure away from the shooter.
l'm pleased your friend is safe and unhurt, he'll be able to put it down to experience, and hopefully carry on in his shooting sports.
Factory ammunition is produced "Within Spec' - according to either SAAMI or CIP, depending upon who made it, and as such needs to be able to chamber in each and every rifle it is to be used in.
Here lays the potential problem... The indicator is there on the case situated on the left.
A cartridge case is made thicker towards to rear, or case head, this has several attributes. lt allows to case mouth and shoulder to expand first, so sealing the chamber at the throat to retard gas leakage, then as the pressure builds the expansion progresses down the case until it reaches the case head/bolt face, where it finally stops.
This is all initiated by the firing pin pushing against the primer, which forces the whole case forward until it makes contact against the front/shoulder of the chamber, and here's where the 5-7thou' comes into significant play.
As the firing pin pushes the case forward, and the primer ignites and sets about burning the powder column, back pressure is already building. This can result in the primer acting as an independent piston to push back against the firing pin (into that 5-7 thou' gap between the case head and the bolt face), and begin to move backwards out of it's aperture in the case head, and now that the firing pin energy is spent, pushing the firing pin back into the bolt face. This you can see on the un-pierced primer (case head on left) which shows primer cup flow into the firing pin aperture (sometimes the whole primer will look as if it has been riveted flat into it's aperture) .
This can sometimes cause the edges of the primer cup to shear and a small section enter the firing pin aperture (sometimes causing a firing pin jam, or not) and in so causing gas flow back into the bolt chamber, and away through whatever system has been put in place by the manufacturer to deal with such an event taking place.
Conclusions....
Weak primer cup, firing pin aperture to large, too much headspace on the live round/rifle chamber, all of the afore mentioned, we only know that it does happen from time to time, in some rifles exhibiting curtain characteristics.
The bolt held, and the shroud popped off, as it's meant to do in such circumstances having deflected much of the pressure away from the shooter.
l'm pleased your friend is safe and unhurt, he'll be able to put it down to experience, and hopefully carry on in his shooting sports.
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