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Author Topic: Neck size diameter  (Read 1295 times)
John K
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« on: January 31, 2018, 11:33:34 AM »

Hello everyone,
My first post here so please bear with me.
I am having a rifle barreled in 308 with atight neck .
How much under my chamber neck diameter should I turn my brass
for the initial fire forming?

Many thanks
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phil
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« Reply #1 on: January 31, 2018, 12:31:26 PM »

HI,
   Turn your necks to achieve a clearance of 1 1/2" to 2 thou. under your chamber neck dimension.
   This should be measured by seating the bullet of your choice in the turned neck and then measuring
   the outside diameter of the  neck where the bullet base sits.

Phil
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John K
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« Reply #2 on: January 31, 2018, 04:46:03 PM »

Thanks Phil,
Ill start there
Much appreciated.
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Mr. Jeffery
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« Reply #3 on: February 08, 2018, 02:48:14 PM »

Hi Guys,
My first post here also.

Im curious about this as well. Is there a set standard to neck clearance inside the chamber?  Does this vary from caliber to caliber? My savage 10 (in .308) leaves my cartridges very dirty on the necks after firing whereas my weatherby vangaurd ( also .308) does not. Both rifles are discharging the same cartridge. unturned unmodified federal brass. Of course one wont accept the other but that is beside the point.

I ASSUME that one tries for the least amount of clearance as possible? The less the neck expands at time of discharge the better?  Correct me if I am misguided. I am only playing with this in my own head.
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DanO
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« Reply #4 on: February 10, 2018, 09:08:38 AM »

As was stated above neck clearance of .002” is a good target.
If you accurately measure the brass from each chamber you will see how much it differs from each other.
We normally measure the neck, shoulder and base dimensions of the fired and sized brass as the way to match up the sizing die to the chamber. One measurement that doesn’t get the attention it should is the shoulder (tapered part) bump back. It is common to see the dimensions over sized which effects the headspace clearance on sizes brass.
In a target type rifle this should again be in the .002” range of bump back. In a hunting rifle a bit more bump back is recommended to handle adverse environmental conditions.
The closer you can match the sized brass back the chamber dimensions the better it aligns the projectile to the bore.
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Mr. Jeffery
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« Reply #5 on: February 14, 2018, 09:54:51 AM »

My bad, Thanks for reiterating information already posted. I did see that post I guess I should have been more concise in my description of what I am going off about.

Lets say you have a cartridge that is"perfectly sized" 0.002 neck & 0.002 shoulder. It shoots lights out. Why mess with a good thing right?  When you discharge the round the neck expands and allows the projectile to leave the chamber with little interference.  Im splitting hairs..

If you have a cartridge with lets say the bare minimum of neck clearance.. 0.268 in a 0.269 chamber... wouldnt this be enough to allow free exit of the projectile as well as keep the brass from being over worked thus prolonging its life and possibly increasing accuracy? Or is that 0.0015 -0.002 what has been shown or proven and is known to work?  Or am I right out to lunch as the cartridge then becomes too difficult to manipulate? Some one out there must have tried this at some point?   Sorry if this is a ridiculous question. Im just curious and searching for 2nd opinions.
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rpollock
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« Reply #6 on: February 14, 2018, 05:22:52 PM »

Mr J,

hard to quantify what works, a bit easier to say what doesn't work. In short range BR shooters have been experimenting with fitted/stepped necks (more or less zero clearance) up through about .004 - .005 total clearance. This has been going on for about 40-50 years. Experience has proven that .002" - .0025" total clearance is where the magic happens. Anything less will require fanatical attention to details and may prove quirky after just a few firings as brass migrates around a bit. Anything over about .004" and accuracy suffers.

While you are pondering this it is important to figure out what the actual chamber neck diameter is. It won't necessarily be what is printed on the reamer. You can use pin gauges, bore gauge to figure neck diameter. Hope this helps.
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Mr. Jeffery
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« Reply #7 on: February 14, 2018, 09:08:35 PM »

Yes Sir it does. Thank you.
I am right on the money. I am 0.269 on the chamber and the brass is turned to 0.2675. My rifle shoots fairly well but i have yet to make it shoot like it should as my skill level is still that of a beginner.  

