This is a discussion on Using Hornady OAL Gauge & Modified Cases within the Ammunition forums, part of the M14 M1A Forum category; I just came across an interesting thread on The Firing Line:
http://thefiringline.com/forums/show...59#post5208259
Attempting to summarize, the Hornady OAL gauge measures the distance between the case ...

Attempting to summarize, the Hornady OAL gauge measures the distance between the case head of the modified case and the ogive (or meplat) of your chosen bullet. It does not take into account the fact that the headspace dimension of the modified case probably is different from the headspace dimension of your resized cases.

Please correct me if this is wrong:

You need to use the OAL gauge to get a measurement for COAL with the modified case, then measure the headspace dimension (shoulder location) on both the modified case and your typical resized case. Add or subtract that difference to the initial OAL measurement with the modified case.

The key data point we want is the distance from the shoulder of OUR case, not the modified case, to the ogive of our chosen bullet.

Whoa whoa, your getting ahead of yourself. I have one of these tools. It does not measure headspace. It measure the size of your chamber, this way you know how long you can seat your bullets. I'm seating my bullets in my bolt rifle 0.015" off the lands. Therefore my rounds are 0.015" shorter than my chamber.
What you do is put a bullet in the modified case, which is attached to gauge. You insert it into the chamber and push on the slide until you feel the bullet make contact with the lands. You then tighten a set screw on the gauge and remove the assemble out of your chamber. You then measure the length of the modified case and bullet. It's fairly simple. I'm not sure if I've done a very good job at trying to explain it or not.
If your wanting to check headspace, there's a different tool for that.

Good point, I had never given this a thought but it looks like this is a valid concern if your ogive to lands distance was critical and very close. I think that we get away with not having a problem for two reasons;

Our magazine limits the overall length to the point where the ogive is far enough off the lands that it really doesn't matter.

Hornady's standard modified case has a head space dimension that is close to what most of us size our cases to so the ogive ends up within a couple of thousandths of where we expect it to be.

I checked my modified case and the head space measures 1.627", I usually size my cases to have a headspace of 1.629" so the ogive will end up about 0.002" closer to the lands than I have assumed it would be. But, since my bullets don't any closer than about 0.025" to the lands, it doesn't really matter.

If I were a bench rest shooter and needed to be accurate to within 0.001" or so then I'd worry about it, but I don't have a need for that kind of accuracy.

I'm using my gauge in a custom R700. My chamber is on the minimum side of spec, so OAL is critical in my rifle. Us my best accuracy seems to be 0.015" off the lands. The Hornady gauge I have is for bolt rifles only. I'm not certain if they make them for other actions or not. But in a bolt rifle and loading for all the accuracy you can get, it's a very useful tool.

Good point, I had never given this a thought but it looks like this is a valid concern if your ogive to lands distance was critical and very close. I think that we get away with not having a problem for two reasons;

Our magazine limits the overall length to the point where the ogive is far enough off the lands that it really doesn't matter.

Hornady's standard modified case has a head space dimension that is close to what most of us size our cases to so the ogive ends up within a couple of thousandths of where we expect it to be.

I checked my modified case and the head space measures 1.627", I usually size my cases to have a headspace of 1.629" so the ogive will end up about 0.002" closer to the lands than I have assumed it would be. But, since my bullets don't any closer than about 0.025" to the lands, it doesn't really matter.

If I were a bench rest shooter and needed to be accurate to within 0.001" or so then I'd worry about it, but I don't have a need for that kind of accuracy.

What he said!
Interesting tidbit though... I can load a 168 Nosler CC to just .008" off the lands for a COAL of 2.845" and I THINK a base to give length of 2.226" and it still fits my mag W/ probably .010" to spare. I'm not at home and don't have access to my records, but I'm positive about the COAL length being 2.845". I guess I have a long mag, but it functions flawlessly with at least 700 match rounds through it. I never thought about the headspace difference until I read this. Good point! I guess its just not much of an issue for us. Even if we jammed a bullet into the lands a few thou the difference in pressure would be worth .5 to maybe 1 grains, but, due to the design limitations of our gas system, we load our match rounds a good bit lighter than published max loads. So, even if we jammed into the lands just a smidge we would still not likely suffer catastrophic failure.

Exactly. The measurement we really need is from the shoulder to the ogive. The Hornady tools together will get you that measurement.

Slayer - the Hornady OAL gauge does not measure your chamber's overall length. It allows you to measure the shoulder to ogive dimension. If the modified case is very close to your resized case in terms of the headspace dimension, then the OAL gauge works fine.

You make a good point about and cartridge short enough to fit in my magazines. In either of my M1As or my AR-15 you can't get the bullet close to the lands.

I drew up a picture to make sure I had it squared away myself. The Hornady OAL gauge measures the long dimension from the case head to the ogive (or meplat) of the bullet. (The 2.520" dimension in the drawing is only an example.) However, the critical dimension for the distance between the bullet's ogive and the lands is from the ogive to the shoulder. The shoulder stops the cartridge from moving any further forward in the chamber. Therefore, if your headspace dimension (case head to shoulder) is shorter than the modified case, then your bullet will be closer to the lands than you expect.

My guess is that Hornady deliberately uses the minimum headspace dimension for their modified cases. That way you err on the side of having the bullet's ogive farther away from the lands rather than closer/on the lands.

As others pointed out, this effectively makes very little difference in service rifles. However, it could make a difference in your bolt action rifle or in a rifle with a "match" type chamber.

Please note that I exaggerated a variance in the headspace dimension since .308 Winchester maximum is 1.634" with a variance down to 1.627" per SAAMI drawing.

Last edited by TheTinMan; September 5th, 2012 at 10:07 AM.

Attempting to summarize, the Hornady OAL gauge measures the distance between the case head of the modified case and the ogive (or meplat) of your chosen bullet. It does not take into account the fact that the headspace dimension of the modified case probably is different from the headspace dimension of your resized cases.

Please correct me if this is wrong:

You need to use the OAL gauge to get a measurement for COAL with the modified case, then measure the headspace dimension (shoulder location) on both the modified case and your typical resized case. Add or subtract that difference to the initial OAL measurement with the modified case.

The key data point we want is the distance from the shoulder of OUR case, not the modified case, to the ogive of our chosen bullet.

heres a demo of the tool... and a bonus of a different tool for the same job...