Sine marks on fired case necks

B

Brian

Guest
I've noticed the up and down carbon marks on case necks for many years but never new what exactly caused them and never worried because I was always told they were normal. I've been chambering my own barrels for the last 6 or 7 years and always had them until just recently. A couple weeks ago I chambered a new 6ppc .262 neck barrel and after fireforming some new brass I notice the necks have a perfectly round ring not the up and down sine pattern they have always had. I cut my brass a bit on the thin side (loaded round measures .259) but that is what I have always done. The barrel seems to shoot pretty good and the cases don't seem to show any signs of high pressure. Should I just keep shooting this barrel or do I need to be concerned.

Thanks.....Brian
 
I experienced the exact same thing with one exception...my barrel wouldn't shoot well. It seems, however, that something is wrong there. Check the measurements again to make sure it all adds up OK...check everything involved.
 
Brian said:
I've noticed the up and down carbon marks on case necks for many years but never new what exactly caused them and never worried because I was always told they were normal. I've been chambering my own barrels for the last 6 or 7 years and always had them until just recently. A couple weeks ago I chambered a new 6ppc .262 neck barrel and after fireforming some new brass I notice the necks have a perfectly round ring not the up and down sine pattern they have always had. I cut my brass a bit on the thin side (loaded round measures .259) but that is what I have always done. The barrel seems to shoot pretty good and the cases don't seem to show any signs of high pressure. Should I just keep shooting this barrel or do I need to be concerned.

Thanks.....Brian
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The general consensus is if you get a solid black circle, you don't have enough neck clearance.

The trend in recent times has been to run a little more clearance than in past years. I used to try for .001 total, but quite a while back decided to shoot close to .002. With that I do get that sine wave pattern, on both my 6PPC and my 30BR. With .001 or less, you won't.

Close fitting necks came from the practice of years ago when shooters never even sized their brass, the natural spring back gave the bullet enough grip. I suppose the powders back then lent themselves to this. Today, it seems like everybody full length sizes, and gives necks more clearance. Maybe the advent of VV 133 has a lot to do with that.

I think the consensus is if you are going to err, be sure it's on the loose side.
 
Ditto Jackie... I use the sine wave pattern as a way to judge my neck turning performance. A solid line up on the neck means the necks are too thick, down on the shoulder means they are too thin. Good clear sine wave means just right.

The sine waves correspond to the lands and grooves in the barrel. Four groove has a different pattern than a 3 or 6 groove.

Rod
 
Gentlemen

That makes three of us with the same issue only I have same issue with two separate barrels!

I'm willing to bet the only thing common with all of us?

Is new blue box lapua brass it appears to me the necks are a little soft...

Only thing is my two barrels are shooting outstanding!! {Sorry Wilbur}

Russ
 
Soft brass will cause a just a ring...

I have gotten the solid ring at the top just after annealing my brass. After the first firing it usually disappears. If your necks are already on the thin side them Mr. Stiner is probably correct.
 
On both your sine wave barrel and the one without, use a small hole gage
, full ball or half ball, then measure the actual neck bore and that barrels brass with bullets seated.

Find a difference in actual neck clearance between the two?

.
 
Mr Jarvais,

Thanks For the clarification for possible cause of carbon ring on brass!

I made a new batch of brass for both new barrels and your right I neck turned them thinner this time for more neck clearance {.0025}

I measured loaded round neck Dia. three times on five different rounds. all .3275-.3278 For 30BR .330 neck.

Jerry

Your right that we can never assume! always measure the best you can with the equipment that you have.
All I have Is {Micrometer, Calipers, Tubing Micrometer, and chamber gauge made with chamber reamer}

Russ
 
One item to remember is that the actual neck size of the barrel may not be what's marked on it. That doesn't mean it's a bad chamber job but rather the neck diameter is different than what's marked on the barrel. A good barrel, is a good barrel, is a good barrel....and we don't know exactly why it's a good barrel.

As an aside subject.....there's something there that the barrel makers are onto though because there are more good shooting rifles now than there has ever been....perhaps rifle builders as well. Once upon a time, there were just a handful of winning rifles and now there's two or three handfuls.
 
Hey guys, I'm still learning rifle handloading (1 year experience). Could someone post pictures of these lines and what each line indicates? Thanks!
 
MislMan said:
Hey guys, I'm still learning rifle handloading (1 year experience). Could someone post pictures of these lines and what each line indicates? Thanks!
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One of my fired 30 BR cases. It has .002 clearance in my .330 neck. Since it shoots quite well, I figure this is about right.

I have a Custom 3006 with a neck size that gives on average .0035 clearance over a Lapua non neck turned loaded round, and it gives the same pattern.



http://benchrest.com/attachment.php?attachmentid=19374&stc=1&d=1493682409

http://benchrest.com/attachment.php?attachmentid=19375&stc=1&d=1493731191
 

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dnal said:
Don' t sine waves match up to Lugs on bolt ?
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Yes if you have straight brass and lugs that are well matched you should see 2 waves or dips. It's just telling you that as the bolt lugs settle in on the receiver lugs the brass has contracted enough to let some gas escape, hence the wave pattern. If you have a 3 lug you should see 3 small waves or dips. Lug engagement and brass run out will determine how even the wavy pattern is. If your necks are too thick, you won't see a wavy line, just a carbon ring straight across all the way around the neck.
 
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As a rule, the sine wave becomes more pronounced as neck clearance increases.

In my experience, a few other things that happen are:
- Wave lengthens as necks become work hardened.
- Wave shortens when switching to a 'faster' powder.
- Wave lengthens when going to a 'slower' powder.
- Wave can vary depending on how much you're really pushing the shoulder back.

Good shootin'. :) -Al
 
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