Why not the same?

[coldtrail]

I use a Hornady oal case gauge to measure different bullet types in my rifle and get different results. I would think that if the ogive of the bullet is stopping when it's diameter matches the lands diameter it shouldn't matter what bullet you measure with. The oal should be the same regardless of bullet shape. When the diameters meet, it's always the same diameters touching, right? I have been told by a knowledgable reloader that is just the way it is and accept it but I can't resolve the conflict in my mind. So, what's the explanation?:banghead:

 

[NeuseRvrRat]

your question is very confusing to me. not sure what you're asking.

you will need to use the COAL gauge for each type of bullet you're loading. "ogive" simply refers to the curved surface of the bullet, as opposed to the "shank", which is the cylindrical portion, and the "meplat", which is the point or tip. bullet comparators use a specific diameter that will lay somewhere along the ogive. the location where the ogive diameter is this size will vary from bullet type to bullet type. some bullets have a secant ogive and others, mainly spitzers, have a tangential ogive.

your results from the COAL gauge are normal. i put a bullet in the modified case and put the gauge in the rifle and push the bullet in until it contacts the lands. lock down the gauge, remove it, and use the appropriate comparator to measure off the ogive. when i do this again with another type of bullet, i get a different measurement. the reason is the second bullet type has a different ogive radius than the first bullet type. no big deal. it takes all of 5 minutes to do the measuring.

 

[jr_roosa]

It's because the throat and rifling of your rifle's chamber is one shape and the hole in the comparator is another (just a hole with a sharpish corner to it).

Basically your comparator measures to the point where the bullet is about .300" or so (if you look closely at most bullet comparators, the edge that touches your bullet is radiused a little). The beginning of the rifling in your gun probably has a bit of a taper to it, so a long skinny ogive will stick in little deeper and a short fat one less so.

If you had somebody make you a comparator that had the same shape as the beginning of the rifling in your barrel, including whatever throat erosion you have, then you'd find that the numbers matched really well and would stay consistent from bullet to bullet. Otherwise, you need to check each style of bullet you use in your chamber and then read the seating depth with your comparator, just like you're doing.

-J

 

[coldtrail]

Here is where I am having trouble with the procedure. First, I know how the gauge works and what is happening inside the chamber. Next, the diameter of the comparator, allowing for a radius or no, and the inside diameter of the lands, allowing for taper or no, are constants as is the bullet ogive diameter. So, if I take any bullet of the proper caliber it should stop at that point where the diameter on the ogive, whether secant or tangent ogive, contacts the same inside diameter of the lands. Shape of bullet should have nothing to do with it. When you take the bullet and measure it in the comparator, which is a constant and not necessarily touching the ogive in the same manner as the lands, there should be no difference in the measurement result from bullet to bullet type. It's diameter of bullet, constant, to diameter of lands, constant, to diameter of comparator, another constant.

For instance, if for the sake of example, you had a bullet that was of proper diameter for the bore but was a foot long. The ogive would touch the lands when the diameters were equal. You might have eleven inches of bullet in the bore but the distance where the ogive touchs to the base of the gauge case should be the same as for any other bullet. Or, if the ogive were such that the bullet contacts the lands an inch form the tip of the bullet then, allowing that you can't really have eleven inches of imaginery bullet in the case, the measurement should from the case base to the point of contact be the same. Even though the taper of the ogive is different from bullet type to bullet type the diameter at contact is the diameter at contact. The comparator, regardless of whether it has a radius or not is a constant and gives you a comparative measurement to adjust your seating depth with.

I know the reality is different than my thinking but it just bugs me that they are not all the same. I will continue to use the measurement as taken for each bullet in adjusting my reloading but will still ponder why the difference.

 

[helotaxi]

You're assuming that the datum diameter on the comparator is the exact same as the diameter of the lands of your barrel. It isn't. That is why the shape of the ogive matters and why every bullet is going to be different.

 

[Joe's]

coldtrail,
Be sure and check your seater die stem profile. Sounds like your trying to seat a lead tip (SP) bullet and it's hitting on the point instead of the ogive. What style of bullet are you trying to load and what brand of seater are you using.

