Just how accuarate are OAL gauges?

[ovenpaa]

I have built a few OAL gauges of different types and they work very nicely however I was talking to ex-skydiver about this today and we both agreed they are still to vague as they tend to reference from the shoulders (Unless it is a rimmed case) So far from ideal as it is dependant on the case shoulder to chamber fit.

Time for something new. I have my idea, Ex-skydiver has his so I am going to scurry off to my man cave to have a play with the lathe.

So does anyone have a view on this or interested in knowing their true OAL?

[rox]

I have built a few OAL gauges of different types and they work very nicely however I was talking to ex-skydiver about this today and we both agreed they are still to vague as they tend to reference from the shoulders

If you are measuring the chamber in order to produce rounds with a particular jump/jam, isn't a measurement based on shoulder-datum to ogive the key dimension anyway?

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[ovenpaa]

If you are measuring the chamber in order to produce rounds with a particular jump/jam, isn't a measurement based on shoulder-datum to ogive the key dimension anyway?

For the vast majority of gauges it is exactly that which is fine as you have a very precise datum in the shoulder to work from. The one thing you are not controlling as well is the head to shoulder distance.

If you look at a Hornady OAL gauge as an example, you screw on a case, ideally fired through your rifle and shove it into the chamber, you then push a rod through until you feel the bullet stop moving as it meets the lands. Lock the rod up, remove everything and measure from the head to the ogive using a comparator insert or measure from the meplat.

Great, but that is very dependant on the head to shoulder dimension. which is going to vary depending on the case you used. The most accurate measurement has to be from the bolt face which pushes the cartridge in to the ogive when the bolt is closed.

My guess this dimension is going vary by a few thou from the traditional means of measuring OAL's with the possible exception of the slit case neck and bullet type method which from personal experience gives wildly varying results.

For many shooters the current methods are more than adequate but for the accuracy nuts it is a very different matter....

[Dougan]

I have built a few OAL gauges of different types and they work very nicely however I was talking to ex-skydiver about this today and we both agreed they are still to vague as they tend to reference from the shoulders

If you are measuring the chamber in order to produce rounds with a particular jump/jam, isn't a measurement based on shoulder-datum to ogive the key dimension anyway?

..

Yes, if it's a case shot in the rifle, surely there will be very little movement of the case in the chamber when fired - so the reference to the ogive is good... :?

I got the chance to try a couple of F/TRs on Saturday - One was a Barnard action, in .308, with a barrel with 3 groove rifling and a trigger better than my Anschutz The owner used an OAL gauge (he drills his own cases, like yourself) to work out his seating, and is confident he's got a 15 thou jump - It was too easy at 200 yrds (zeroing), and annoyed the guys down the butts, as they ran out of pins for the spotting disks

Anyway, I look forward to more details on this new idea
:geek:

[rox]

Great, but that is very dependant on the head to shoulder dimension. which is going to vary depending on the case you used. The most accurate measurement has to be from the bolt face which pushes the cartridge in to the ogive when the bolt is closed.

I'm unconvinced.

At the moment of ignition the case has been driven against the chamber shoulder by the firing pin (if not already held there by a plunger type ejector). As pressure builds the case expands and grips, stretching at the head to take up any excess headspace. Doesn't that make it all about the shoulder to ogive measurement? I still don't see why the head to ogive (or meplat) measurement is of interest. Of course, case headspace is interesting, but that's a different matter.


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[rox]

Great, but that is very dependant on the head to shoulder dimension. which is going to vary depending on the case you used. The most accurate measurement has to be from the bolt face which pushes the cartridge in to the ogive when the bolt is closed.

aaah - maybe I see now; things will be different if jamming is your bag (not jumping). Is that where you are coming from?

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[ovenpaa]

aaah - maybe I see now; things will be different if jamming is your bag (not jumping). Is that where you are coming from? .

It could apply for both jump and jam. I personaly spend a few minutes checking my build length every time I reload and even have white witness marks on my dies to visually show they have not moved. I am sure others do so as well. For me the first stage of an accurate load is get the charge right, second stage is OAL and I do wonder just how inaccurate my measurements have been.

It is easy to argue that as long as you have found the ideal OAL for your rifle why worry, but as the throat erodes and you remeasure and try to reproduce your favoured .008" jump or whatever I suspect inaccuracies will creep in hence the need to accurately determine the OAL. My new method would remove all doubt and introduce a new level of accuracy not usually enjoyed.

Lathification time I think!

[spud]

Dave you should be able to control the distance from case head to shoulder by using a stoney bump gauge. As for same seating depth once the throat has erooded, say you seated your tip out a little further to keep the same jump. In reality the pressure in the case will drop a little as the powder on ignition has more space to initialy start the firing process. Personally id say that if my throat was 10 thou erroded id probably just double check the velocities on a chrono as well. Some bullets will prefer different jumps at different speeds. The berger vld's like to touch to jam at certain speeds but drop the speeds by a couple of hundred fps and some will perform better with a slight jump. Ive had vlds that were tack drivers at 2890 in my 6.5 and at 2760 they were crap jammed unless you jumped them 10 thou. Another good reason i see for not mixing brass or different lot numbers of same brass.

[zeroveez]

Lathification time I think!

Well, what happened, or is it posted elsewhere?

[ovenpaa]

I am on around MkIII of my development and I am sure I will overcome the perceived issue at some point. Right now I am building an OAL gauge with a Dial Gauge to give a quicker reference between tested bullets.

It is a labour of love....