Gap between end of case neck and chamber

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  • Last Post 07 November 2010
CB posted this 31 October 2010

Because of manufacturing tolerances and conservative manufacturers there is always a sizable gap between the end of the case neck and the end of the chamber. This can be as long as 0.045” with both the case and chamber in spec and longer if one or both are out of spec.

Lead and gunk collects in this gap in varying amounts. Is that a concern? Does it affect accuracy with cast bullets when this collection of lead and fouling varies over the course of several shots?

Some competitors think it may be one cause of those occasional fliers and they use longer brass to resize to a longer length than specified to shorten the gap (30-06 cases formed to 308 with a neck that almost reaches the end of the chamber's neck for instance.)

If this is over done the case mouth could be pinched down on the bullet by the end of the chamber and theoretically increase pressures so care is called for.

Has anybody done any testing to prove or disprove that the gap does any harm and thus it might be worthwhile to fit cases to shorten the gap -- or not?

John

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runfiverun posted this 31 October 2010

i haven't much testedd it but have seen accuracy fall off from leaving the gap. you push hard on the base of a boolit and it tries to expand out and fill what it can fill. and is shaved off. short trimmed cases is probably the most overlooked area in reloading for accuracy with either type of bullet.

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shooter93 posted this 31 October 2010

With jacketed bullets it seems to take a very good shooting rifle to tell any difference, if any differnce at all. With paper patched lead it can at times wreak havoc as well as a too sharp a transistion angle into the leade. Softer alloys and/or over sized grooved lead bullets seem to be effected at times too although that needs more testing and anymore I generally don't shoot many lead bullets out of factory barreled rifles.

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billwnr posted this 31 October 2010

Too long of cases caused “two grouping” in my .22-250 years ago. The condition disappeared once I trimmed my cases to the proper length.

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CB posted this 02 November 2010

"Too long of cases caused “two grouping” in my .22-250 years ago. The condition disappeared once I trimmed my cases to the proper length."

What is “proper” length. The specified length leaves a huge gap in most chambers. Something over specified leaves a smaller gap or none at all.

None and all is probably asking for trouble and maybe two groups.

Are you sure your “two groups” problem wasn't caused by inconsistent length and not because any were too long and touching end of chamber? Were your longest cases touching the end?

John

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billwnr posted this 02 November 2010

Proper length is SAAMI standards. Mine had grown as they were untrimmed. The “two grouping” condition continued until a Forster case trimmer was purchased and the cases were taken down to less than max allowed length.

This was back in the 1967 era when I figured rifles that shot groups under 1 MOA were magical.

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CB posted this 04 November 2010

"Proper length is SAAMI standards."

Can't argue with that, at least in the legalistic sense of “proper."

However, if you have a chamber at the long end of SAAMI spec as most of the ones that I have measured are, and IF excessive gap really does cause trouble with cast bullets as some CBA record holders think, then proper (in the sense of what is correct for best results with that rifle) might be cases longer than SAAMI spec.

What I was trying to ask was are you sure that your two group problem went away because you increased the gap or because you took out variation in the gap from one case to the next. It seems to me that it might have been either. Certainly if some of your cases were bumping into the end of the chamber and squeezing the bullet that could certainly cause inconsistency.

I am just trying to find information on this gap question. A big gap may not be a problem but I hope somebody has tested to see if it makes any difference with cast bullets. It seems to me that would require having cases set up for minimum gap (under 0.005") and similar cases trimmed to SAAMI length which might result in a gap as big as 0.050 and see if there was any difference in accuracy over several groups. If no one else has done it I guess I will have to if I want to know.

I'm just lazy and don't want to invent the wheel if I can avoid it.

John

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joeb33050 posted this 05 November 2010

John;

All dimensions below are ~, the brain is letting me down. 

There is a report of tests I did and comments by and about others in 6.2 of the book. Any chamber I've measured, for years, is cut for a case quite a bit longer than published length-max length-trim to length. (I just recently trimmed a set of 308 Win cases to 2.025” after much firing. Still short of the chamber. See the specs.)

I have never detected an accuracy difference connected to case length, nor have I found a record of tests with short and long cases showing that accuracy varied as case length.