Joe's

 

[Blue68f100]

Look at a round nose bullet then look at a sharp pencil. If you take a set of mic and measure the point of a set dia on each (just for the kicks of use 1/8"), it will be a different distance from the top of the two. A different reading on your comparator, too. So in order to get the same distance off the land you will need to change your seating depth. The pencil point can have a very long OAL and may not even fit the magazine, where the round nose will be very short.

And like Joe said, the seating dye will have a different profile too. Which makes no difference if you adj for the different bullets.

 

[coldtrail]

the diameter of the comparator is a constant regardless of it's shape. Even if it were slightly larger or smaller than the diameter of the lands it would give you a measurement to compare (comparator) your eventual seating depth to.

The type of seating die for the bullet is inconsequential to how I arrive at the eventual seating depth. You adjust according to final oal desired.

From the base of the gauge case to that point where the diameters of the lands and bullet are the same should always be the same regardless of bullet shape. I reject reality and insert my own. However I will adjust accordingly when reloading.

 

[NeuseRvrRat]

From the base of the gauge case to that point where the diameters of the lands and bullet are the same should always be the same regardless of bullet shape.

so, what you're saying is your experience and the experiences of multiple other handloaders using the gauge does not jive with your theory on how you think it should work, but you still think your theory is correct?

you made a thread asking a question, multiple members answered your question and tried to explain it to you, but then you have the audacity to tell us we're wrong?

good luck with your loads

 

[coldtrail]

What I am saying, Neuservrrat, is that no one has explained it to me in a way that supercedes my impression of how it should work. And my loads work just fine, thank you. Audacity has nothing to do with it.

 

[NeuseRvrRat]

imagine if the bullet in this pic had a much larger ogive radius. the spot on the ogive that is the same diameter as the lands would be much closer to the lands. that would mean this imaginary bullet would not allow you to push the gauge in as far.

 

[Dobe66]

Given that the case length is fixed, the distance from case base to begining of the freebore throat is fixed, the diameter at the lands is fixed, the diameter of the comparator is fixed, and the freebore throat lead is fixed, I have yet to read a response that explanes why any bullet's distance from the base of the case to the bullet's ogive (at the point equal to the comparator's diameter) should be different. Granted that in NeuseRvrRat's example the increased radius wouldn't allow the bullet to go in as far, and it's overall length would therefore be shorter, but it doesn't explain why the distance between the case base and it's ogive contact point has changed. The one variable is the relative shape of the comparator "hole" and that of the lands - which may contain milling lead. This appears to be somewhat of a streatch to explain the amount of differences in actual measured distances. That being said, empirical data appears to contradict theory so the mystery remains.

 

[coldtrail]

Ditto, Dobe66

 

[NeuseRvrRat]

as was explained earlier, the diameter of the hole in the comparator is different than the diameter of the rifling where it engages the bullet. each one of these diameters will occur somewhere on the ogive of the bullet. the distance between these 2 diameters will vary based on the ogive radius of the bullet.

it's no mystery.

 

[Gillie Dog]

I’ll take a shot at it.

The diameter of which the bullet comparator touches the bullet is different than the diameter of the lands the bullet touched with the OAL gauge.

Depending on the shape of the bullet the off set will be different so measured OAL will be different.

Below is a depiction. Say the left line is where the bullet touches the lands and right hand line is where the comparator hit the bullet. The diameter of touching the lands is the same on both bullets. The diameter of the comparator touching the bullet is the same on both bullets. But the measured AOL is different by 14.2 thousandths. On the round nose the distance is small because of how steep the angle is. On the other the distance is large because the angle where measured is much flatter. There are several things to cause this: shape of bullet, hole size in comparator, radius on hole of comparator, etc. But it does not matter as long as you use the OAL gauge and same comparator for each bullet measured you will always know the accurate length to the lands. And thus will always know what actual jump is.

Thus you must use the individual OAL gauge results for each individual bullet or your jump will be different and not quantifiable.

That is my best shot at answering what I think the question is.

GD

 

[coldtrail]

At last the light comes on. A simple enough explanation. Thanks for the responses.