Reloading question. brass issue.

[BuckSnort]

Looks like low pressure and or a chamber out of specs... After looking at the brass pic really good I'm gonna lean toward sloppy chamber... Is this the first few rounds out of new chamber? Be careful with under pressure it can be very dangerous!!

 

[YBPS]

The manufacturer recommends that the S bushing be sized a few thousands smaller than the fired case neck (can't remember the exact amount but it's in their instructions), obviously you can sneak up on that bushing size but it sounds like you need a little more neck tension. Typically I use a bushing that is about .003" smaller than the outside neck diameter of my fired cases.

My loaded cases are .296 and I am running a .294 bushing. I have a .292 so I am going to try it next. Thanks for the tip.

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I have over 100 rounds through the chamber. This is the third time it has happened. I really don't think I am low on pressure. I am getting 3250-3280 FPS with a 140VLD verified on two different chronographs. Thats pretty dang fast, but is it correct to think that high velocities dont exist with low pressure? My bolt was a little heavy at 68.0 with some loose primers and my speeds where 3350 plus, so i backed it off in .5 grain increments and then settled at 65.2 based on accuracy. 65.0 is the recommended starting load per Berger. I would really prefer my speeds to be more like 3050-3100... but i am getting nice groups.

 

[jorgensen.travis]

There's no way pressure is too low. The speed seems way too fast. Even with a 28" barrel. What seems weird to me is that I've seen brass fired out of fluted chambers that don't dent. I still think it's over pressure. With all the friction of the long bearing of the VLD at that speed, pressure has to be high. A loose seal and a sloppy chamber usually results in a bulge, not a dent. We need a physics major. As the bullet travels down the barrel, (from what I speculate) pressure remains equal throughout the pressure chamber. The pressure chamber being the inside of the brass to the back of the bullet. As the bullet travels down the barrel the pressure chamber expands. The brass should remain pressed against the chamber walls and sealed. If there was a blemish in the chamber that allowed gas between the chamber wall and brass, I would think it would happen every time. Is there any point that there could be negative pressure inside the brass? From the growing pressure chamber as the bullet gets towards the end of the 28" barrel? At the point of exit?
I don't know. I never even made it to high school physics.

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[rayporter]

we're talking about pressure being too low for Ratumbo's super slow burn rate right?

correct.

it is not low pressure necessarily that causes this, it is that the pressure builds slowly and the bullet releases before the neck is fully expanded to seal.

think of most powders having a pressure spike as the bullet leaves the neck versus the spike hitting the bullet half way down the barrel.
theoretically you can have the same pressure and the same velocity from each powder burn rate.

 

[Shrek]

New brass is short for the chamber and retumbo isn’t building pressure fast enough to expand the case and seal it. Gas is getting by occasionally. I suspect the problem will go away on the second firing with the brass expanded to near chamber dimensions. Your chamber is probably long and new brass short. Belted magnums headspace off the belt technically not the shoulder so when checking the chamber it can appear the chamber is within spec on a go gauge but be actually long to the shoulder. Belted cartridges suck imo.

 

[treillw]

If there was a blemish in the chamber that allowed gas between the chamber wall and brass, I would think it would happen every time. Is there any point that there could be negative pressure inside the brass? From the growing pressure chamber as the bullet gets towards the end of the 28" barrel? At the point of exit?
I don't know. I never even made it to high school physics.

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The pressure is initially outward on the brass, as you state. The bullet starts ripping down the barrel and the pressure will be outward on the barrel, propelling the bullet down the barrel. In the split second that it takes the bullet to blow out of the barrel and relieve the pressure of the explosion, the explosion can create an inward pressure on the brass if the shoulder is not sealed and the gas can escape around the brass and blow out through the bolt. This is why you see powder burns where they don't belong in his picture.

It is like ramont and others are saying "the bullet is moving before the case expands in to the chamber walls. The dent is created by the combustion gasses that have moved back in to the area of the shoulder."

I saw the same thing (although not as drastic) when I was fireforming brass for my 300 win with the cheapest 110 grain bullets I could find. Jam those same bullet into the lands and the problem disappears because the brass has time to fully expand to the chamber walls before the pressure can get onto the outside of the brass and shrink it.

I suspect that the problem will go away after the new brass is fireformed.

 

[jorgensen.travis]

The pressure is initially outward on the brass, as you state. The bullet starts ripping down the barrel and the pressure will be outward on the barrel, propelling the bullet down the barrel. In the split second that it takes the bullet to blow out of the barrel and relieve the pressure of the explosion, the explosion can create an inward pressure on the brass if the shoulder is not sealed and the gas can escape around the brass and blow out through the bolt. This is why you see powder burns where they don't belong in his picture.

It is like ramont and others are saying "the bullet is moving before the case expands in to the chamber walls. The dent is created by the combustion gasses that have moved back in to the area of the shoulder."

