Case Length?

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  • Last Post 22 March 2016
billglaze posted this 16 March 2016

All:I've got a problem with my new 6mm BR.The thing shoots like a house afire, but I'm concerned because I can't get bullets to slip into the fired cases.  Looks as if it might be a case trimming problem, but the Lapua brass is very uniform at 1.540.  SAAMI is no help, or possibly I'm not finding my way down their dark hallways, but bottom line is I haven't gotten a dimension. In another forum, the shooter says he trims to .550.  I saw another soul who said something about 5.20.  Seems awfully short.  I realize that if I used any published figure, the problem might still remain.  So---What to do?  Only thing I've come up with so far, involves cutting the cases shorter until a bullet slips into the fired neck. But, I hate to start hacking into matched cases that are over a buck each, until I get some other info. If the other folks shooting this cartridge chime in, I'd sure listen.  BTW:  While my loads are modest, even then I'm getting absolutely no pressure signs.  And the rifle has been shooting some loads into one small,ragged hole.   Like it's been said:  go figure. Bill

In theory, there's no difference between theory and practice. In practice, there is. My fate is not entirely in Gods hands, if I have a weapon in mine.

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Duane Mellenbruch posted this 16 March 2016

Chamber cast?

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mckg posted this 16 March 2016

Bill, can you tell where the bullets hang? Do you have some form of crimp on the fired cases or is their neck uniformly tight?

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Bud Hyett posted this 16 March 2016

Which chamber do you have"<>6 mm BR Remington<>6 mm BR NormaThere is a difference in these chambers. Contact the rifle maker to get their information on their chamber dimensions.<> What is the neck thickness and what is the chamber neck diameter:<>The question above concerning a chamber cast, get the actual dimension you are working with<*>Roto-Metals has chamber cast alloyChamber casting is a lengthy procedure, but done correctly the cast will yield useful information. Allow .003 inch total clearance, .0015 on a side.A chamber cast is the best method to know what the dimensions are.

The other possibility is case length growth due to repeated firings. Each lot of cases is a rule unto themselves for alloy composition and hardness; factors which influence case length growth. You may have a lot number which grows faster.  I am glad you are asking these questions; there is nothing like the momentary excitement of a cartridge exceeding the rifle's limits. The person sitting close to you on the next bench is the one often catching the shrapnel. I have both R-P and Lapua cases, the Lapua are more uniform for case neck thickness and length. The R-P cases were bought first and necessitated both outside neck turning and case length trimming. I hesitate to give you a trim-to length until you do a chamber cast and know what the chamber shoulder, neck and lead dimensions are. 

Farm boy from Illinois, living in the magical Pacific Northwest

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billglaze posted this 16 March 2016

Duane:  No chamber cast yet--I'm wondering if it would tell me a lot, inasmuch as there could be a lot of variability as to where I am to measure from--could be some guesswork involved, but your idea is on the top of the short list. mckg: The bullets hang up at the very case end--if I muscle them in, they freely slip into the neck easily, friction free, for the entire length of the existing neck, which makes me think the neck is just a skosh too long, even though you can tell that Lapua took some time to get them uniform; they did a beautiful job. BHyett: Thanks for your previous help; it is much appreciated.I'm sure case stretching is not a factor; the cases have been fired only twice, and show no stretching from new, according to my notes; Savage stamped the barrel with “6mm Norma B.R.” So I have good reason to believe that's what it is.  I have been informed that the Remington BR and the Norma BR differ only in the overall case length. (I sure hope I'm not misinformed.) One of the problems here is that I have no numbers for either of the BR's, let alone which is longer/shorter.I think the first thing I'll do is sacrifice a case to the cause; I'll start a couple of thousandths at a time until the bullet achieves a free passage, and see what I've got; meanwhile, contact Savage for their story, and get some chamber cast material. Gentlemen, I can't thank you enough for your participation in my latest puzzle. Bill

In theory, there's no difference between theory and practice. In practice, there is. My fate is not entirely in Gods hands, if I have a weapon in mine.

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Paul Pollard posted this 16 March 2016

Chamfer the case mouth. It might relieve it enough to let the bullet in.

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Ken Campbell Iowa posted this 17 March 2016

bill :: PLEASE stop thinking and do a chamber image. soft lead is good .

if it takes over 20 minutes you are taking two coffee breaks instead of one 5 minute one.

deprime a case ...drop a few mj bullets into it upside down until they come to just about the shoulder ...that leaves 1/4 inch to hold a long lead slug .

for the slug ... need softish lead.... without grooves ...sorta roundish ... that will slide in the bore ... long enough so that when you upset it in the barrel it engraves about half inch forward of the throat . you can cast a long stream then hammer it into a long ugly worm ...

seat the worm in the case down against the bullets stop and chamber the case. stick a stub end cleaning rod down the barrel and give it a hundred or so light whacks with a hammer . you are beating your bolt lugs so be gentle ... maybe 150 whacks...

gently unchamber the case and you should see a bullet upset that looks like your chamber throat and a bit of rifling . notice you can see how much you need to trim your cases also what your throat looks like .

see, that was easy .. better than talking about it ...

store your sample ... you will look at it a lot in the future.

i finally bot some bullet swaging wire just for that ... pm me if you want some ...

go for it ...

ken

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45 2.1 posted this 17 March 2016

It doesn't matter what SAMMI or any other source says. Your rifle dictates what it wants. A throat slug with attached case will tell all you need to know. Just trim a case to about 2/3 neck length, fill with lead to base of neck and put in a cylinder of lead about 3/4” long and upset it. Carefully remove case and slug in one unit and measure it. The full neck diameter and proper case length can be determined easily from it.

