Neck Turning - Wall Shrinkage Over Time

[PMA3]

You can tell it's winter...a lot of reloading that raises a LOT of questions.

I initially turned my 6x47 necks to .0108" to use in conjunction with a current LOT# of .2433" diameter bullets for a .267 tight neck chamber which gave me a theoretical clearance of .0021" (.0108 x 2 + .2433 = .2649). Those bullets are about gone and the new LOT of 3500 measure .2436" which puts me in a range I don't like to be...clearance of less than .002" (.0018"). I decided to measure the neck thickness to see if I had to neck turn again in order to reach at least a .002" clearance. Before measuring, my initial thought was that the case necks would be THICKER from brass flowing forward ...but...they were all THINNER. These 80 cases have been fired 6 times each and 85% measured .0104"-.0105"...the others were either .0106" or.0103". The good news for me is that the loaded round should definitely give me the +.002" clearance I want but raises the question of why the case walls get thinner vs thicker after repeated firings. I was hoping to get at least +20 loadings per case but now wonder how much thinner the walls will get after more firings and the resultant effect on neck tension (which could also be a current problem based on the numbers).

Any input on similar experiences would be greatly appreciated.

 

[Shynloco]

Questions - What kinda of brass are you using and are you FL resizing each time and re-trimming your brass each time? BTW, I've never seen brass necks thickness get thicker over time. Thx

 

[Terry]

I'm not sure .0001 is discernible even with the best micrometers. 1/10,000 of an inch is hard to repeat when measuring.
As to shrinkage, it would be unusual to have a case become unusable

 

[4xforfun]

.002 vs .0018....two tenthousands of an inch!! Put your loading stuff away and watch a game. You are way overthinking this over such a small amount...you will be fine. If you haven't re-adjusted your turning tool since your first batch, then don't.

Now, if you were running .0005 clearace, then I would worry!!!

It's funny how when the temp drops 100 degrees peoples minds start to wander!!

 

[searcher]

Not to worry

 

[PMA3]

Shynloco,
I used new Lapua 6.5x47 brass necked down to 6x47. I F/L size every time with custom dies made from the same reamer and also trim every time.

 

[LCazador]

Never seen case neck walls get thinner or thicker after turning, however this doesn't mean it couldn't happen. Maybe I discard too quickly. Are you sure your measurements are accurate? I wouldn't worry about a couple of ten thousands difference. If you are using a digital micrometer, check the batteries in it. It's always good to verify digital readings manually before recording data.

 

[DeltaBravo]

Is it possible that the brass is flowing forward and thinning out in the process? You said you "trim" every time, which leads me to believe that this might be the case.

Dennis

 

[brxbrad]

If it aint broke...Why are you trying to fix it? If your rifle was shooting good last season and you haven't changed your bullet, powder, brass, or primer mfg's, then when you go to the next competition, your scores will either go up or down depending on the "kind of day" you are having.....I agree, cold weather does strange things to the mind. Get out of the reloading room before you loose your mind.....You'll have plenty to worry about when the weather gets better.

Happy New Year
Brad

 

[PMA3]

I use a manual Starrett 1" ball mic, the same one Sinclair sells on their angled stand.....I have mine mounted on a Hart third hand tool. I have placed plastic washers over the anvil (glued in) so I measure the neck at the same spot each time. I take 3 measurements for each case by rotating the case neck as close to 3 equal revolutions as I can to insure I'm within .0001". If the wall thickness only decreased from .0108" to .0107" or even .0106", I could understand where a measuring error of a couple tenthousands could take place, but to go to .0104" or .0103" is a big difference in clearance and neck tension (.0005" worst case). The only digital tool I trust is my GD-503 balance.

 

[liljoe]

I was told when I started turning brass from a veteran shooter, that your brass will measure a TINY bit smaller after a couple firing because the necks smooth out after the high pressure and the bushing.

If you look at a piece of brass that is fresh turned with a 10X loupe, the surface is not perfectly smooth, it has a very slightly rough finish. If you look at a piece that has been shot 5 times and sized with a carbide bushing, it is almost a mirror finish.

Remember you are talking about .0002" or .0003", that is tiny amount. A sheet of paper is about .003", divide this 30 times and think how small that really is.

Another thing to think about, is your neck diameter actually that stated size, I bet it isn't. Get a couple X class gage pins that are your neck size and then +.0002,+.0004 and try them in your chamber. I bet it is slightly bigger.

I have the pins and have done this. You can also use the pins to check your micrometer.

