Measuring cartridge overall lentgh.

[Alpha1]

I am not happy with my recent measuring of cartridge overall length to many variables my engineering back ground makes me think there as to be a better way to do this.
Any ideas before I go off and start spending stupid amounts of money on measuring tools.

[rox]

I am not happy with my recent measuring of cartridge overall length to many variables my engineering back ground makes me think there as to be a better way to do this.
Any ideas before I go off and start spending stupid amounts of money on measuring tools.

Overall length is always going to be a little variable (well, maybe not if you use turned monolithics), but if you are interested in achieving (and therefore measuring) consistent *seating depth* and thereby consistent jump/jam then here are some options:

1) Calipers + Hornady/Sinclair comparator.
2) Calipers + SHED 2-part or 3-part comparator or Davidson 2-part [base].
3) 'Thimble' type, e.g. RCBS precision Mic (using the 'Land Nut').
4) Die/chamber & dial gauge type, e.g. Redding Instant Indicator, Dolphin Accuriser.

I switched from a Hornady to a SHED 2-part and observed a step-change in repeatability. I also have a precision Mic (3), and while I use this as my reference measurement for case headspace it is too much of a faff for measuring seating depth when other methods give equally good results much more quickly (the Precision Mic 'set' can be used to measure case headspace, bullet seating depth, and can also replace the 'Stoney Point' type gauge, but I prefer using a Stoney Point). The SHED and the Dolphin are the Rolls-Royce of their types (the Redding probably comes-in at 'Bentley'), although note that the Dolphin measures from shoulder datum to ogive whereas all the others measure from case head to ogive (if I'm not mistaken).

..

[John25]

I switched from Stoney Point to a Dolphin about four years ago which resulted in quantum leap in consistency. I haven’t tried a shed one because David’s came later than Mik’s which I have to confess was a pre-production model obtained at an excellent rate (if you discount all the beer!)

I believe that measuring to ogive produces a round much better suited to my chamber. Using my Dolphin also gives me an indication of throat wear and allows me to adjust length accordingly, until the point of no return when the throat is gone completely.

Only testing at the various ranges will show how much jump (or depth into the lands) will best suit and produce the best results.

Shed or Dolphin is a matter of choice but I would certainly recommend one or the other over yesterday’s technology.

[ovenpaa]

The Stoney Point/Hornady, Dolphin and SHED systems all effectively measure off the shoulders and this process is fine however I strongly believe the most accurate method is going to be by measuring from a fixed datum that is the equivalent of the bolt face which of course poses all sorts of issues given the variance from rifle to rifle let alone manufacturer to manufacturer.

[The Cupcake Kid]

The Stoney Point/Hornady, Dolphin and SHED systems all effectively measure off the shoulders and this process is fine however I strongly believe the most accurate method is going to be by measuring from a fixed datum that is the equivalent of the bolt face which of course poses all sorts of issues given the variance from rifle to rifle let alone manufacturer to manufacturer.

I absolutely see where you're coming from but ultimately the ACTUAL measurement from bolt face to lands doesn't need to be established in order to tune a load.

As long as the same measurement procedure is carried out each time with the same tools and the brass used in the rifle is dimensionally the same for every round, the actual measurement is irrelevant to most shooters.

As long as you have a method which will repeatably load cartridges so the bullet ogive ends up in the same place in the chamber every time, then you'll have the consistency you're after.

[ovenpaa]

I do feel that greater accuracy could be gained if the measurement datum was the bolt face, I have built a couple of gauges based on this however the designs is far from perfect. I will keep visiting the idea until I get it right.

[Alpha1]

In my limited experience I can see to many variables bullets distance to the ogive is different not uniform
OAL is not consistent with the overall length gauge. Different bullets of supposedly the same weight are not uniform in length.
I don't know how you guys can work out a accurate length unless you just take a average of your measurements.

[ovenpaa]

You have to work with averages in some areas such as OAL. I work on the basis that what I deem to the OAL may not be a strictly accurate measurement however if it gives me good repeatable results then all is good. The problem is when the throat moves slightly and the OAL has to be adjusted to maintain the optimum jump or jam.

Once the OAL is determined and built to you should be looking at bearing surface length and the relationship of the bearing surface to the base of the bullet. Both measurements are important and batching bullets will pay dividends with the vast majority of makes. SMK's are fairly patchy in this area, Lapua are better and even Berger benefit from being checked.

[Alpha1]

What's the difference between building to jam or jump.
Is building to jam a good idea.
When building to jump is there a rule of thumb as to a starting jump measurement.

[ovenpaa]

What make and weight of bullets?