I was curious "where the magic happened"  And this is exactly the kind of reply I was hoping for. I knew I couldn't be the first one to think about this. I just didnt know that there are 40-50 years worth of people ahead of me.  Haha!

Cheers!

 
« Last Edit: February 14, 2018, 10:19:22 PM by Mr. Jeffery » Logged

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rpollock
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« Reply #8 on: February 14, 2018, 09:49:45 PM »

Organized benchrest has been around since just after WWII. The level of experimentation we are talking about is still ongoing today the same as it has for decades. Most of the experimentation is at the minutia level, but still going on.
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Mr. Jeffery
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« Reply #9 on: February 14, 2018, 10:17:50 PM »

 Well experimenting is right up my alley.. So i should fit right in.. !

 Oh yeah.. the other thing I was curious about... Blow by or blow back. The necks on my bress look nasty and sooty when thew come out of the chamber. Is that considered normal? Im not sure whats an appropriate level of carbon build up on the outer surfaces.
« Last Edit: February 14, 2018, 10:19:09 PM by Mr. Jeffery » Logged

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rpollock
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« Reply #10 on: February 14, 2018, 10:52:39 PM »

Normal to see some carbon on the neck. Not a lot though. Less than 50% coverage. Shouldn’t see any on the shoulder. What cartridge and load are you using?
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Mr. Jeffery
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« Reply #11 on: February 14, 2018, 11:00:12 PM »

I am loading 6ppc In lapua russian.   ive tweaked my load to 26.9 using 68 gr bergers jammed 6 thou. this is giving me the best group. so far......  Nothing to write home about but my last test yielded a quarter inch group at 100yds.  Still need to tweak some. Yodave changed up our targets and we now have a much smaller X target to shoot at. This Sunday will seperate the men from the boys. So if I can get this load or something very close to it to shoot in the high tenths.. I'll be a happy camper.
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phil
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« Reply #12 on: February 15, 2018, 06:51:38 AM »

26.9 gr. of what ?  This sounds really light, and may account for the sooty cases.
Try going up in charge weight to the next node, this will probably expand the cases
enough to seal the chamber. Work up to it.
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Mr. Jeffery
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« Reply #13 on: February 15, 2018, 12:23:21 PM »

uhg.. I always forget something...  good thing its not my head....  N133.  Ive gone as high as 27.8.  Not much difference. other than accuracy. It seems to shoot best at 26.9
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cyanchycki
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« Reply #14 on: February 15, 2018, 01:38:26 PM »

OMG. ...if you are shooting 133 that is Light........

You are positive it is133?

You are talking 133 when you are around 30grs........try starting at 29.1, 29.7, 30.3 grains...okay that is if in a custom action.  If a sleeved 700 start slightly lower and watch for the usual pressure signs.

Lighter loads may work at 100 but will definately get pushed around at 200.....

« Last Edit: February 15, 2018, 06:15:42 PM by cyanchycki » Logged

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duceman
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« Reply #15 on: February 15, 2018, 08:01:07 PM »

next node sitting in line..............
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Mr. Jeffery
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« Reply #16 on: February 16, 2018, 02:10:01 PM »

  this is 26.9 at 300 yards. mild wind -1-4 mph.  as you can see by the one flyer that meessed up the group. I got second place for this.. 1st place was 0.856 or some such nearby number

« Last Edit: February 16, 2018, 02:13:48 PM by Mr. Jeffery » Logged

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danyboy
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« Reply #17 on: June 20, 2018, 07:28:50 AM »

 I am new in benchrest. When you guys say '.001-.002' neck clearance, do you mean total (for both sides) or per side ?  Just built myself a 6BR with a Shilen Barrel that has a .272" chamber neck. Loaded case neck using brand new Lapua Brass is .270". Fired case neck is .271". I bought a Forster bump/neck sizing die and currently trying to purchase the wright size bushing. Virgin Lapua Brass neck OD is .267". 
 Would a .267" bushing be appropriate ?
« Last Edit: June 20, 2018, 07:35:58 AM by danyboy » Logged
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