Joe Hynes

 

[liljoe]

Also, I would not trust 100% the measurement with a ball mic, measure a bullet and seat it in a case and measure the total with an outside mic. That is the important size.

Joe Hynes

 

[LCazador]

Not sure how you can make a die from the same reamer, as the sizing die needs to be slightly smaller than the fired case in order to size back. If the sizing die is a bump die how can it support the lower part of the case with the same dimensions as the chamber? You've got some serious stretching going on if you trim every time, how much do you trim? And yes, if you went from .0108" thickness to .0103", a difference of .0005", it might change bullet tension slightly, but not enough to worry about. And for sure it's nothing that couldn't be remedied by a .001" smaller bushing!

 

[lightman]

I've had better luck measuring the loaded round than measuring the wall thickness and doing the math. Sometimes the math just does not work out right, for me. I also turn case necks with a cordless screwdriver, and touch the case neck with a piece of steelwool before I take it out of the shellholder. Does anyone else do this? Lightman

 

[CatShooter]

I've had better luck measuring the loaded round than measuring the wall thickness and doing the math. Sometimes the math just does not work out right, for me. I also turn case necks with a cordless screwdriver, and touch the case neck with a piece of steelwool before I take it out of the shellholder. Does anyone else do this? Lightman

I use a Forster neck trimmer with a 1/4" power drill, and I feed the cutter very slowly - so the finish of the neck is very smooth without cutter ridges.

Then I put the cases in a Dillon vibratory polisher and the necks are mirror smooth.

 

[mikecr]

PMA3
When you said the theoretical loaded neck was (.0108 x 2 + .2433 = .2649), did the loaded neck ODs also measure this?
What does the loaded neck OD measure with the new lot of bullets seated?
Did you turn the necks at ~26cal diameter, or at sized to 24cal diameter?
Where on the necks does your stop put the anvil ball(depth from mouths)?

When you FL size enough to invoke the need for trimming, this should be caused by lower brass rolling up into necks -making them thicker over time(atleast in the donut area). With this a taper in thickness should form.
So I wonder if your original 'loaded' necks were actually thinner than they are now. That is, thinner than you may have thought.

I have plenty of faith in a ball mic with a stop, but your described condition doesn't make sense yet.
I'm also familiar with your mic setup as I used to run with the same. But there are times when depth of measure other than a standard is called for, so I now use an adjustable stop with the ball offset to spindle so that I can read into donut area. I'll try to post pic.

 

[PMA3]

My rifle took about 20 months to build (BAT action mix up) and during that time I made a lot of inquires about loading for a 1000 yard BR rifle and took a ton of notes. I went back over these notes this morning and found something similar to what liljoe posted; a long time shooter at Williamsport told me to take off just enough material during neck turning to insure the round chambers as the case walls will get thinner over time....I never thought to ask him HOW MUCH THINNER and at what point (approx. # of firings) it should stop.

In my original post, I said I should get a THEORETICLE clearance of .xxxx. That's because I always find the (neck dimension + bullet diameter) number to be LESS than the loaded round number by 1 or 2 tenthousandts....another thing to look out for in tight neck chambers. As far as the dies go, I was trying to generalize ... my smith makes the dies from the die reamer.

For trimming, I do 40 cases at a time....some may be zero, some .001" and an occasional .002"....I want them all the same OAL.

 

[mikecr]

Don't reach for anecdotal or you'll miss out on the understanding.
Stick to facts.
Are my necks really thinner, and why?
I did that & this is what I see now thinking.

 

[PMA3]

mikecr,

I wasn't being anecdotal; I was just providing answers to some of the previous posts. My necks are definitely thinner than they were after my initial neck turning (to .0108"). It seems that this is what they do (become thinner) but the WHY still escapes me. In answering your questions that I missed:
1.The loaded round measured approx. .0002" more than adding the numbers.
2.I turned the necks AFTER necking down from 6.5.
3.The anvil ball measures at mid-neck, just like on your picture.

I'll be breaking in another 6x47 this year and will keep tabs on the neck diameters for comparisons down the road.

 

[jlow]

I actually don’t think thinning necks is such a strange thing. We all know that brass from the case body will migrate up to the body, the reason being that pressure will flatten the case against the chamber and it has to go somewhere. Usually this ends up against the neck shoulder junction to form that infamous donut. So in the same manner, when the gun is fired, the case neck will also fatten against the side of the chamber and it too has to go somewhere which is to lengthen the neck. If the neck lengthens, brass has to come somewhere and although one can argue that most of that comes from the body/shoulder, is it reasonable to say that none of it comes from the neck? If some comes from the neck then the neck has to thin, there is no way around this.