Question about measuring length to ogive

[Smithcollector]

I recently bought a Hornady OAL gauge and am trying to measure and work up seating depths a little more precisely. I started with my AR with a 69 SMK and measured an average +/- 1.965 over several measurings. I then used a 80 SMK and got 1.985 average. So my question is: why does it matter which bullet I use, shouldn't the distance to where the ogive touches the chamber be the same no matter which bullet used? I realize OAL varies from by bullet weight, but the position of the ogive on the bullet shouldn't impact what I am measuring, right? How am I looking at this wrong?

 

[thejim2]

Re: Question about measuring length to ogive

My understading of ogive measuments is it changes (even if sometimes very little) from bullet to bullet, weather its from the same box, different weight, different brand, etc...

 

[Charlie Papa]

Re: Question about measuring length to ogive

If you're taking the measurements using a comparator, remember that the comparator's bore is not the same diameter as your barrel's bore, so the different shapes of the ogives will cause your measurements to vary. If you had a comparator that was an exact copy of your throat, you would get closer to the same numbers.

I hope this helps.

 

[doc76251]

Re: Question about measuring length to ogive

The reason you will get different ogive lengths is simply due to the differences in ogive on the boolet. Secant vs. tangent is the most obvious. If you wanted to REALY do it right you would have the comparator hole the same size as your seater base then you could adjust your seater w/o doing math. Unless the bullet tip hit first of course.

Ogive is an arbitrary number and has nothing to do with your bore or any other measurement which is why you want yo make sure you use the same insert for the same cartridge or at least boolet diameter. The closer you can get it to the major diameter of the boolet the better you will be also.

Cheers,

Doc

 

[Wannashootit]

Re: Question about measuring length to ogive

What you're measuring with the OAL gauge is the length where a <span style="font-style: italic">particular</span> bullet touches the lands.

Every bullet type is going to give a different length. In your example, you used two different bullet weight- so their length, and consequently the length of the point where they contact the lands is going to differ.

It's not just limited to different weight bullets. Shape is everything...

I was surprised to find a HUGE difference when comparing the <span style="font-weight: bold">same weight</span> bullet for our 6.5 Grendel, using identical weight 123 grain bullets. One was the 123 Amax, the other a 123 grain Nosler Custom Competition. The Hornady is a much "sleeker" bullet, with the ogive further from the bullet tip.
The Nosler is "stubbier"- more along the shape of a SMK.

Identical weight bullets, but two different lengths to seat them to the ogive.

The OAL gauge and Comparator only provide <span style="font-style: italic">relative</span> measurements for a specific bullet in a specific rifle.

I disagree that a different design would be any more accurate.
The diameter of the collar used for the measurement- whatever it is- is going to seat on any given bullet at the same location. Whether it seats higher or lower, than the exact point of the ogive is irrelevant. The point is that it will seat at the identical location on every bullet of that type.

You're determining the point where the ogive hits the lands with the OAL gauge- and locking the bullet into the case at that length. The collar just provides a point of reference to transfer that measurement to identical bullets. So, if I used a collar that was slightly larger in diameter, it would seat slightly further down on the bullet, and provide a shorter <span style="font-weight: bold"> relative</span> measurement.

 

[Huckleberry75]

Re: Question about measuring length to ogive

It also give a much more reliable number than measuring OAL off of the meplat.

 

[waste_knot]

Re: Question about measuring length to ogive

What diameter bore is in the comparitor. My 308 Hornady comparitor has a 0.300 bore which is the exact dia. of the rifle lands. It's also the smallest dimension in the whole barrel. The freebore will always taper down to this. This means the bullet secant will jam at this ring diameter. I don't see how this can vary. A VLD would just have a longer OAL but the distance to a point on the secant/tangent equal to the rifling should be the same.

 

[Smithcollector]

Re: Question about measuring length to ogive

Makes sense, many thanks