I saw the same thing (although not as drastic) when I was fireforming brass for my 300 win with the cheapest 110 grain bullets I could find. Jam those same bullet into the lands and the problem disappears because the brass has time to fully expand to the chamber walls before the pressure can get onto the outside of the brass and shrink it.

I suspect that the problem will go away after the new brass is fireformed.

Why dosen't brass dent when fired in the fluted chamber of a CETME? Is it because it's happening 360 degrees around the case rather than all in one spot?

I don't see how seating out to the lands is relevant. The bullet is always going to hit the lands before it exits the brass. Would it really make a difference it it jumped a couple of thousandths first?

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[rayporter]

seating into the lands causes higher pressure. even high pressure with the wrong powder.

seating with some jump lowers pressure. think weatherby.

 

[Cahunter805]

As stated I’d suspect it’s a pressure problem and likely won’t be a problem with once fired brass. Setup your sizing die to just bump the shoulder back about .002. Most new brass is undersized and belted brass typically grows a lot in the first firing especially in the shoulder area.
Have you measured any of the fired cases? What is the fired neck diameter? I would also agree your neck tension might be a little light depending on neck thickness. I’d load up some of your once fired brass sized with the 292bushing and see what velocity and accuracy you get.

 

[LcMoeller2]

I would agree with the light neck tension and a bad or light seal for gas "relief" down the barrel and compressing gas

 

[hwy1strat]

Those dents do look like the classic dents caused by excessive lubricant on the case or in the resizing die.
They are safe to fire and the dent will fire form out.
However, you mention that you suspect that it was caused during firing. Do you have some pictures of the cases after foreign and before resizing?
I have seen dents caused by the extraction/ejection process.
Another trick is to use a sharply to mark the case. Index the mark on some reference point on the rifle (or place the mark straight up). Fire the cartridge and see if there is any correlation or consistency as to where the dent shows up. If there is a defect in the rifle’s chamber, the dent will form in the same spot in relation to the index mark.

Hope this helps.

I agree. This looks exactly like too much lubricant

 

[YBPS]

All the fired cases are .299-.300.

The case with the dent measures .292 in one spot which is where the gas snuck around.

I am going to size these once fired cases and see what happens. I suspect that the theory that the new brass being undersized is the issue along with lighter load.

I talked to Berger and they told me a little more powder should be fine. I just don't want any more speed. For some reason this tube is fast.... First I am going to try the resize brass.

Thanks to everyone that has responded

 

[Cahunter805]

I would size some with the .292 bushing as well.
Do you know the reamer specs?

 

[YBPS]

I don't know the specs on the reamer. Only thing hats not spec is the throat... got a longer lead to seat my bullets out. I just sent the bolt in to get the firing pin bushed. That may help eliminate the pressure sings on the lighter load...

 

[Laelkhunter]

As others have said, it is probably caused by lube collecting at that spot while resizing. To prevent that on my brass, I put a small dab of lube on my fingertip, and rub it around the body of the case, then a little is left over on my finger that I put on the neck of the case spinning it between my finger and thumb. No lube goes on the shoulder, but make sure some goes on the body and on the neck. I rarely get those oil dents using this method. They are unsightly but harmless, and will go away when you fire them.

 

[Wildstreak]

Are you sure this happened after firing? The dent is the picture looks classic for over lubing the case while sizing. Look for it in a reloading manual and you'll see what I'm referring to...
Hope it helps

Exactly what I thought.

 

[ckleeves]

There’s no way that is a case lube dent. #1, the OP has clearly stated that it was after it was fired. #2 you would need about the entire bottle of lube to make one that big and #3 if for some reason it was a lube dent it would have come out when it was fired.


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[tdhanses]

If you had it built I’d have the gunsmith look it over that did the work and maybe a second independent one if the builder is shady about it.

 

[YBPS]

Update:
I determined that it was in fact gas getting out of the case and into the chamber before the brass expanded. I have done a few things and this problem has gone away.

I had the firing pin bushed by GRE-TAN Riffles, and all of the primer hits are very uniformed and show no pressure signs. Very happy with this work. Shot up to 70 grains of Retumbo and the primers are perfect. No crater, not flatted, pretty nice. Prior to this work, even the lighter loads still had some cratering. If you dont know about this process look up his video on You tube. He did the work for 100$ and had it back to me in 6 weeks.

At the 65.2 load, which is a minimum load according to Berger, it did happen one more time, after the firing pin work and sizing down on my bushing. But I have since bumped up my charge to 69.5 and have shot 30 rounds without an issues with a mix of new and 1x and 2x, fired brass.

Accuracy is good and the speed is very fast 3400 +/- 20 FPS, but the primer pockets are holding well, the bolt lifts fine, and no suit or dented brass. The speed is faster than I like, but this is just a project and I will just keep shooting it until its over.

Thanks for everyone's input.