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Measuring OAL


andybrock

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Could someone explain the 'jam' method??

I presume this means jamming the bullet in the lands, but if so how far would you "jam" the bullet in the lands??

Cheers

Jam1e

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With the 6PPC, we turn the necks and run quite low neck-tension. Load the bullet well out and jam the bullet into the lands and that is your starting point. Work back from there. Saves all those 'measuring devices'! You couldn't use the 'jam' method with a normal cartridge/rifle.

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When I chamber a new barrel, there is always an inch or so to cut off the muzzle end. This goes in the scrap bin - or does it!

 

You can easily make an interesting little gauge - which answers all of the questions in the posts on this thread.

 

Run the reamer into the barrel-stub to just short of the shoulder/body junction. Now, in the milling machine make a cut about 10mm wide to half the depth of the barrel to expose the end of the chamber neck.

 

Now you can actually look inside your own chamber at this critical point. If I knew how to post a pic I'd show you one.

 

You can check for case growth, seating depth, clearance (if running a tight-neck) and you can use it to accurately measure case-length and 'bump'. To me, it's an absolutely essential bit of kit.

 

Takes me about half an hour to make one - ask your gunsmith the next time you have a barrel fitted - can't charge more than about £40.....................

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With the 6PPC, we turn the necks and run quite low neck-tension. Load the bullet well out and jam the bullet into the lands and that is your starting point. Work back from there. Saves all those 'measuring devices'! You couldn't use the 'jam' method with a normal cartridge/rifle.

well i hope you can run the jam method, because once i sort my 20 tac out i am intending to jam about 10thou in ...

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TGP, the easiest way of posting a picture is to upload it to photo bucket then copy the img code associated to that picture in your post here. Alternatively email it to me and I'll post it.

OK - give me your e-mail. Mine's vinceb@6ppc.fsnet.co.uk

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Thanks for getting pics up MJR,,,,,,,,,,TGP,,,yep and interesting improvement to the semi chambered barrel stub which I have found useful as well.Tell me,,, I can understand most of the uses you would get from your modified stub but where are you actually getting the measurements from when arriving at a "Jam" COL measurement or OG Jam measurement.From the pics you seem to be just measuring base of case to end of stub??

What I can think is that you withdraw the case with bullet lightly seated and after coming in contact with the lands?Sorry I know there is going to be a simple answer but a pre run of new years eve last night has interfered with my normal thinking!!,,,,,thanks Onehole.

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Well, as I said in an earlier post - you are looking for a starting point with bullet seating depth.

 

It's easy to do this visually with this gauge - then only testing on the target will establish 'into' or 'off' the lands and how far. Like the 'jam' method, you are simply looking for a starting point.

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Well, as I said in an earlier post - you are looking for a starting point with bullet seating depth.

 

It's easy to do this visually with this gauge - then only testing on the target will establish 'into' or 'off' the lands and how far. Like the 'jam' method, you are simply looking for a starting point.

 

Ok ,,,with you now,,,,cheers Onehole

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Well, as I said in an earlier post - you are looking for a starting point with bullet seating depth.

 

It's easy to do this visually with this gauge - then only testing on the target will establish 'into' or 'off' the lands and how far. Like the 'jam' method, you are simply looking for a starting point.

i think this sort of measruring device should come with all rifles its an excellent idea...

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Boys you are talking about two different measuring devices ?

One is an Oal gauge that measures from the base of the case and with a bullet inserted you then put it in your rifle push the bullet until it reaches the rifling and stops, take it out, measure and that is your Oal to the rifling ,, as Vince has said this is your starting point, hopefully??, my rifles all seem to like this ,,,

Now the shooting shed comparator is a nice little tool that let's you measure the Oal of a seated bullet in the case, ie base to tip. Of the bullet and it gives a very accurate measurement,,

Now I use this to check that all of my rounds are all the same Length in my box,,

 

Hope that this has cleared things up a little

Darrel

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Boys you are talking about two different measuring devices ?

One is an Oal gauge that measures from the base of the case and with a bullet inserted you then put it in your rifle push the bullet until it reaches the rifling and stops, take it out, measure and that is your Oal to the rifling ,, as Vince has said this is your starting point, hopefully??, my rifles all seem to like this ,,,

Now the shooting shed comparator is a nice little tool that let's you measure the Oal of a seated bullet in the case, ie base to tip. Of the bullet and it gives a very accurate measurement,,

Now I use this to check that all of my rounds are all the same Length in my box,,

Hope that this has cleared things up a little

Darrel

But if you were only measuring from the base of the cartridge to the tip of the seated bullet couldn't you just use callipers? Please correct me if I'm wrong but I thought all OAL comparitors take their measurment from the base of the case to the ogive of the bullet?
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The stoney point gauge when used with a drilled and tapped case gives you a measurement from the base of the case to the tip of the bullet.

The hornady cartridge overall lenth gauge that takes the inserts and the shooting sheds gauge measure to the bullets ogive.

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I ment to say to the orgive, I stand corrected!!,

Also they are calibre spacific and fit like a glove,

 

Not like the Stoney point one that you have to wiggle about to get a repeatable reading,,

 

The shooting shed one also is good for getting a very acurate case lenth measurements with out the bullet seated ,,

 

David Bonwick ( shooting shed) does actualy make a very good Oal gauge,

And a realy good concentricity gauge.

 

But as Vince says they are just good starting points

 

Darrel

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One thing about "OAL Gauges" that has been missed by all so far, is that they do not actually gauge from the base of the case, but from the point of contact of the supplied case with the chamber shoulder.

