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Measuring distance to lands

So a friend and I were experimenting with the Hornady bullet comparator and OAL gauge. I would think that using the OAL gauge (with the modified case and a bullet) and measuring it with the bullet comparator would give you the distance to the lands in your chamber. I would think this measurement would be consistent from bullet-to-bullet since it is a measurement of your chamber itself... However, this measurement changes if you use two different bullets in the same chamber. We verified this numerous times to be sure and made sure we were not pressing the bullet too hard (engraving it) or too soft (not touching the rifling).

My only theory of why the measurement changes is that the Hornady bullet comparator does not actually measure the part of the ogive that touches the lands - it must measure some other point of the ogive geometry.

Can anyone confirm this or offer another explanation as to why this measurement changes with these tools?
 
Josh,

What you are experiencing is an actual difference in dimension in the bullets themselves. Until we learn different, we ASSUME that all the bullets we pull out of a box are exactly the same.

This is NOT true.

When you measure the distance to the lands, you need to use the same bullet each time. So the bullet you are using, should be marked and kept for just that purpose.

If you made the measurements with five different bullets from the box, odds are that you would get five different readings.

Try taking a sharpie and coloring the major diameter of the bullet at the ogive. That way it makes it easier to see when the rifling begins to touch the bullet.

HTHs.

Bob
 
Josh, I have noticed the same thing. As I measure different bullets (different brand, bullet weight, etc.) I get different lengths as well. It was confusing to me when I first was using it. I came to the same conclusion as you - the comparator ID is slightly smaller (or larger?) than the lands of my rifle and therefore on a different part of the ogive.
 
Josh, you are correct. The land contact for 6mm's is either .236" or .237" but my bullet comparator measures .233". With that said, my 105 Hybrids and 105 Amax bullets have a slightly different CBTO to lands, a .004" difference.
 
Nomad, I have been dealing with the inconsistencies for a while and it caused me grief at a recent match!!! The reading I now take is the one that I can reproduce 4 times. I have tried fresh resized case with new bullet, I have tried resized case with neck expanded with mandrel and new bullet, have tried the hornady oal tool, removing firing pin, soft vs hard close of the bolt, etc. I found the tension on the case varies a tad to not reproduce same readings in a row although very close, I also noticed that using the same bullet a few times with this method also varied a little and got better consistency using a fresh bullet each time but you need to make sure the bullets measure the same from base to ogive with whatever tool you decide to use for measuring.
My most recent method was to take the bolt out, use a fired/resized case that has been expanded with a mandrel and I pick 4-5 bullets out of the box that measure the same from base to ogive. I place the bullet in the neck, guide the case into the chamber and then I use a wooden dowel and a small jeweler hammer or bullet remover type hammer and tap the dowel against the primer pocket slowly pushing the case into the chamber. The case shoulder can only go so far so the next step is for the bullet to softly seat into the case. The dowel and hammer make a very distinct sound and the feel is different once the case and bullet reach the maximum length and touch the lands. After that I take another dowel form the muzzle end and carefully tap the case/bullet out.
Using this method I was able to reproduce the same measurement 4 times in a row with each of the 4-5 bullets and all measurements were the same. It was very consistent and a lot easier to do.

Someone mentioned bullets being different hence the different reading you are getting. This variance isn't only in a box of 100-500 but it can be very different between lot numbers. I am using 7mm 180 hybrids and from one lot to another the base to ogive measurements have been as far as 6-7 thousandths apart between lots. This noted difference has made me determine a base number (ogive to land contact point) with each lot of bullets because if you trust the lots, your seating depths could be way off !
 
take the two bullets, and use the oal caliper and measure the ogive length to base of the 2......

keep a bullet in the tool box ans always ues the same bullet.

it is a reference number not a true measurement.
 
That reference will only be good for that lot of bullets assuming there isn't a lot of variation. 20 thou jump with that bullet might not be the same for the bullets in a different lot which is why it's best to establish a reference point with each lot.
 
I just sort bullets (same lot) by beating surface. Then I grab 5 random bullets. Use the comparator tool(with cleaning rod through muzzle pushing gently against the bullet to help feel the lands) and measure each one 5 times. Then I will take the avg reading of that and use as my touch.

When I take 5 readings with same bullet I don't have any more than 0005 variance.

You get different readings with different weight and style of bullets because each bullet is made differently and have different ogive shapes and bearing surface lengths.
 
I don't consider myself as an expert, and I can tell only about my experience;

rooky mistake (myself!!)
I measured the distance to fine tune my load
I used 1 bullet and wrote down the measurements that gave me a 2.458 length. As the test resulted in JAM -.014 would be the best setup I archived that value = 2.414

It worked pretty good, and one day "out of the blue" 1.7 inch groups and pressure signs (hard bolt action)
You can imagine my surprise and I was wondering and thinking what did go wrong, and worse thoughts .... from mistakes in my load to damaged rifle......

here is the deal, I got new bullets same brand and lot number. However after a lot of troubleshooting I used instead of the marked bulled for my measurement one of the new bullets and here my surprise the JAM length was 2.415

So the result of this was; now if I open up a new box I take out 10 bullets and measure ten times the length.
In my experience I have a number that comes up 5-7 times in the measurement. That is my JAM number or touch lense. Btw. I have +- 0.0005 difference in the measurements.

maybe I'm wrong but it helped me....
 
The tool measures the same place from bolt face to lands regardless of bullet used, the problem is that the place where their comparator measures the bullet is not the same place as where the bullet contacts the lands, therefore transferring inconsistencies into the final measurement.
 
Then you have to take into account that the throat erodes over time and that causes your seating depth to change and you have to start all over again.
 
Note that with bottleneck cases headspacing on their shoulder, it's the case shoulder that's the reference to use for distance to the bullet ogive point that first touches the rifling. And that point on the bullet will be at least bore diameter.

Any spread in case headspace (head to shoulder reference) will be transferred to where the bullet's contact point is in the chamber. With all bullets seated to the same head to bullet dimension, the bullet's jump to the rifling distance will have the same spread as case headspace.
 
FWIW:
I have started using the SINCLAIR INSERT STYLE BULLET COMPARATOR.
They are stainless steel rather than aluminum. I have found that they measure different bullets to the lands most of the time the same for when using the Hornady gauge.
They won't work for deeply seated bullets oftentimes because they slip over the bullet a ways.
 
I mentioned this in another thread not too long ago, but I verified my Hornady tool was accurate by polishing a bullet and seating it long and chambering the round so I could see the rifling marks. Then I polished the bullet and seated further into the case in fine increments and checked the rifling marks. Kept repeating this method until the rifling marks just went away. Then polished the bullet and seated in 001 increments until I just started to see the rifling marks. This is what I called my touch and wouldn't you know it was within 001" of what my Hornady tool was telling me.
 

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