Primer cratering.. SOLVED THANKS FOLKS!

[Boatschool02]

My last five boxes of 6BR purchased over the past two years have yielded loaded rounds in the 0.269-0.2695 before turning. I experienced pressure earlier than expected with the 95vld when first starting out, because I made the same assumption in a .272 chamber. I fully acknowledge all of those shooters who've had success running 0.271 necks and/or not neck turning... but for my testing & personal preference, I choose to turn to 0.004 clearance. I have not cataloged the highs & lows to four decimal places, but your numbers alone would sell me on turning if not including an interior mandrel step. (The same effect will be apparent when you slowly increase the cutting depth.) I fiddled around with 0.003 on my most recent BR-improved barrel during forming and first firing and just don't feel that any gain is worth running things that close when there are so many other areas for improving & tuning that offer easily discernible up-sides without such obvious negative consequences. Once I heard that John Whidden runs as much as 0.010 clearance in his HOT 308 loads and has NEVER had a neck wear out before primer pockets, I stopped worrying about trying to find accuracy in minimum clearance. On the order of priority list, I would place uniform clearance and the potential for consistent tension much higher than uber-tight clearance... given that with sufficient freebore the bearing surface will be centered to a much higher degree of precision, anyway.
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To be honest; however, I guess all of this assumes perfectly concentric dies and resulting resized brass. From a tolerance stacking perspective maybe the extra clearance is a good practice for anyone (like me) who hasn't, yet, verified concentricity is within 0.001?

 

[ckaberna]

Thanks “boatschool02” ! I think I am at fault here due to not verifying the throat in this chamber compared to my brass.

 

[johara1]

May have just discovered the problem as it relates to the randomness of my issue. I was told this barrel was a no turn well apparently a .272 is NOT a no turn for my lapua brass. Some brass measurements are .0143 wall while some are right at .0132. Matter of fact the ones with the issues are the thick ones. How STUPID of me. I will learn, maybe this post will help somebody else at some point. Thanks for all your input and feedback maybe one day I can return the favor!
Kevin

What was the diameter of the bullets at the pressure ring? At .2434 it would be a loaded round of .272 this is part of the problem, The spring is the other or/ and too much firing pin clearance is the other part. it looks like the pin is rebounding..... jim

 

[ckaberna]

.2433 just checked

 

[AlNyhus]

The bullet diameter added to the case neck thickness (X 2) rarely gives an accurate loaded round figure. It's best to simply measure the case neck over the pressure ring of the bullet. -Al

 

[ckaberna]

The bullet diameter added to the case neck thickness (X 2) rarely gives an accurate loaded round figure. It's best to simply measure the case neck over the pressure ring of the bullet. -Al

Yes I am going to load some more and measure them. Just have not had time. I am also going to pull barrel and measure the actual throat as well. Will post results.

 

[Webster]

May have just discovered the problem as it relates to the randomness of my issue. I was told this barrel was a no turn well apparently a .272 is NOT a no turn for my lapua brass. Some brass measurements are .0143 wall while some are right at .0132. Matter of fact the ones with the issues are the thick ones. How STUPID of me. I will learn, maybe this post will help somebody else at some point. Thanks for all your input and feedback maybe one day I can return the favor!
Kevin

Someone probably said what I just posted but I don't feel like reviewing many replies.

Your right on. I have a no turn 6BR Lapua cases.

0.014" + 0.014" + .243" = .271" in a .272 no turn chamber. No margin for error. I turn my necks to 0.0125".

With .0143" necks the loaded round is 0.2716' in a 0.272' chamber.

 

[AlNyhus]

Yes I am going to load some more and measure them. Just have not had time. I am also going to pull barrel and measure the actual throat as well. Will post results.

Nothing beats a pin gauge for accurate chamber neck diameter measurements. That said, most fired case necks will be within .001 of the chambers neck diameter....depending on brass hardness, etc, etc.

 

[ckaberna]

I have been using the small hole gages by starret with a micrometer. Accurate enough? Don’t have a set of pin gages,....yet.

 

[AlNyhus]

I have been using the small hole gages by starret with a micrometer. Accurate enough?

If it's a good 'tenths' (.0001) micrometer, you bet.

 

[mr45man]

I'm no expert but.
If there is 001 clearance the neck no longer has a grip on the bullet as the power turns to gas and sends the bullet on its way down the bore.
Maybe pressure issues occur when chamber to bore alignment is not perfect.
Ready with flame suit.

 

[ckaberna]

I'm no expert but.
If there is 001 clearance the neck no longer has a grip on the bullet as the power turns to gas and sends the bullet on its way down the bore.
Maybe pressure issues occur when chamber to bore alignment is not perfect.
Ready with flame suit.

