Primers backing out

[MtGun44]

Happens with light loads. Primer pops out under pressure like a piston,
pushing case hard forward, in some instances even setting the shoulder
back slightly. Then the pressure isn't enough to drive the case head back
against the bolt, the only thing holding is friction of the front 80% or so of
the case against the chamber wall.

No real harm, but may eventually shorten the brass a good bit.

Primers pop out on all loads, but full pressure loads stretch the case out and
reseat them as the case head is pushed to hit the bolt face.

Bill

[Shiloh]

What's the history of the rifle? Might not be a bad idea to borrow a go-no gage.

+1

Particularly on well used military relics.

SHiloh

[whelen fan]

Took some pictures.

Backed out primers, fired brass, 17gr unique with 230 cast and group at 100 yards.

[44man]

All has been said, no comment from me. they key was loads too light.

[Ballistics in Scotland]

You didn't say how far they were backing out, which could be important. It happens to the extent of the headspace excess of chamber (to the shoulder) over the same measurement in the cartridge. If it was excessive it could be caused by an over-deep chamber (perhaps a little dangerous when you switch to full power) or stretching or cracking of the action (worrying at any time.)

Now, however, those pictures look like the headspace is within safe limits.

I think this is primarily an effect of gas pressure, rather than primer pressure within its pocket. Try popping a primed but unloaded case and you might find the primer set back, but more often you won't. It doesn't matter very much though, as the things that happen happen a tiny fraction of a second later.

It happens because the adhesion of case to chamber, caused by the gas pressure, is too great to be overcome by the light pressure of this round. It actually indicates a very safe condition. With hot loads in the same rifle, a few things would happen which aren't very dangerous when it is on this scale, but might limit brass life. The head is forced back against the bolt face, stretching the brass. With a very tapered case that stretching occurs in the last fraction of an inch, but with the very straight-bodied Improved types stretch most of the way back to the shoulder.

I never did believe that cartridges like the .300H&H and .303 necessarily produce less accuracy or shorter brass life, and people used to get extremely high standards in both from rounds like the .30-40. But they need more accurate headspacing to do it.

Another effect of the primer being extruded and subsequently the case stretched backwards, is that it tends to mushroom the primers, leaving them with sharper edges around the edge of their pockets. That can falsify a sign of higher pressure than you in fact have. But it would produce an error in the direction of safety.

[WALLNUTT]

My primers back out in my 30 06 with light cast. It always fires, shoots good,what's to be concerned with? I neck size them and shoot them again.

[whelen fan]

All has been said, no comment from me. they key was loads too light.

If the loads are too light, how much unique can you safely use to make this not happen?

[whelen fan]

You didn't say how far they were backing out, which could be important. It happens to the extent of the headspace excess of chamber (to the shoulder) over the same measurement in the cartridge. If it was excessive it could be caused by an over-deep chamber (perhaps a little dangerous when you switch to full power) or stretching or cracking of the action (worrying at any time.)

Now, however, those pictures look like the headspace is within safe limits.

I think this is primarily an effect of gas pressure, rather than primer pressure within its pocket. Try popping a primed but unloaded case and you might find the primer set back, but more often you won't. It doesn't matter very much though, as the things that happen happen a tiny fraction of a second later.

It happens because the adhesion of case to chamber, caused by the gas pressure, is too great to be overcome by the light pressure of this round. It actually indicates a very safe condition. With hot loads in the same rifle, a few things would happen which aren't very dangerous when it is on this scale, but might limit brass life. The head is forced back against the bolt face, stretching the brass. With a very tapered case that stretching occurs in the last fraction of an inch, but with the very straight-bodied Improved types stretch most of the way back to the shoulder.

I never did believe that cartridges like the .300H&H and .303 necessarily produce less accuracy or shorter brass life, and people used to get extremely high standards in both from rounds like the .30-40. But they need more accurate headspacing to do it.

Another effect of the primer being extruded and subsequently the case stretched backwards, is that it tends to mushroom the primers, leaving them with sharper edges around the edge of their pockets. That can falsify a sign of higher pressure than you in fact have. But it would produce an error in the direction of safety.

My primers do not back out with full power jacketed loads and on cases that have 5 firings there is no indication of incipient case head separation when checking with a bent paper clip.

[44man]

If the loads are too light, how much unique can you safely use to make this not happen?

Don't know. Why Unique when you have slower powders? Big case suited to much better choices.

[whelen fan]

Well I have about 30 lbs of unique in a steel drum from the 50s and the whole reason for me having the whelen is because I can get brass for free and shoot it dirt cheap.

[Ballistics in Scotland]

My primers do not back out with full power jacketed loads and on cases that have 5 firings there is no indication of incipient case head separation when checking with a bent paper clip.

You have just answered your own inquiry in your previous post. With those Unique loads you don't have to stop it happening.

[casac47]

I agree with Don Fischer. Under normal pressures, the casing expands to fill the chamber. Once the shoulder can't move, the rest of the brass swells backward until it slams up against the bolt head. If the primer was backing out, this action will re-seat it in a hurry. You would never notice it unless there is leakage around the primer.

[Yodogsandman]

How about using that Unique load and seating the boolit out far enough to jam in the rifling and push the case back against the bolt face hard. So that the bolt closes with some resistance. This will increase initial pressures, sealing the chamber with the case better and preventing that case from moving as much when the primer is fired.

