This problem may be related to a weak spring but is just as likely to be due to the large hole and pin diameter. The certain cure is to bush the bolt and reduce the diameter of the pin. A switch to a different brand of primer can help but that's a stop gap measure.
M70 EW cratering primers
- Thread starter Ruger007
- Start date
The first case on the left shows pressure signs compared to the other two, of coarse firing pin diameter can be verified. Check also for the very faint ejector slot marks.
I would segregate the cases by weight and retry a series of loads working up to max.
Those cases are strong but heavy and you can go overpressure in a heartbeat with normal loads, they don't show pressures well.
BB
The first case shows signs of a head clearance issue; not pressure. Anyone who shoots post-64 Model 70's a lot will, sooner or later, come across this primer blanking problem. The firing pin diameter and the firing pin hole are too large and too much of the primer is unsupported. Even Remington 700's, which have a smaller diameter pin and hole, are borderline with small rifle primers. Mausers, Remingtons, Savages, Rugers, pre-64 Model 70's, you name it, all have smaller diameter pins and holes than does the post -64 Model 70.
Another thing: later Model 70's have a lighter striker assembly. This decreases locktime (good), increases firing pin impact velocity (good) but also reduces the inertia of the pin which would help to support the primer cup (bad).
I have to mention as well, when one primer blanks, the firing pin tip is often gas cut and becomes more likely to pierce primers.
I have one Model 70 which has a heavier spring and has never blanked a primer but another, heavier spring, new tip and all, still blanked until I finally gave up and bushed the bolt and reduced the pin diameter.
Another thing: later Model 70's have a lighter striker assembly. This decreases locktime (good), increases firing pin impact velocity (good) but also reduces the inertia of the pin which would help to support the primer cup (bad).
I have to mention as well, when one primer blanks, the firing pin tip is often gas cut and becomes more likely to pierce primers.
I have one Model 70 which has a heavier spring and has never blanked a primer but another, heavier spring, new tip and all, still blanked until I finally gave up and bushed the bolt and reduced the pin diameter.
Head clearance is the space between a closed bolt face and that particular chambered case. It is often incorrectly referred to as headspace... a chamber can be set up using steel gauges to have the correct headspace but when a cartridge is sized slightly too much and the shoulder is pushed back, you will have a chambered cartridge with head clearance. This is different than a chamber that has been incorrectly chambered so there is excessive headspace measured with steel gauges.
You got it...
Often with slight head clearance or headspace.. and a normal load... the primer can back out of the case on ignition and when the pressure builds up the case will back up.. flattening the primer making it look like high pressure did it... but there are usually no other signs... easy extraction, not overloaded, etc.
Sorry Bill, I don't mean to be answering for you...
OP,
In a Winny of any vintage check to ascertain if the chamber is ROUND by attempting to rotate a fired piece of brass in the chamber.....most will NOT.
With your overbored 264WM
Clean the throat of any & all carbon deposits or your pressure issues will persist & cause fits.
.
In a Winny of any vintage check to ascertain if the chamber is ROUND by attempting to rotate a fired piece of brass in the chamber.....most will NOT.
With your overbored 264WM
Clean the throat of any & all carbon deposits or your pressure issues will persist & cause fits.
.
There is a slight drawback in that the primer strike has to be more precise but in the real world, it's not that critical. The other drawback may be that the pin tip is simply not as robust. Lee Enfields, P14's Remington 788's and a host of custom actions have smaller pins. As I said before, only the post-64 Model 70 has a pin and hole this big.
Something I did not notice before but it smacked me in the face now.
You state that you had around 100 rounds before it started cratering and had no issue during the work up and using new brass.
So what did you change to cause this after 100 rounds ?
BB
