Pressure signs on one round, not others

[coloradokevin]

I'm getting close to honing in on a load for my new AIAX 260, and was shooting a load today that was comfortably below the maximum I've tested in this gun so far (without pressure signs).

However, one round gave me some notable high pressure signs, such as: a cratered primer, extractor marks, difficult bolt lift, and a shot that landed slightly higher than the others in the group. I'm trying to figure out why this may have occurred.

Here's what I can tell you for sure:

1) All of the rounds were loaded with 41.4 grains of H4350 powder, measured with an RCBS Chargemaster, and I'm confident that none were overcharged according to that scale (and I make it a habit to lift the pan off of the scale and put it back down to verify the measurement on each load).

2) The brass was not trimmed prior to this firing, but a random sampling of at least 1/3 of the pieces in this batch were well below the maximum cartridge length prior to this firing. The problem case now measures 2.034", post-firing, which is still .001" below maximum length shown on my cartridge diagram (granted, trim-to length is 2.025").

3) None of the other shots seemed to be exhibiting pressure signs, and I've fired at least 50 rounds at this charge weight, and 50 rounds in the past at higher charge weights.


So, what might be going wrong here? This was an isolated incident, but I certainly strive for consistency in my precision rifle hand loads, and I'm trying to determine why one cartridge behaved so differently from the others.

Thanks!

 

[243winxb]

Pressure spike

Bullet and primer info might help?

We tested loads at both maximum normal pressures and at the starting loads (some labs calculate start loads—we shot them). Standard primers caused no ignition issues at the max load but posted higher extreme variations in pressure and velocity in the lower pressure regimes of the start loads. In extreme cases, the start loads produced short delayed firings—probably in the range of 20 to 40 milliseconds but detectible to an experienced ballistician. Switching that propellant to a Magnum primer smoothed out the performance across the useful range of charge weights and completely eliminated the delays.

Read more: http://www.shootingtimes.com/2011/01/04/ammunition_st_mamotaip_200909/#ixzz305YTt6yd

Can you double check the RCBS Chargemaster with another scale?

 

[WestKentucky]

Weigh a few of your empty cases. A case with thick walls leaves less volume inside. Heavier=thicker. I have never seen it cause a pressure spike like you are describing but it's all I can think of. Bullet info would be good to know. Is your bore clean, can you guarantee there wasn't a bore obstruction giving extra drag and creating extra pressure?

 

[steve4102]

How do you clean your cases? Maybe Media stuck/left inside?

Ya sure you didn't have an odd overweight bullet?

 

[Joe's]

Wouldn't heavier=thinner?

Be Well,
Joe's

 

[rcmodel]

No, heavier = Thicker.

If the outside of the case is the same size, which it has to be to chamber?

And it weighs more?

The extra weight can only be taking up room on the inside.
So it has to be thicker somewhere.

rc

 

[ironworkerwill]

Ya sure you didn't have an odd overweight bullet?

Or the wrong bullet?


The brass may be thicker and heavier, but, I doubt if made by the same mfg as the rest of the lot it would be by very much. That USUALLY gets culled. The scales will tell the tale.


Did you happen to have a loose 6.5 bullet or an open box near by?

 

[coloradokevin]

Thanks for the responses so far!

Here's some additional information, per your questions:

1) Bullet was a 139 grain Lapua Scenar. No chance that other bullets from other boxes were mixed in, as these were alone on the bench, and loaded from the box directly.

2) Brass is Lapua .260 Rem brass

3) Winchester LR primers

4) Walnut hull media was used to clean this brass, but prior to removing the old primer. As such, I can't see how anything would have remained in the flash hole after decapping.

I'll take some measurements and weigh the problem brass piece against some other cases later today. I'll also double-check the Chargemaster scale against my other scale (I don't anticipate any problems there, though, since the other 49 rounds were fine).

I should also add that these cases were all on their third firing, and had been neck sized after being fired in this same gun.

 

[Joe's]

WestKentucky,
My apology. You were correct.
I was thinking of case volume. Thicker=Less Case Volume/Capacity.

Be Well,
Joe's

 

[gamestalker]

I had a similar incident with thicker brass, so brass can definitely cause some extreme pressure variances.

GS

 

[steve4102]

I was not referring to media stuck in the flash hole, I was referring to a bunch of media left in the case itself.

 

[spitballer]

Cases that are difficult to extract from the chamber are enough to make me nervous. I once had a batch of reloads that did this, and the only thing I noticed was that the entire batch of cases had oversize flash holes. These were cases from store-bought ammo that I'd reloaded.

I'm convinced that outsize flash holes make a difference, but others don't seem to agree with me on this. (see thread "no pressure from oversize flash holes")

If size DOES matter, then an inconsistent flash hole is a possibility.

 

[higgite]

Any chance one bullet got pushed back either while chambering or by recoil when in the magazine? Or gremlins?

 

[GLOOB]

I was not referring to media stuck in the flash hole, I was referring to a bunch of media left in the case itself.

This is what I would suspect.

Doesn't matter how carefully you weigh your powder, you still need to physically inspect each case. You can look in each case before and/or after you charge it. I inspect a block of 50 cases after they are all charged. It's a lot easier to look down those tiny necks when the cases are all in the block. Unless you have really bright overhead lighting, you will probably need to hold a flashlight right up against your head to get a good look. It's easier to get it all done at once. Plus, if there's any difference in powder level (whether due to bad throw or foreign matter in the case), it's easier to notice when you are directly comparing it against 49 normal cases.

 

[coloradokevin]

I was not referring to media stuck in the flash hole, I was referring to a bunch of media left in the case itself.

Definitely not. I handle each case pretty extensively while reloading for precision rifles. There couldn't have been much (if any) media in a case that would have gone unnoticed.

I also look in the cases after each powder charge is added, and I weighed each charge individually. Obviously it's impossible to double charge a rifle case like this, but even an overcharge should have been noticed with the way I do my loading.

Any chance one bullet got pushed back either while chambering or by recoil when in the magazine? Or gremlins?

This is possible. I don't crimp my precision rifle ammunition, and I suppose there is a possibility that the bullet got set back in the case somehow. Admittedly, the .260's recoil is pretty darn tame in a 17lb rifle, but there's certainly a possibility that the bullet could have pushed back while chambering (if it did, I obviously didn't notice it).

 

[loose noose]

That's what I was thinking the bullet got pushed back in the case even just slightly and caused the pressure to go up.

What kind of rifle are you shooting them thru, at 17 pounds?

 

[KingM]

Not that this caused your problem but you know a fired case is shorter then a sized one?

 

[jwrowland77]

Any chance that a odd case was over length and bullet was pinched?

 

[coloradokevin]

What kind of rifle are you shooting them thru, at 17 pounds?

It's this one:

It's an Accuracy International AX in .260 Remington (believe it or not, this awesome rifle was given to me as a gift):

Not that this caused your problem but you know a fired case is shorter then a sized one?

True. But, being that these cases were neck-sized only, I don't know that the length would change with them in the same manner that they do when full-length sized. To be honest, I've only ever noticed the case length grows after full-length sizing… But, does the case length actually decrease between a loaded round and a fired round? I don't think this is the case, but that could make a difference, potentially.

Obviously the case length being too long is one of the first theories I had on this subject, but I'm not sure if I could prove or disprove that this was the case (except to say that none of the cases I sampled before loading were too long, and this case wasn't too long when I checked it after firing)

 

[KingM]

Yes a fired case is going to be shorter then the loaded one but I doubt it is long enough to have been pinched. I would assume you would see the neck thinned just a bit at the mouth if it was pinched.