Measuring .308 cartridge overall lentgh

[Alpha1]

98% of my rifles are military calibres so precision re loading is some thing I don't normally do.
I have a .308 rifle that I am going to re load for. I have bought 100 Lapau cases trimmed them all to a uniform length and cut the primer pockets.
I have drilled and tapped one of the cases to fit the stoney point gauge. I have purchased a comparator body and a .308 insert.
I am using 168grain Sierra match king bullets. So I have mounted the case on the gauge inserted a bullet and pushed it in to the chamber and pushed the bullet up until it engages the rifling. I have measured this length.

So what do I do next how does the comparator body and insert fit in to the equation.
What do I need to do with the measurement and is there a figure that you use to back it off the rifling.

[ovenpaa]

The comparator should be machined to suit the rifle so typically between two parameters, I have spent the last year or so diligently researching rifle bores by make and measuring samples when I get a chance so I have a good idea of what comparator bore to match to barrel however in the absence of data it should be around .009" to .011" less than the bullet.

This should give a reasonably repeatable method of measuring from the cartridge head to the ogive. I size mine to match up where the bullet would be engraved by the lands. This measurement unfortunately has no bearing on the overall length (OAL) of the loaded round as it is bullet dependant so you will need to firstly make sure the bullet is a snug fit in the cartridge neck (I prefer to use brass that has been fired and neck sized twice and then polish the inside of the neck and finally push a mandrel down the neck to ensure a good fit with minimal sideways movement of the bullet.

Now check the OAL 5 times with a bullet and record, repeat this with a total of 5 bullets and take the average length Meplat to head and the Ogive to head. You will now have an average OAL and OOAL (Ogive OAL) Keep a note of these two measurements for future builds. If you change bullet make and/or weight repeat the process and if you really want to keep on top of the game check from batch to batch.

One final thing. The bullet base to bearing surface upper length will change by anything up to .015" bullet to bullet dependant on make and that has a HUGE impact on vertical dispersions.

[tackb]

Load to OAL as per your reloading manual and crack on !

Then you can start messing with jump to lands if you need to ?

You might be surprised/satisfyied with the level of accuracy you can achieve without spending hours rubbing powdered unicorn horn on your bullets in a darkened room !

[Alpha1]

I have loaded to over all length as per the reloading manuals manufacturers data this rifle shoots tight groups with factory ammo it shoots even better with my re loads I am wanting to take the next step. But I have never re loaded to F class or Bench rest standard for instance but this rifle deserves it so I am going to have a go.
I am not compromising on this one. It will get the best ammo I can build and it will only be the best components I can afford.

Any way back on track thanks for your reply ovenpaa but I think I am going to have to re read it a couple of times because to be honest it sort of lost me.

Lee Enfield's are a lot less complicated.

[ovenpaa]

If you are already seeing tight groups the next step is to fine tune the overall length as it may be the rifle/bullet combination works best. What bullets are you using?

[tackb]

ok, right , sorry i got the impression you were new to reloading and to be honest quite often begginers start to worry about things that are a bit further down the line than they need to ?

I've found the 168 smk to be fairly length tolerant like the amax but i would start at 20 thou off the lands and then try going either way 5 thou at a time , shoot your groups and record your results ?

[The Cupcake Kid]

One small point which might help prevent confusion later...

The overall length of the cartridge measured from the tip of the bullet is the "OAL" but when you use your comparator and measure from the ogive, this is going to be a different (shorter) measurement and I find it helpful to use "COAL" (comparator overall length) when I record or refer to that measurement. It would probably also help if we used those terms in this thread too.

The OAL really isn't particularly relevant in accuracy and reloading terms, other than when you need a figure to enter into Quickload software (if you use it) or to check that your handloads will feed from a magazine, if you're using one.

