6BR pressure signs low loads

[Dolomite]

There could be other reasons, but one is not enough pressure to expand the case neck.

I am hesitant to advise anyone to run an "experiment", but if it was mine, looking at the primers and the neck not expanding and low velocity, I might run the powder up and see what happens.

The case may not be "sticking" to the chamber (with the lower pressure) and sliding back against the bolt face, and causing swipe.

Jim

Thanks doesn’t hurt to experiment and see what happens with a few rounds.

I wonder if my chamber is too smooth not allowing the brass to grip it….. hmmmm

 

[Dolomite]

Just so I get this right. No pressure signs on virgin brass / 28.5 gr of Varget but resized than if I read this correct’ you start at 25.gr and go till 28.5 experiencing pressure on each round ?

yes correct. The load testing was done on a dirty barrel around 39 rounds fired prior. this was another batch (different from the brass with ejector swipes) of virgin brass fired during break in. 28.5 grains of varget.

 

[NZ_Fclass]

This is a tough one. Couple of ideas. 2575 is slow for 28.5. I suggest that the barrel is on the tight side - and far from being run in yet. Which could , in itself cause pressure. a check down there with a borescope may put you at ease there anyway.

But I would also check the headspace. A case moving back will give an increased primer mark and imprint off the bold. Check the difference between factory specs and fired cases - or a chamber gauge - a go gauge and "crush" gauges should indicate any issues. I had a 6.5-06 that had that issue.

Good luck BTW, what's accuracy like?

 

[Dolomite]

This is a tough one. Couple of ideas. 2575 is slow for 28.5. I suggest that the barrel is on the tight side - and far from being run in yet. Which could , in itself cause pressure. a check down there with a borescope may put you at ease there anyway.

But I would also check the headspace. A case moving back will give an increased primer mark and imprint off the bold. Check the difference between factory specs and fired cases - or a chamber gauge - a go gauge and "crush" gauges should indicate any issues. I had a 6.5-06 that had that issue.

Good luck BTW, what's accuracy like?

thanks this is mind boggling. I did scope the barrel, I didn't see anything odd but then again I just examined the throat and for cleaning purposes. The barrel doesn't foul with copper at all which I'm assuming is a good thing or maybe not.

Anything I should be looking for that shows a tight bore? I know when I bought the Bartlien 1:8 it suppose to be a .237 bore, is that tight or normal?

The difference between the virgin Lapua brass and a fired case is .004-.005.

.005 would a heavy bolt lift, not smacking the bolt just heavy. Heavy enough that its hard to chamber a fired case without some effort.

The 28.5 at 2575 shoots really good low .2's at 100 on a good day with only 129 rounds down the pipe in this barrel.

I'm just concerned about the heavy bolt lift and ejector marks.

 

[jimmymac]

Didn’t read the whole thread, but if you can’t slip a bullet down in to a fired case, that would concern me because you could be starting off with a bit of an interference fit between the neck and the chamber regardless of your chamber dimension. What is the neck wall thickness of this batch of cases? What is the neck OD of a loaded round?

The other thing I’d be looking at is the brass being too long for the chamber. If you have a reamer print, you should be able to figure out what your do not exceed dimension is for overall brass length. I’d start by measuring OAL of a fired piece of brass, then measure again after you size the case. If you exceeded the max OAL, that will cause issues as well.

If it were me, I’d be looking hard at both of the above before firing more rounds.

 

[jimmymac]

Okay...Had coffee, went back and read your first post again. The dimensions are there for OD. If a bullet doesn't slip in a fired case, I'd still be concerned. Are you certain the chamber is a .272? What is your max cases length per your reamer print?

 

[Dolomite]

neck wall thickness is around .0127 - .013 for a total of around .0257

Lapua Case OAL virgin 1.556
Lapua Case OAL fired 1.554
Lapua Case OAL trimmed 1.551
max according to reamer is 1.570


Loaded round OD is .269 chamber supposed to be .272

 

[boltfluter]

Not sure if I can help, but here goes. First take a good look at the bolt face and make sure there are no burrs. If you have a different lot of Varget try it. Some are much hotter than they should be. I have access to a lathe, and this is a last ditch effort when I have to resort to this. Sacrifice one case and start by the cutting neck back .050. Then remove the shoulder, just below the body/neck junction. Chamber round after each step. One of these cuts will have the brass chambering much easier, giving you hard evidence where to look for the problem. Has never failed me yet. Also try running the brass into a .243 or .308 die as they generally match a small base die. Do this before you start cutting. That's all I have at the moment. Good luck!

Paul

 

[jimmymac]

OD fired case .2705-.271
OD fired round .268-.269

From your first post... When you say OD of fired round and OD of a fired case, I'm not sure what the difference is and what you are referring to. What is the neck diameter of a freshly fired case when it comes out of the gun? Are you measuring with a micrometer?

I ask because if the brass is coming out of your gun at .268 or .269 I'm wondering if your chamber is actually a 272.

On my 268 neck PPC, the brass comes out of my gun at .267 for the most part.

 

[243winxb]

OD fired case .2705-.271
OD fired (means loaded?) round .268-.269Chamber

Thinking its not a .272" chamber neck, smaller? Turn 3 brass so a loaded rounds OD measures .267" as a test. Will need smaller bushing.

The ejector hole may need a slight bevel, if it has a burr or roughness.

How is the weight of the brass, compared to other lots. If heavier, pressure will be higher.

