New brass over pressure?

[OU812]

I encountered something at the range the other day that I found puzzleing and hope some one can either confirm or deny my thoughts on the problem. This is the problem. I loaded up some 107.6 and 107.7 grain loads for my 338-378. I have shot these loads several times prior with no pressure signs what so ever until the other day. Upon firing my last 3 rounds of the 107.7 the bolt would rotate just fine but did not want to open. The powder is RL-25 and is the same container I've been using with no issues what so ever, the bullets are NP 250 grain from the same box, all of my brass is Norma, primers are Fed 215GM, case length is 2.903, COL 3.720,

So here is what happened. I loaded up 5 107.6 grain shells with good clean, resized, trimmed to length, debured fired brass. I did the same for the 107.7 grains shells BUT I was short good clean brass so I pulled 3 new pieces of brass. Checked the OAL, deburred and loaded.

This is what I think was my issue. I believe that since I did not size the new brass the case neck had a tighter bite on the bullet which caused a pressure spike. Does this seem reasonable?

 

[Guest]

Could be. Could also be that you have a pretty tight chamber, and the new brass was a little snug going in, and got moreso at ingition. Or, one other possibility I can think of is that the brass itself was a little long, and got a little bound up against some crud in your chamber (from the first shots) and created a pressure spike. I would back down and work up again from a grain or two below.

 

[rick smith]

Also consider that new brass has less internal volume than fired brass.Rick.

 

[OU812]

Also consider that new brass has less internal volume than fired brass.Rick.

I checked the internal case volume today between the fired brass and the new brass. I used water to do the test and repeated it 3 times and came up with an ave difference of 4.2 grains less in internal case volume with the new brass compared to fired brass.

Is this enough difference between the 2 to cause what I saw? How does this 4.2 ave smaller case volume relate to over all pressure? Is it equal to haveing an extra 4.2 grains of powder in the load?

 

[rick smith]

I would think 4.2grs would be plenty to cause your problem if you are working near max. The loads I use in a 280AI would not be safe in a standard 280.Rick.

 

[Richracer1]

I always FL size new brass. You never know what your getting from the factor, regardless of how reliable you may think they are. Just my .01

 

[OU812]

I would think 4.2grs would be plenty to cause your problem if you are working near max. The loads I use in a 280AI would not be safe in a standard 280.Rick.

Rick,

That is part of the problem, max. I've seen max listed as 107.5 up to 110.5. And in other places as high as 112.2.

How does a smaller internal case volume of 4.2 grains of water weight corralate to the pressure increase? I understand that if the same powder charge is dropped in to a smaller internal case volume pressure will increase.

I guess what I'm looking for if it's possible to correlate the pressure I saw with 107.7 grains of RL-25 in a case that is 4.2 grains of water weight smaller than a used/expanded case. What powder chaarge would be needed to recreate the same pressure in an expanded case?

 

[zupspoon]

I'd seriously doubt that it's a bullet pull problem unless the case neck is being pinched in the chamber, and even mild chamber pressure easily overcomes a very tight crimp. There are formulas that guesstimate pressure increases based on cartridge internal volume, but if you don't know where your pressure is to begin with, it's pretty useless info. Insufficient headspace could cause the problem, but you'd probably feel the resistance when chambering the round. If all other parameters are in fact identical except for the case volume, that's probably the culprit.

 

[alpinecrick]

Also consider that the new brass is most likely a different lot of brass. Different wall thickness, different internal volume, the web may be thicker or thinner than the previous lot. Although brass is more consistent today from lot to lot than it was in the past, I still treat a new lot of brass just as though I was changing brands of components.

Casey

 

[FOTIS]

Also consider that new brass has less internal volume than fired brass.Rick.

Yup!
Also if you're shooting these on a hot day .......

 

[rick smith]

Remember that you should always drop back and work up when you change any component. You are putting the same pressure in a smaller case which means you are creating more pressure. I would not venture a guess as to how much powder it would take to make this happen in the old brass nor would I suggest you try to find out. I see that you have posted the same basic question in another forum and have received some good suggestions as to the basic question.
As to various max charges in different manuals, all of them are using different components and have the proper pressure testing equipment. If you are new to loading and don't have that equipment, you should use the lowest value for max. Rick.