There is a man on the ASSRA forum who makes and sells a case-stretcher apparatus for lengthening 45/70 and other cases. He, and I assume the buyers of this apparatus, believe that with black powder case length affects accuracy.

At higher but far from excessive velocity I can reliably shoot a cast bullet and leave a ring of lead in the chamber. These bullet-diameter rings have small thicknesses, suggesting that the bullet expanded, filled the case mouth-chamber end gap, and were sheared off. Production of these rings is accompanied by loss of accuracy and leading.

I am still working with long 38/55 cases formed to 30/30 and trimmed to ~ .005” shorter than chamber max, looking for the accuracy bonus that may be hiding there.

And then there's the supposed danger, which makes me a little nervous.

joe b.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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JetMech posted this 05 November 2010

joeb33050 wrote: I have never detected an accuracy difference connected to case length, nor have I found a record of tests with short and long cases showing that accuracy varied as case length.

At higher but far from excessive velocity I can reliably shoot a cast bullet and leave a ring of lead in the chamber. These bullet-diameter rings have small thicknesses, suggesting that the bullet expanded, filled the case mouth-chamber end gap, and were sheared off. Production of these rings is accompanied by loss of accuracy and leading. Not to start an argument, Joe. You do alot of testing and provide great information, but those 2 statements seem to contradict each other.

In BPCR shooting, we typically use bullets cast from softer alloys such as 20:1, 25:1 and 30:1. BHN runs around 9 or 10. With these softer alloys the bullets readily bump up into the gap and quickly destroy accuracy at 1300 to 1400fps. That why, in my H&R 38-55, I had to replace the original cases that measure 2.080 and buy the longer 2.125 cases. 10 round group size immediately settled down and leading eliminated.

I think with the harder alloys the gap has less effect, with bullets from straight lino or heat treated showing the least tendency to expand in the gap.  

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CB posted this 05 November 2010

Joe,

It's not just your brain. I suspect i have read that section of your book since I think I have read all of it. I will reread.

Joe and Dollar Bill,

Thanks. I think you have at least provided the limits of this problem. I have never seen evidence that jacketed bullets are affected but it looks like at the other end with soft enough bullets there is at least some evidence that it is worth worrying about. The darned middle,as usual, is the question. How hard and at what pressures should we fret over this?

John

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billwnr posted this 05 November 2010

joeb33050 wrote: John;

All dimensions below are ~, the brain is letting me down. 

There is a report of tests I did and comments by and about others in 6.2 of the book. Any chamber I've measured, for years, is cut for a case quite a bit longer than published length-max length-trim to length. (I just recently trimmed a set of 308 Win cases to 2.025” after much firing. Still short of the chamber. See the specs.)

I have never detected an accuracy difference connected to case length, nor have I found a record of tests with short and long cases showing that accuracy varied as case length.

There is a man on the ASSRA forum who makes and sells a case-stretcher apparatus for lengthening 45/70 and other cases. He, and I assume the buyers of this apparatus, believe that with black powder case length affects accuracy.

At higher but far from excessive velocity I can reliably shoot a cast bullet and leave a ring of lead in the chamber. These bullet-diameter rings have small thicknesses, suggesting that the bullet expanded, filled the case mouth-chamber end gap, and were sheared off. Production of these rings is accompanied by loss of accuracy and leading.

I am still working with long 38/55 cases formed to 30/30 and trimmed to ~ .005” shorter than chamber max, looking for the accuracy bonus that may be hiding there.

And then there's the supposed danger, which makes me a little nervous.

joe b.

 

Welcome back, Joe.

John,  I saw evidence of it back in 1967 what with what I term “two grouping".  I suspect it was because jamming too long of a case into the leade causes chamber pressures to rise.  Higher chamber pressures unbalance a load and cause a tuned load to go south (or north)

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CB posted this 07 November 2010

"John, I saw evidence of it back in 1967 what with what I term “two grouping". I suspect it was because jamming too long of a case into the leade causes chamber pressures to rise. Higher chamber pressures unbalance a load and cause a tuned load to go south (or north)"

That certainly sound like a plausible mechanism for inconsistency and two groups. According to the conventional wisdom, it might also raise pressures enough to be a concern with hard alloys and hot loads.

John

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