Check chamber length, for a short chamber.

Other wise, its a barrel problem. I am out of guesses, for now.

 

[OMW57]

in the case of the ejector marks, could you possibly have some dirt or debris in the ejector pin hole causing it to not compress the spring fully, would really drag the ejector hard on case head going in & coming out?

 

[Dolomite]

neck wall thickness is around .0127 - .013 for a total of around .0257

Lapua Case OAL virgin 1.556
Lapua Case OAL fired 1.554
Lapua Case OAL trimmed 1.551
max according to reamer is 1.570


Loaded round OD is .269 chamber supposed to be .272

From your first post... When you say OD of fired round and OD of a fired case, I'm not sure what the difference is and what you are referring to. What is the neck diameter of a freshly fired case when it comes out of the gun? Are you measuring with a micrometer?

I ask because if the brass is coming out of your gun at .268 or .269 I'm wondering if your chamber is actually a 272.

On my 268 neck PPC, the brass comes out of my gun at .267 for the most part.

sorry I’ve edited the info.

No I used my mitutoyo calipers to measure.

Neck OD fired case .2705-.271
Neck OD loaded round .268-.269

Chambering empty case new and sized is fine

Chambering a fired case is heavy bolt closure.

Snug fit when trying to insert a bullet into a fired case.

 

[Dolomite]

Thinking its not a .272" chamber neck, smaller? Turn 3 brass so a loaded rounds OD measures .267" as a test. Will need smaller bushing.

The ejector hole may need a slight bevel, if it has a burr or roughness.

How is the weight of the brass, compared to other lots. If heavier, pressure will be higher.

Check chamber length, for a short chamber.

Other wise, its a barrel problem. I am out of guesses, for now.

I was trying to avoid neck turning and maybe my last resort.

I did feel the ejector hole and ran some 1000 sandpaper to remove any burs.

the weight of the brass was all within 1 grain or less for 100 pieces.

I scoped the barrel with the brass inside and there is lots of room for growth in length.

 

[Dolomite]

in the case of the ejector marks, could you possibly have some dirt or debris in the ejector pin hole causing it to not compress the spring fully, would really drag the ejector hard on case head going in & coming out?

Good point, I pulled the ejector and cleaned it out. I also thought it could be the heaviness of the spring so I swapped out the stock spring for a medium weight spring from Ernie the Gunsmith. I didn’t need my brass ejecting 3’

 

[Dolomite]

Could it be the chamber was cut cleanly that it needs to be lightly polished up so the brass grabs the side walls? I’m guessing here but the chamber is a really nice.

I’ve also read that powder and humidity can cause over pressure.

 

[pdhntr]

Just so I get this right. No pressure signs on virgin brass / 28.5 gr of Varget but resized than if I read this correct’ you start at 25.gr and go till 28.5 experiencing pressure on each round ?

yes correct. The load testing was done on a dirty barrel around 39 rounds fired prior. this was another batch (different from the brass with ejector swipes) of virgin brass fired during break in. 28.5 grains of varget. View attachment 1312355

So the above were virgin brass at 28.5 first firing, right? And no ejector swipe, or pressure signs. I would think they should rechamber just fine, but they are tight, correct?

Then you resize and get pressure signs, (swipe and hard bolt movement) at the same 28.5?

Thanks.

Jim

 

[Dolomite]

Not sure if I can help, but here goes. First take a good look at the bolt face and make sure there are no burrs. If you have a different lot of Varget try it. Some are much hotter than they should be. I have access to a lathe, and this is a last ditch effort when I have to resort to this. Sacrifice one case and start by the cutting neck back .050. Then remove the shoulder, just below the body/neck junction. Chamber round after each step. One of these cuts will have the brass chambering much easier, giving you hard evidence where to look for the problem. Has never failed me yet. Also try running the brass into a .243 or .308 die as they generally match a small base die. Do this before you start cutting. That's all I have at the moment. Good luck!

Paul

Bolt face has no burrs I double checked. I’ll try another lot of varget.

I’m not sure why I get heavy pressure and low velocity. is that from the powder being too fast?

I wonder if I could color up a piece of fired brass insert it into the chamber and look for any rub marks? If not I’ll try and find a way to cut up the brass as you mentioned.

I do have a 308 small base die which I could try. Would it be sizing the upper body of the brass too small? The 308 small base die really squeezes the upper portion of the brass down a lot.


I believe the blown out shoulder is causing by the heavy pressure which is causing the heavy bolt lift.

 

[pdhntr]

Could it be the chamber was cut cleanly that it needs to be lightly polished up so the brass grabs the side walls? I’m guessing here but the chamber is a really nice.

I’ve also read that powder and humidity can cause over pressure.

If you can load 28.5 in a case and fire it and not get ejector swipe, (using the same powder) than neither of the above are likely in this situation.

Jim

 

[Dolomite]

So the above were virgin brass at 28.5 first firing, right? And no ejector swipe, or pressure signs. I would think they should rechamber just fine, but they are tight, correct?

Then you resize and get pressure signs, (swipe and hard bolt movement) at the same 28.5?

Thanks.

Jim

Yes that is correct. The sized brass would chamber and the bolt would drop nicely, following the Alex Wheeler method.

 

[Dolomite]

If you can load 28.5 in a case and fire it and not get ejector swipe, (using the same powder) than neither of the above are likely in this situation.

Jim

It’s the second firing that got the heavy bolt lift and ejector marks.