How did this happen?

[amlevin]

Just recently I had a rash of "misfires" with my .308, something I haven't experienced in several thousand rounds.

All were the result of a light primer strike.

Investigation showed that-

Firing pin protrusion was more than adequate
Firing pin spring and bolt channel were free of gunk and grease
Primers were properly seated

Only thing out of "tolerance" was that the cases involved had excessive headspace (or more appropriately, the headspace measurement was too short).

All of my freshly annealed, fired cases, measure 1.625", using the Stoney Point/Hornady gauge. The cases involved in the misfires measured 1.612" in the same manner.

Primers were seated to a recess of .007". All of my BR-2's measure from -.005" to .007" with this particular brand of brass (BHA Match) and hasn't been a problem with cases that measure the 1.625" "Headspace" measurement, just the "short ones".

Now for the big question, "How did this happen to these few cases". I've checked all my sizing dies, a Redding Body Die, a Forster BR FL sizing die, and even a Lee .308 FL die. All are adjusted properly and don't bump the shoulder beyond the .001" I use when FL sizing after the cases get a little "tight".

Any ideas out there that might cause the extra .013" or so that causes the case to go far enough into the chamber that it won't fire????

Most of the time I neck size only with either a Lee Collet or RCBS Neck Size only die. Again, all cases seem to come out without being shortened like these few.

Needless to say, I'm now case gauging every case now with a Dillon gauge. Adds some time but I get tired of practicing "Flinch Control" with the unexpected "duds".

All Ideas, Suggestions, and even comments, that might put me on the track of what caused this would be appreciated. All the cases involved were not new to me and have been sized/loaded/shot several times in the same rifle. It's something new that's happened and I just can't get my mind around it.

Thanks in advance.

 

[Eddie Harren]

From looking at your original "headspace" measurements 1.625" after firing vs1.612" before firing, I'd say .013" of headspace. If your bullets were seated deep they wouldn't hold the case back against the boltface.

 

[jcampbellsmith]

... If your bullets were seated deep they wouldn't hold the case back against the boltface.

Building on this suggestion, have you opened up a new box of bullets? I changed bullets recently and the bullets in the new box were significantly different from the bullets in the old box. Both old and new were labelled as the same bullet and had the same part number, but clearly the manufacturing spec had evolved.

Just a thought. Regards JCS

 

[amlevin]

I have a "magazine length" limitation with this rifle so I can't jamb the bullets into the lands. For that matter I can't even get close. The measured freebore, using mag length OAL is over .150". For me to get a "jamb" I'd have an OAL that first wouldn't fit the magazine and second, not have enough bullet seated in the neck to keep it straight while handling/chambering. This rifle isn't a bench rest quality rifle, just a really accurate "tactical" yielding groups in the teens when it's operator gets his "stuff" together. 8)

With this rifle, the controlling measurement is the headspace from the .400" point on the shoulder to the case head, not the ogive to case head as it would be with a custom chamber.

I realize I might find a real long bullet, seat it out to close to the lands, single load it, and save the case but for the half dozen cases I had this problem with it wouldn't be worth the extra bullets I don't need.

I'm kind of hoping to find out what caused the cases to have their shoulders set back that far to begin with when all my dies checked out OK for settings. I've even measured my shell holders to see if I had one that was shallower than others but they all read the same depth. ??? ???

 

[jlow]

I know from what you posted, it sounds like a headspace problem. However, there is one other thing that you have not looked at and that is the hammer spring. From personal experience, these do get old and weakened which will give you a light primer strike and the symptoms will be “a rash of “misfires””….

You stated you have fired several thousand rounds out of the gun, what type of trigger do you have? The lighter the trigger, the more susceptible you are to the problem I stated above.

 

[6BRinNZ]

Once I started annealing I had to reduce the amount I was FL sizing. Reading you post I can't quite make out if it is related, something to think about either way. Do you use a machine to anneal or by hand?

 

[DanSavage]

I've heard of guys having too much lube building up in the die while sizing causing hydro lock, which would bump the shoulder back too far. Just a thought.

Regards,
Dan

 

[BoydAllen]

Did you use the same die setting for cases before and after annealing? If you did, that is the source of your problem. There is a huge difference in spring back going from work hardened brass to annealed brass. Shooters that load at the range and use a small set of brass over and over are very familiar with this. Any time you go to new brass, or anneal, you need to reset your Fl die. As brass hardens from being fired and FL sized the die will have to be screwed down into the press slightly. Keep in mind that a full turn of a 14 pitch amounts to about .071", so go slow. In the world of 6PPC short range benchrest, it is common for shooters to have their shell holders ground or turned shorter so that they can set the die where it needs to be for .001 bump on cases that have had a lot of firings. After accidentally bumping a new case too much because the die was set for old, hard brass, I now make a practice of resetting my FL die for each loading session. This is not as time consuming as it sounds. I have a Lee lock ring locked to the die with a Hornady, split type ring above it. I turn the die in until the lock O ring that is built into the Lee lock ring touches the top of the press and then start sizing and measuring, making very small changes. With a starting place that is very close, it does not take long.

 

[243winxb]

All of my freshly annealed, fired cases, measure 1.625", using the Stoney Point/Hornady gauge. The cases involved in the misfires measured 1.612" in the same manner.

The strike from the firing pin set the shoulder back, causing the misfire. The brass is to soft from the annealing. If the round fires, you would never know the shoulder had been set back.

 

[amlevin]

I know from what you posted, it sounds like a headspace problem. However, there is one other thing that you have not looked at and that is the hammer spring. From personal experience, these do get old and weakened which will give you a light primer strike and the symptoms will be “a rash of “misfires””….

You stated you have fired several thousand rounds out of the gun, what type of trigger do you have? The lighter the trigger, the more susceptible you are to the problem I stated above.

Brand new Gre Tan fluted firing pin (complete assy). Trigger is Timney set to 1.5 lbs.

As this thread ages, it seems like there were only a few cases that had this "headspace problem". As for the firing pin knocking the shoulder back, I checked some loaded rounds with the case gauge before firing and the short ones wouldn't fire . They were short before the primer struck the primer lightly.

Still a mystery as even my freshly annealed cases, when I size and measure now, don't get as short as the ones that caused my problem.

Bad thing is that I still haven't uncovered the cause. Good thing is that all my cases now are "gauging" to a proper headspace.

I've even checked all my shell holders on the chance I had a "shaved" one. All measure the same from top to where the head of he case seats.

I'm going to stop scratching my head before I get a bald spot that I've been able to avoid so far.

Thanks for all the suggestions and views. Much appreciated.