 

There is no guarantee that the case used in the gauge will the same "headspace" as the cases used in your loaded ammo. I have found some gauge cases to be out by 10 thou or more!

 

George

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My problem is that I am being persecuted by an acronym. COAL.

SAAMI(1926,USA standardisation of sporting rifles and ammo) and CIP(1914,European ammo safety) have ensured that fifles and ammo work safely together;(eg a 243Win commercial cartridge should fire safely in any commercial 243Win chambered rifle).('Wildcats', of course,by definition were never so standardised).

However,COAL (Cartridge Over All Length) has become much more problematic than chamber specifications-at least in some of it's useages- and is really best considered now as a specification for magazine length,better yet,a guide -as some manufactures exceed it in their magazines,and there has been some move to 'one size' can be made to fit all (using spacers) in the interests of manufacturing economy. There is a more general mischief possible-indeed,probable,if COAL is taken as the starting datum measure for precision oriented handloaders,rather than a final rather derivative,measure.

Handloaders quite quickly realised that reducing seating depth freed up space within the cartridge case,which could accomodate ever more efficient powders,and a much wider choice of bullets per calibre added to this seating depth flexibility (there are over 200 6mm bullets available,eg).With care,there might be no serious pressure increases either,and bullet retention was less critical ,than in magazine sporters.

COAL became a variable,not THE datum.

The search for more velocity led to improved BCs- bullet designs became more 'pointy' as in VLDs,and Eric Stecker muddied these waters a little by using the term COAL to refer not to SAAMI Coal,but rather the length from cartridge base to where the bullet ogive(curve on the bullet) touched the rifling.That is indeed the crucial measure-how much 'Pointy' bit sticks into the empty space of the barrel hardly matters (except for magazine users,of course),and thankfully the current top ballistic honcho at Berger,Bryan Litz,has specifically addressed this by the term 'Cartridge Base To Ogive"-CBTO.This is what we want-the measure of cartridge base to the point where the bullet ogive-where the curve ends and the now cylindrical part of the bullet- just touches the lands/rifling.It is very well established that close to this will be the most accurate seating depth,so if we know that,we can vary it,mainly by seating a little more into the case -we are talking a few thousands of an inch 'jump' here,normally less than 30 thou,often 5-10.Going into the lands lands a bit is 'jam',and usually nearer the smaller figure,as it increases pressure. Handloaders should of course know all this- we have to measure CBTO,and there are various ways to do so.

But herein lies the next set of issues.

When I read physics,I learned that if something can be measured,somebody will measure it

When I read Psycholgy,I learned that if a mistake can be made,somebody will make it.

From both,measuring instruments AND measurers vary-tolerance/individual differences.

So our desireable CBTO measure is achievable,but is vulnerable to variations in rifles,chambers,throats,bullets,measuring tools and human sensitivity....at least-we can't really expect uniform agreement-even even with the same batch of bullets,rifle model,and tools,including seater,shell holder,etc....let alone measurer(you!)

Good news,is that mostly,this does not matter-except when forum discussions forget it! What each handloader wants is the crucial starting datum for his rifle,chosen bullet,measured with his tools,by himself.I helps-as ever-to be as accurate as possible,but really it is repeatability that matters-repeatability of measures for that set of 'variables'.It does not matter if others get equally ' different measures for their set ups. Differences in length in same box bullets of the order 5 thou are not unusual-note that what is far more critical is that 'bullet base to ogive' has as low a variation as possible-good bullets probably do this-Berger specifically address this.Different measuring tools willl show variable tolerances-manufactuters want to keep costs down,generally,and probably know about the other variables...though some will be 'better' than others..there is also considerable anecdotal,but corroborative evidence,from women,that men's digital sensitivity varies!

Thee bottom line is that you can get a decent,certainly ueseable,CBTO measure,perhaps unique to your setup,maybe not what others get/advise;but it's what you need.Ypu can then seat your bullets to that CBTO,or use that as your known starting point (just touching the lands) to vary seating depth,as you fine tune your set up.

Of course,once you have your CBTO,or fine tuned,you have fixed the COAL too (subject only to variations-in bullet manufacturing tolerance,but that is just 'pointy bit' in the barrel.CBTO (or tuned) is the definitive measure. If your loads don't fit the mag-you at least know why,and increase seating depth til they do.You might reasonably expect quite close similarity to others datum measures,the more so the more variables are shared,but there will be variation,it does not generalise. It does not even have to be 'accurate',as long as repeatable."Measured size" matters,but only yours,since it fits your rifle optimally.

 

Gbal

 

ps 'hybrid' bullets help-the tangent ogive tip gives high BC,the secant ogive towards the 'lands touching' mid part of the bullet is far more tolerant of variation in seating depth-compared to VLDs eg-so a few thou difference in seating depth around this 'sweet spot' probably isn't too critical,even in one rifle.Seems a good win/win trade off between absolute precision,and realistic ease of achieving it!

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Basically gbal, I think you are saying what I said a few posts earlier - all we need is a reliable 'off the lands' starting point - the target tells us the rest.

 

We shouldn't get wound-up about actual COAL (or now CBTO) measurements - these will be peculiar to each shooter's rifle/cartridge/bullet combination.

 

If we have learned this, then that is a step forward and should hopefully do away with all those "Can anyone give me the overall length....................?" posts. Wanna bet?

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This thread has gone on longer than War and Peace. Did he die in the end?

That's a given,Les.

But did they do the math properly on his coffin?

 

G

ps Can I borrow your super duper new accurate comparator when it arrives?

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