Flattened Primers - 308??

[Dave P]

I shot 20 carefully weighed and made hand loads for my 308 garand. Same load I have used many times before. 41.5 gr, xmr4064, Nos168, Win case, cci primer. Nice generic, mild load.

Did load them long: about 10 mils from the lands, for accuracy (2.88" oal).

All primers were flatter than normal, indicating high pressure. 2 primers had the firing pin detent blown back into the f/p hole, indicating very high pressure.

Temps were OK; I can't think of anything unusual. Even if the bullets were jammed into the lands, I would not expect these high pressure signs.

Ideas?

Thanks
Dave

 

[HankL]

Any lube left on the cases? Just thinking out loud so to speak.

 

[50 Shooter]

Are the shoulders set back to much? If you have any of the ammo that you loaded, place it next to a factory load as a quick ref.

If they are, check your dies as they might have moved. Any signs of pressure near the base, like a ring around it?

 

[warrior23]

Do you have other brass fired from the same gun to compare?Sometimes slightly flatten primers doesnt necessarily mean high pressure,usually this with other signs would lead to high pressure.

 

[Dave P]

Forgot to add: these are virgin Win cases, so the case volume is slightly less than a fired case, which should increase pressures just a tad.

Virgin cases: no lube issues.

5 unfired rounds: weight is with 2 gr of freshly loaded rounds, and they fit the case gauge perfectly - headspace between min and max.

No pressure or stretch rings around the base (this rifle is kind to my brass, not like my M1A!)

Warrior, no other fired cases today, but I don't recall flattened primers in the past (new bbl 3 wks ago).


Thanks for the brain power, guys. I guess I will double check the OAL to the lands, and then just keep an eye on things!

Thanks
Dave

 

[Poodleshooter]

Longer seating close to the lands or possibly touching would be my guess. Sometimes I see weird velocity changes when I get the ogive close to or touching the lands, even though the bullet is seated further out. Longer seating should reduce pressure, but that's not always the case.
Also, any possibility that the Garand action is jamming the bullets a bit deeper? Try extracting a round after feeding and measuring it.

 

[hps1]

Virgin brass results in lower muzzle velocities in all of the rifles I have checked. Consensus of opinion is that the stretching of the brass as it expands to fit the chamber acts as a shock absorber, thus less velocity. I would suspect that it would also result in a slightly lower breech pressure as well, but that's just my guess. Can't remember where I ran across a thread discussing this, but if I can locate it, will post a link.

Seating bullet close to the lands often results in higher pressure as Poodleshooter has stated. Many magnum rifle manufacturers freebore their rifles to allow for more powder/higher muzzle velocities while keeping breech pressure at manageable levels.

One other cause of flat primers can be excessive headspace which can be a result of over-sizing your cases. If you set your dies to size properly for your rifle using brass which had been reloaded several times (work hardened brass), and then size the new cases w/same die setting, these new cases will have more headspace than the work hardened cases since they do not spring back upon removal from sizing die. The reverse is true, also, therefore, it is a very good idea, when loading for a gas gun, to routinely check loaded rounds for proper dimensions using a cartridge case gauge. (Insufficient headspace can cause slamfire in gas guns and damage the rifle and/or injure shooter.) Edited: Just noticed you have checked your unfired cases in a case gauge....looks like OAL may be the culprit.

A case which has the shoulder pushed back too far is driven forward all the way into the chamber by the firing pin, forward part of the case expands but head of case is not supported by the bolt at this instant, allows primer to be pushed back against bolt face which flattens primer, then the case stretches to fill chamber and pushes primer back into p. pocket.

Regards,
hps

PS Here's the link to the discussion on velocity differences between new and once fired brass:
http://www.nookhill.com/cgi-bin/ubbcgi/ultimatebb.cgi?

 

[HankL]

Dave, This wasn't your first outing since the rebarrel was it? Still thinking out loud so to speak.

 

[Dave P]

first outing ? No, I have about 200 rnds thru it already.

I will make a dummy round tonight (same OAL, new brass), and let the bolt slam it home, then see if there are any scrapes on the ogive ...

 

[Mal H]

Don't know if anyone has asked this yet, but what kind of crimp are you using, if any? Garands, and almost all semi-autos, need a heftier crimp than a bolt action which, in general, doesn't need one at all. As soon as you said "let the bolt slam it home" above, I started envisioning the bullet sliding forward enough to touch the lands.

Perhaps you could use a little Magic Marker around the bullet at the case mouth. When you try your experiment to see if the bullet is touching the lands, you will be able to see if the crimp is holding tight even if the bullet doesn't make it to the lands. If a tiny gap in the paint shows up, then it's a good possibility that the suspect rounds may have had loose bullets also.

 

[Lucky Jim]

.308 loads

I ran into a problem using 4064 @ 41 grains and 168 grain Sierra HPBTM bullets in my Remington LTR .308. I was using new brass, Federal primers and bullets were uncrimped. I was loading single rounds in the bolt gun. The powder I started out with was nearly 5 years old and had been properly stored. At 41 grains using the old powder I had no indications of pressure at all and it was very accurate. I bought a new 1 pound cannister of 4064 and loaded 100 rounds using the newer powder. I immediately noticed that the primers were flattened a little. No really big deal and still accurate. The old cannister of 4064 using same brass, bullets, etc. did not flatten primers at all. They looked like any factory round. I could only figure that maybe the powder was just a little different. My bullets were also seated long, about .010" from rifling. I know the gas gun is a whole different scene as far as reloading and the replies you have received probably have solved your problem. I just thought that what has happened with my load might be something you have not considered. I just dropped back 1.5 grains and all is well and still accurate. I had always thought that powder would be very close to the same, lot to lot and year to year. But maybe it is not.

 

[warrior23]

Dave, I would lean towards the OAL then,if you didnt have any prior problems with the same load only shorter.The only other thing I would try is either firing a couple factory rounds down the tube or if ya have some of the shorter OAL rounds and visually inspect them to see if the primer is flattening still.I have a Remy LTR and with handloads and factory ammo it seems to slightly flatten the primers no other signs of high pressure.

 

[HankL]

Dave, Are you going to leave us hanging?

 

[Dave P]

Uh, sorry guys - busy day at work! I shortened the rounds about 20 mils, and shot them with no flattened primers, no craters etc.

So surprising for such a mild load, but it appears that the bullets were hitting the lands and raising the pressure to un-acceptable levels.

Thanks for the help!

Dave

 

[Edward429451]

Interesting.