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BR Discussions => Centerfire Discussions => Topic started by: John K on January 31, 2018, 11:33:34 AM

Title: Neck size diameter
Post by: John K 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
Title: Re: Neck size diameter
Post by: phil on January 31, 2018, 12:31:26 PM
   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.

Title: Re: Neck size diameter
Post by: John K on January 31, 2018, 04:46:03 PM
Thanks Phil,
Ill start there
Much appreciated.
Title: Re: Neck size diameter
Post by: Mr. Jeffery 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.
Title: Re: Neck size diameter
Post by: DanO 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.
Title: Re: Neck size diameter
Post by: Mr. Jeffery 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.
Title: Re: Neck size diameter
Post by: rpollock 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.
Title: Re: Neck size diameter
Post by: Mr. Jeffery 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!


Title: Re: Neck size diameter
Post by: rpollock 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.
Title: Re: Neck size diameter
Post by: Mr. Jeffery 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.
Title: Re: Neck size diameter
Post by: rpollock 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?
Title: Re: Neck size diameter
Post by: Mr. Jeffery 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.
Title: Re: Neck size diameter
Post by: phil 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.
Title: Re: Neck size diameter
Post by: Mr. Jeffery 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
Title: Re: Neck size diameter
Post by: cyanchycki 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.....

Title: Re: Neck size diameter
Post by: duceman on February 15, 2018, 08:01:07 PM
next node sitting in line..............
Title: Re: Neck size diameter
Post by: Mr. Jeffery 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

Title: Re: Neck size diameter
Post by: danyboy on June 20, 2018, 08:28:50 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 ?
Title: Re: Neck size diameter
Post by: hendershot on June 28, 2018, 12:34:40 PM
Hey danyboy,

When the guys talk about clearance, they are referring to total clearance. .002" clearance would be .001" per side for example.

With regards your particular situation, the neck clearance of your Lapua brass looks like a good place to start - .002" No need to turn the neck diameter down unless you want to clean the necks up a bit and/or feel like adding more clearance for whatever reason. As for neck bushings, I would try a few different sizes and see what kind of neck tension your rifle/load likes. Perhaps .267" on the tight end to .269" on the loose end? I don't shoot a 6BR so can't really say what a 6BR likes for neck tension. Another thing to keep in mind is that your bullet could have a pressure ring at the base of around .0005" The bullet makers in the crowd can confirm or deny this dimension. What this means is that your actual loaded OD could be .2705" measured at the pressure ring with the bullet seated. If you're using longer bullets in your 6BR, the bullet's base could be down below the neck/shoulder junction. Just measure the bullet at it's base and compare that measurement with a measurement taken just a tad further up the jacket. Some will argue that tight neck tension is a good thing because if you're running a compressed powder charge, the tighter neck tension will keep your bullet in place where as minimal neck tension could push the bullet out some. Seating depth considerations are an important step in load development and consistent seating depth is one of the critical aspects to shooting small groups. Consistency is key, even if you're doing things all wrong, do so consistently.  :D