The COAL figure is the one I always look at and use to work out where the bullet is in relation to the rifling when the cartridge is sitting in the chamber. It sounds like you have a good starting point in that the published data for your load is giving you good accuracy, but using that data and bearing in mind that production rifles usually have a longer throat than a custom chambered rifle, your bullet may be having to jump a fair old way before contacting the rifling right now and you might see an improvement if you can reduce that jump.

It's easy to lose track of all these numbers, so once I've got the OAL and COAL measurements for a particular barrel/bullet/cartridge, I enter the figures in a spreadsheet which is set up to automatically calculate some fixed 'jump & jam' values so I can then easily read off the measurements I need when I'm handloading. It looks like this for one particular load I have:

0.03” Jam 0.02” Jam 0.01” Jam Hard seated 0.01” Jump 0.02” Jump 0.03” Jump 0.04” Jump 0.05” Jump

OAL 2.883 2.873 2.863 2.853 2.843 2.833 2.823 2.813 2.803
COAL 2.302 2.292 2.282 2.272 2.262 2.252 2.242 2.232 2.222

(the formatting is a bit out once copied into here, but you should get the idea)

As Ovenpaa has said, these measurements will change from bullet to bullet and even from one batch of bullets to another. They will also change as the chamber throat erodes so you need to keep checking the COAL measurement for your favorite loads when anything changes or every few hundred rounds through a particular barrel.

[Alpha1]

My current load is a 155grain Hornady A max with 42.3grains of vitavouri N140.

For this exercise I am using a 168grain Sierra match kings with 100 new Lapua cases. Powder to be determined later.
Does any body have any preferences for the 168grain match kings powder wise in .308.

Do you use excel for your spread sheet if so what sort of calculations are you using.

I am still not sure of the procedure for using the OAL/COAG lets see f I have got this right.
I put my drilled cases on the stoney point gauge insert my 168g bullet.
Insert the gauge and push the bullet in to the rifling. Take a measurement and this is my OAL.
Then I make a dummy round to this measurement and measure the round using the comparator with the .308 insert.
This measurement is my COAL. Is this correct so far.

[The Cupcake Kid]

My current load is a 155grain Hornady A max with 42.3grains of vitavouri N140.

For this exercise I am using a 168grain Sierra match kings with 100 new Lapua cases. Powder to be determined later.
Does any body have any preferences for the 168grain match kings powder wise in .308.

Do you use excel for your spread sheet if so what sort of calculations are you using.

I am still not sure of the procedure for using the OAL/COAG lets see f I have got this right.
I put my drilled cases on the stoney point gauge insert my 168g bullet.
Insert the gauge and push the bullet in to the rifling. Take a measurement and this is my OAL.
Then I make a dummy round to this measurement and measure the round using the comparator with the .308 insert.
This measurement is my COAL. Is this correct so far.

You don't need to make the dummy round to measure the COAL. Just measure it directly off the stoney point gauge. (I'm assuming your comparator fits onto dial calipers, if not then your way is OK but could lead to small inconsistencies).

But yes, correct so far. Now this measurement you have is the COAL of a round where the bullet is just touching the rifling. From this, you can calculate longer or shorter COALs in order to try different jumps or jams (shorter means the bullet will have to jump, longer and the bullet will start 'jammed' into the rifling).

Yes, I'm using Excel and the formulas are pretty easy: i.e.
=SUM(E41+0.01), =SUM(E41+0.02) etc. to calculate LONGER COALs in 10 thou' increments
=SUM(E41-0.01), =SUM(E41-0.02) etc. to calculate SHORTER COALs in 10 thou' increments
Where E41 in my spreadsheet is the COAL you measured with the stoney point gauge.

If you try 41.0gn N140 with the 168SMK, you'll be starting at approx. the same pressure as 42.3gn N140 with a 155, but that's a very rough calculation on my part not knowing your actual OALs, barrel length etc. etc. so as always, start lower and work up if you don't have anything else to go on.

[tackb]

my 308 loves 46g of varget with either 168g matchking or my fav the 168 amax .

as ever , work up to these loads !