.001 clearance per side is about minimum from my understanding

 

[matorre]

For your consideration.

Less than .0007 of clearance on either side of the neck.
And yes all three were in the bolt. one had to be pounded out of the firing pin hole. The gun was still fireing.

 

[ckaberna]

For your consideration.

View attachment 1063967 Less than .0007 of clearance on either side of the neck.
And yes all three were in the bolt. one had to be pounded out of the firing pin hole. The gun was still fireing.

That is .0014 clearance. Too little in my book. Need at least .002.

Got the spring in from Bat today but only sent the "heavy" I wanted to try the "light one" first. Will call back tomorrow.

 

[johara1]

You are on a path of recovery, but if you have that much variance in neck thickness you need to do something. I hope you are using the same lot of cases, if not you caused the problem yourself. It will not shoot good with a .001 variance, a .0001 is what we consider the norm. Turn necks and get uniform neck tensioning set your loaded round clearance..... jim

 

[matorre]

That is .0014 clearance. Too little in my book. Need at least .002.

Got the spring in from Bat today but only sent the "heavy" I wanted to try the "light one" first. Will call back tomorrow.

its not .0014. that brass had only .0007 PER SIDE, to move out of the way.
Saying that I had .0014 obscures the hazard of the situation.

M->

 

[Texas10]

its not .0014. that brass had only .0007 PER SIDE, to move out of the way.
Saying that I had .0014 obscures the hazard of the situation.

M->

Could you be a little more vague?

Please, for the benefit of those who are not accredited experts, perhaps a little more explanation would go a long ways to satisfy those with an inquiring mind. Just saying.

 

[243winxb]

May have just discovered the problem as it relates to the randomness of my issue. I was told this barrel was a no turn well apparently a .272 is NOT a no turn for my lapua brass. Some brass measurements are .0143 wall while some are right at .0132. Matter of fact the ones with the issues are the thick ones. How STUPID of me. I will learn, maybe this post will help somebody else at some point. Thanks for all your input and feedback maybe one day I can return the favor!
Kevin

perhaps a little more explanation

The fat necks were getting crimped by the chamber on loading/firing. Pressure goes up.

More clearance in the neck area is needed between cartridge neck and chamber.

Hope this helps. .

 

[Texas10]

Thanks, got that part. What I don't understand is the DIFFERENCE between .0014 total clearance and .0007 per side and, most importantly how that obscures the hazard of the situation.

Not just trying to bust your chops here, I shoot a .272 no neck turn 6mmBR, and use Lapua brass that measures .012 to .0125 in the neck wall. I don't turn necks, I've never had issues with overpressure events caused by tight necks, so perhaps you see my concern. I've not popped a primer yet, but now I may have to watch for thicker necks coming from Lapua.

Thanks for any help, 243WinXB! You're expertise in these matters is of great help to those of us who are less than experts.

 

[kvd]

A kinda "double hit" with case not fitting as in fire forming,comes from the initial pin to primer strike, case goes forward, firing happens, case comes back into pin.

JMayo hit the nail on the head - or in this case the firing pin on the primer. I've experienced this condition on a few cases every time I begin fire forming a new batch of 6mm BRA brass from Lapua 6mm BR Norma brass. I've heard it referred to as "top hatting" due to the look of it and for all intents and purposes it is blanking a primer without the cut out piece making it out of the primer cup. I've seen it with CCI 450's, and other types and brands of primers as well.

A stiffer firing pin spring will not cure it. In fact, I believe a stiffer spring actually makes the condition more likely when a case is loose in the chamber as the protruding firing pin has more force pushing it out to act as a punch.

I’ve also had the next worse loose case condition where the case can push so far forward that the firing pin will only dent the primer and not detonate it. A new, stiffer firing pin spring did not help this condition either. These cases are a real aggravation and the only way to get them fire formed is to rework them and start with a relatively soft CCI 400 primer and bullet seated 0.040 into the lands with 0.003 or 0.004 neck grip. Sometimes even this isn’t enough but most of the time it will finally fire.

Some new cases simply are to loose in the chamber for the first firing. Probably a subtle combination of both length and width. Once these few problem cases have been fired, the condition goes away.

My cases need to be resized to 1.1700 inches for a near perfect fit in my 6mm BRA chamber so it may be a little long for some new Lapua 6mm BR Norma brass which typically measures in the 1.1580 inch range.

I have a 0.268 chamber neck so I must neck turn before fire forming. I try to shoot for approximately 0.0105 neck wall thickness to provide 0.002 minimum clearance per side. Given the variability in unturned Lapua brass neck wall thickness, even for a 0.272 chamber neck, a small degree of material removal from the neck wall would be a benefit to accuracy.