[TXGunNut]

Looks to me like some good plinking rounds. Good pics of the case; sooty neck, backed out primer, no bright ring at the web. If you had headspace issues you'd probably be seeing a bright ring just above the web after 2-3 firings, if not after the first. If it makes you feel better get some chamber gauges, like most precision tools they aren't cheap.

[murf205]

Well I have about 30 lbs of unique in a steel drum from the 50s and the whole reason for me having the whelen is because I can get brass for free and shoot it dirt cheap.

That's about as good a reason as one could have. I guess it's safe to say that the powder shortages of Unique didn't even make a blip on your radar. Ever wondered how many primers it will take to shoot up 50 lbs of Unique? You are very fortunate.

[44man]

How about using that Unique load and seating the boolit out far enough to jam in the rifling and push the case back against the bolt face hard. So that the bolt closes with some resistance. This will increase initial pressures, sealing the chamber with the case better and preventing that case from moving as much when the primer is fired.

This is the solution. Or neck size for resistance with the bolt. FL is too much.

[Ballistics in Scotland]

That is a solution. Now what is the problem?

[44man]

It is something I never figured out. Brass grips the chamber but does it stretch back to seat a primer again? or does pressure drop so the brass can move back with barrel pressure? Unique will peak fast and lose the barrel pressure so brass has no more drive. Powders that burn in more length to extend peak down the bore will add push. Brass retracts from the walls as pressure drops.
Some military guns that use blow back have fluted chambers to hold brass longer to prevent gun damage.
Unique is just not efficient in the large case so it peaks instantly to punch the boolit and then does no more.
I would look at 3031, 4895 or even Varget and why not 4350?

[Ballistics in Scotland]

It is something I never figured out. Brass grips the chamber but does it stretch back to seat a primer again? or does pressure drop so the brass can move back with barrel pressure? Unique will peak fast and lose the barrel pressure so brass has no more drive. Powders that burn in more length to extend peak down the bore will add push. Brass retracts from the walls as pressure drops.
Some military guns that use blow back have fluted chambers to hold brass longer to prevent gun damage.
Unique is just not efficient in the large case so it peaks instantly to punch the boolit and then does no more.
I would look at 3031, 4895 or even Varget and why not 4350?

Fluted chambers do just the opposite, by reducing friction to permit movement of the case. In many rifles, such as the CETME, headspace works normally and the fluting simply facilitates extraction. But in the Oerlikon 20mm. cannon the chamber actually admitted the bolt and extractor, permitting ignition of the primer before the bolt had come to a standstill. The first impulse of the gas pressure thus came while it had to overcome the inertia of a forward-moving bolt, rather like kicking a door moving towards you will hurt your toe much more than a stationary one.


I wouldn't draw conclusions from what happens to the primer in the Oerlikon, as it was quite exceptionally crimped in. But it originally required greased cases, to permit the long movement of the case in the chamber, and this was later replaced by a fluted chamber.

A fired case with extruded primer indicates that with that load, the case didn't move backwards at all. As to what happens with a high pressure load, there is no reason why it should be the same every time, but I think it is normally the case stretching rearward. If it was otherwise, the location of head separations wouldn't be as precise as we see it to be.

A primer fired in the gun without powder produces far less pressure than a conventional or perhaps even very light load would do. It is also, being a tiny quantity of high explosive, much more momentary. But it is still not low. If you ignite a primer on a hotplate (cover it, because you don't know which way it will go), the explosion is enough to blow out one side of the cup. It is hard to see how the primer could slide rearward with that pressure locking it in its pocket.

I think primer movement comes later, with the more protracted pressure of the powder gases, and probably the early part, while the primer pressure has vanished and powder pressure is low. The Unique load is enough to produce adhesion of the brass, but a higher pressure load would stretch the case back in line with the rear of the primer. The highest would produce swelling of the unsupported part of the primer, and the moving brass would size the primer down, except for the sign we usually find disquieting, the edge of the primer swollen into the chamfered edge of the pocket.

[44man]

High pressure does stretch the brass, good reason not to set shoulders back, just the very light loads like shown that is hard to figure. Not enough pressure to push or stretch but brass will last longer.
There were a few semis and autos made to start the bolt movement with the primer impact until the crimped in primers came out.
The ruined cases I used to find on the range from an auto, don't remember the make but might have been Israely. I read about it and it said the flutes retarded bolt movement until pressure dropped off.
I shoot heavy 296 loads from my .44 with heavy boolits and primers get pretty flat since they move out and the case comes back to re-seat them. Brass never grows so the case is moving. Pressure drop is faster in a revolver because of the gap, so much so carbon gets on the brass and even lube is blown back. There is enough internal pressure on the primer to push the hammer to near full cock. Primer set back can be used to cycle an action but is not reliable.
Some old Colt SAA's had sharp pins that punctured primers and the gun would run full auto.
Hammer bounce and a cylinder lock that jumped out of the slot from inertia is what caused some X frames to double. It was NOT the hammer going back far enough to unlock and rotate the cylinder because the cylinder turned backwards. To prevent doubling, leave the chamber under the hammer empty or put a strong lock spring in. There is a lot of pressure from the flash hole so it does push the primer out.
Have you ever shot a cheap repro Italian ML with a large nipple hole? It will cock the hammer against a very strong spring and blow the cap into shrapnel. I use only CCI or REM caps even with a good nipple, RWS and some Italian caps will be brittle and stick pieces in your chin or nose. Safety glasses are a MUST.