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billglaze posted this 18 March 2016

Well, following my old dictum of trouble-shooting I decided to start simple.  So, aim started trimming one of the cases.  I trimmed .001” at a time, and the Berger Match 90 gr. Still stopped just above the boat-tail break.  No change. Then, in an utter streak of genius, (yeah, right) I decided to try one of the bullets nose first.Guess what?  It slid smoothly in the fired case until it got almost to the place where it wouldn't go in the case mouth base first, where it stuck firmly nose first.  Another from the same box, same thing.  Tried a Sierra Match King--perfect sliding fit.  Opened another box of Bergers--perfect fit. Took a good, magnified look at the offending bullets; I could just barely see, (and I mean JUST) a small band around the circumference of the bullets.  Definitely some kind of a manufacturing error that left this bulge; I guess it escaped quality control. I don't know why I didn't think to check further earlier, particularly when a good friend told me that he had found four different ogives in the same box of his match bullets. And, just to think I felt that I'd had every weird thing happen that could happen........ Thanks many times over for the kind, helpful replies; I appreciate all your helpful suggestions; sometimes I think I need some kind of keeper. Bill

In theory, there's no difference between theory and practice. In practice, there is. My fate is not entirely in Gods hands, if I have a weapon in mine.

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frnkeore posted this 18 March 2016

Regarding finding the actual neck length and how far to trim the cases:

The chamber cast will do that for you. Your reference point will be the shoulder.

Frank

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onondaga posted this 18 March 2016

http://www.castbulletassoc.org/view_user.php?id=5098>billglaze

Maximum case length for the 6mm BR is 1.560". Pre-set case trimmers like Lee trim .005” less than Maximum for bottleneck cartridges. 1.555” is .005” less than Maximum case length for the 6mm BR and is a reasonable accepted standard. Cutting cases shorter than that doesn't have a benefit and leaves bullets unsupported in your chamber for the length you have shortened your brass unless you have a custom chamber that compensates for short brass.

” but I'm concerned because I can't get bullets to slip into the fired cases" Are you breech loading at the firing line to seat bullets? Then you need that kind of slip fit and custom turned Lee mandrels for their custom Collet Neck Sizing Dies do that. Custom Full Length Sizing dies can be made to do that too. Custom bullet size can do that too. This kind of stuff generally involves neck turning too or your brass won't chamber. Is that what you want to do? Doing all that stuff generally drains wallets and makes rifles shoot worse. You said yours is shooting some loads "into one small,ragged hole" right now. Don't fix it if it isn't broken.

Who told you that a slip fit of bullets into fired brass is necessary or a good thing? It is not necessary or a good thing but does sometimes happen by chance with fired brass and cannot be depended upon from lot to lot of brass and bullets. Your sizing die is designed to size brass for the correct fit and bullet tension to your bullets. Or, do you have a custom under diameter chamber neck to create tight fitting bullets? If you do, then you made this a problem for yourself.

If your rifle and press setup are normal and within SAAMI, only a slight inside case mouth chamfer is necessary for starting bullets with your seating die. Chamferring and de-burring more than 1/4 brass thickness at the case mouth weakens brass case mouths  and causes splitting. If you are crunching case mouths and necks seating bullets with correctly chamfered clean sized brass, then you got a problem.

Gary

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billglaze posted this 18 March 2016

Actually, I have no problem. It's Berger bullets that has the problem.

Bill

In theory, there's no difference between theory and practice. In practice, there is. My fate is not entirely in Gods hands, if I have a weapon in mine.

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Loren Barber posted this 22 March 2016

Bill,If you reference page 30 of the Sinclair International catalog (paper or internet), you will find case length space gauges for about $6.  Choose the gauge that meets your caliber and insert it into a significantly shortened full length sized case for your chamber.  The image is quite self explanatory.  Insert the case, gently close the bolt which pushes the plunger into the case, extract the case and measure the OAL.  This will give you the chamber length within 0.001 or better depending upon your technique.  I was frugal and made ones for my 6 PPC and 30 BR.  The condition of not being able to insert a bullet into a fired case might relate to a tight neck chamber.  Do you have the dimensions of the reamer that was used to chamber your barrel?  0.262 might be expected for a benchrest rifle.  Please check the OND of your loaded ammunition and compare it to the neck diameter of a fired case. I would expect it to be at least 0.001 smaller than the actual chamber neck diameter.  I might think that neck thickness of 0.008+ would be a good value for jacketed bullets.  Please be very certain that you are not chambering a round without any room for neck expansion.  Very rapid rise in pressure could occur and a speck of dirt could make chambering difficult.Loren

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