tailoring a specific neck/chamber clearance in a factory rifle?

[nuthead]

HI Gents,

Now that I'm dipping my toe into neck turning I can tailor my chamber/ neck clearance to a certain degree. I was just wondering if there is any point in tailoring a neck diameter for a factory rifle?

My current neck/chamber clearance is about 0.004 (4 thou) at the moment and I've read that a lot of the benchrest guys use 0.001 to 0.003 depending on the range that they shoot at. I was just wondering if there Is any point in trying to tighten those dimensions up or is it just a waste of time in a factory barrel?

I could probably get it down to 0.002 (but the chamber pressure seems to rise a bit with a thicker neck)

[John23]

I have zero experience on the topic so please forgive me.
How do you increase neck thickness but cutting material away?

JH

My bad - I probably wasnt very clear

currently the cutter contacts 100% of the case neck when turning - if I backed the cutter out so it trimmed only 80% of the neck, the overall neck diameter would increase

[Mega]

So, it;s a Factory Rifle with no doubt a Factory Chamber. You have 0.008" neck clearance at present. How did you determinr this measurement...???

Lets say the easiest way since you will not have a Reamer Print or Chamber Cast to go from. You measured a "Fired Case" from your chamber then measured a "Loaded Round".... gives 0.008" difference. Well, that's a tad out if it's the way you went about it. Most case necks expand to the chamber size on firing but then the brass in the neck springs back as does the whole case otherwise you would have a stuck case in the chamber. The amount of "Spring Back" can vary on the brass type and load you fired but say in general this can be 0.001" to 0.002" or more so you actually have a chamber neck clearance of 0.010" roughly not 0.008".

By neck turning you will increase the chamber neck clearance not reduce it as you indicated. If you take the very lightest of skim off the neck you are probably taking off around 0.0005" at least so that doubled will mean you have just added 0.001" on top of your current neck clearance.

In Factory Rifles it really does not matter this large amount of chamber neck clearance but what you are doing is improving the consistancy of the neck tension on a bullet when loaded around it's diameter.

If you had a Custom Chamber then the Neck Size is most likely smaller and if it's a "Tight Neck" chamber them much smaller.

As I said to you most Benchrest Rifles use a minimum of 0.002" and no less or you can be asking for trouble. Remember I said that most target bullets have a "Pressure Ring" at the base of the bullet and this would typically be around 0.0005" so if you had a neck clearance of 0.001" then as the bullet moves up the neck on being fired it reduces that to 0.0005" which is very dangerous.

sorry mega - i realised I stuffed up my own measurement!! had the wrong decimal place in and didnt measure enough cases to get a proper representation. hence the edited post... its actually closer to 0.004

I measured the neck diameter of a few fired rounds and subtracted 0.001 - they were fired a few days ago if that makes any difference...

thanks for the quick reply though - so 0.004 seems reasonable it would seem. Is there any truth to the comments I've read about 0.003 being preferred for the long range benchrest guys (as opposed to 0.002?) - The thought was that it produces a lower std deviation (extreme spread)

[Mega]

Replies whilst I was answering....

Read above about Factory Chambers and as we discussed you will be just as well off turning about 75% of the neck or less and perhaps gain 0.001" in loaded round neck diameter.

[Mega]

It's all about consistancy of loaded rounds.

The 0.002" is really the absolute minimum clearance anyone wants and quite a lot use more like 0.004" as it's personal experience and their choice of what they work with. My other Custom Chambers that are called "No Turn" use up to 0.005" neck clearance although I personally strive for slightly less and 0.004" in my mind is ideal for those.

Have another look at the photo's in my 30BR Thread and look at the amount of cut on the neck. For your purpose you don't want any more than that to achieve an increase in consistancy of neck tension in your factory rifle. You could in fact come back a tad and you don't need to get the cut to go all the way to the top.

Remember I'm playing with custom cut chambers.

cool - I'm happy to stick with 0.004-0.005 then - if its good enough for benchresters its good enough for me

I actually cut more away than the photos on that 30br thread... but I necked 308 brass down to 7mm08 instead of necking up as you did (the brass seems to become very thick around the neck after doing this)

[Mega]

What you had been doing is probably fine but if you want to reduce the clearance then by all means try a few and see how they perform.

Without a nice Neck Micrometer it's hard to get the exact measurements you need but they are expensive.

These are the two I spoke of .....

[Mega]

I am but only one opinion, others may think and advise differently.

Each to their own and the decision is all yours when it comes to deciding on the best course of action.

Here are some more Neck Turned Cases. They are 30BR but the idea is to show you don't need to turn right to the end of the neck. The thickness of material in these necks from top to bottom varies by less than 0.0005" or there abouts. Doesn't matter what calibre but this is what I call an 80% cut.

In my case the idea was to get rid of the "Donut" created at the bottom of the neck and improve continuity of neck tension. I'm working on a neck chamber clearance of as close to 0.002" as I can get but no less and every case has been checked.

[Mega]

The same as your 7mm08 my .243W is a Tikka T3 just a different calibre.

I went and measured my fired rounds and some loaded rounds. The difference is 0.006" between them. Lapua Cases that have been Neck Turned about the same amount as I discussed here. They receive the same treatment as I would a target round with case preparation and I think it pays off.

Here is the Tikka T3 which was a standard Varmint but in a Super Varmint Stock. The group shot below was during load testing at 100m and is not ideal since it shows a bit too much horizontal spread. Since then the Tikka Action has been "squared" by skimming the front of the Action and the trigger has been very much lightened down to 2oz in Set Mode and 1.5lb in Normal Mode. The barrel btw is very warn and has only a few hundred more accurate shots left according to my Gunsmith after a Bore Scope examination...that's is the Throat is worn, not the bore.

thanks for the help as always mega

thats pretty damn accurate for a nearly shot out barrel!! looks to be about 0.6ish moa still? you going to rechamber that one or rebarrel?

I think its sensible for me not to spend $1000's of set of mics yet :lol: maybe one day when I have all the other bits and pieces I need... thinking about it I dont think you ever recoup the costs of reloading equipment?!

I'm going to try some brass turned to 0.002 clearance to see if they make a difference. I actually havent even had the chance to test any of the fireformed brass yet - about to head off to work to fill in for a driver.... hopefully I get a chance to do some testing on the weekend

[Mega]

Can't Re-chamber, it's a Factory Barrel and there's no Knox Form available to do it. I'm trying to wear it out to the maximum and then it'll just go to the scrap metal pile. There's a near new 6.5x47 Lapua Barrel it gets switched to so it will probably stay at that and down the track I'll buy another .243W Rifle.

Expensive reloading gear is never cost recovered, it's just part of wanting accuracy.

I think you need some sleep.... "turned to 0.002" clearance" ....think you messed with the figures again.

You are welcome as I said to come and wear some of my gear out.

Don't work too hard.

Can't Re-chamber, it's a Factory Barrel and there's no Knox Form available to do it. I'm trying to wear it out to the maximum and then it'll just go to the scrap metal pile. There's a near new 6.5x47 Lapua Barrel it gets switched to so it will probably stay at that and down the track I'll buy another .243W Rifle.

Expensive reloading gear is never cost recovered, it's just part of wanting accuracy.

I think you need some sleep.... "turned to 0.002" clearance" ....think you messed with the figures again.

You are welcome as I said to come and wear some of my gear out.

Don't work too hard.

I absolutely do need some sleep - been up for 26hrs now!

What I mean is that after necking down the brass (before turning) it becomes so thick that theres probably around 0.001 clearance (so if I make a shallower cut I can bring it to 0.002)

i might take you up on that offer mega!

anyway gotta sleep...!