OAL all over the place?

[mikeystew]

you shouldnt really even need a comparitor for accuracy because the seating stem in the die seats them off the ogive anyway, resulting in the same jump to the lands. i like to loosen a collet die and partially size the neck just enough that the bullet can be started by hand with moderate pressure and then chamber it in the gun. the bolt will seat the bullet so you measure and record that OAL, remove and use that very same bullet to seat in your first charged case but seat it .010 less than the recorded OAL for a rather short jump to the lands. keep the seating die locked down and load them all. the OAL's may measure slightly differently but the base to ogive should be the same.
more important than comparing lengths is weighing bullets but thats another topic altogether...

 

[Wissem70]

Interesting thread. So if you have one of these gauges how do you then proceed to do anything different in terms of seating on your press to the spec of say 20 thou off the lands for every round? Does this comparator gauge mimic the spec of the lands? Or grooves. Is the bullet diameter not the same as groove diameter? Is the spec for the distance to the lands standard for each caliber? Where is that data? Or how do you go about determining this for a specific rifle if not? Seems to me that those little caliber inserts should be made to go in the seating die....

Each gun is different in terms of where each bullet would hit said lands. First you determine the length to the lands with the desired bullet using the hornady OAL gauge http://www.hornady.com/store/OAL-Gauges/ with modified case for your caliber http://www.hornady.com/store/Modified-Cases/ and comparator set for your caliber http://www.hornady.com/store/Bullet-Comparator-Kits/ .
And experiment from there. Using the tip of plastic tipped bullets gives you too much difference compared to measuring from the ogive. I use a home made comparator but I constantly use my Hornady OAL guage. Between cousins and buddies we have accumulated a few modified cases of different calibers.

 

[snomad]

Mikeystew, that makes sense. Why would one bother with all the other stuff?
If you were to check all of the rounds loaded the way you describe with a comparator they should all be the same. Have you done this? I wonder that if the bullets are that much out that the accuracy is going to comprimised anyway? And then you mention the bullet weights....

 

[snomad]

Another question here, when you look at reloading charts like Hodgedon's, they give different COL's for different slug weights. This has got to relate to seating distance to lands in the end but as discussed here it would be innaccurate using COL to reference that if the bullets themselves are varying. Or am I assuming something wrong here?

 

[mikeystew]

Another question here, when you look at reloading charts like Hodgedon's, they give different COL's for different slug weights. This has got to relate to seating distance to lands in the end but as discussed here it would be innaccurate using COL to reference that if the bullets themselves are varying. Or am I assuming something wrong here?

i don't measure them with the comparitor often because again, the seating stem touches the same ogive area as my comparitor so if it was off that would indicate a faulty die stem if anything... i usually only bother to measure the first 2 loaded rounds. first for COL to make sure it's correct, and the second to make sure it's the same as the first.
but this is a perfect example why it's best to determine the jump from ogive to the lands by yourself and not from the book. measuring the OAL of all the cartridges is frustrating and inaccurate. the method i described in post #21 is a quick and easy way to find the sweet spot.
take my .303 brit for example, when loaded with a 125 grain bullet, a .010" jump to the lands results in the bullet being just barely seated in the case about 1/8" and is .050 off the max OAL for the cartridge... just does not seem right and is not factory spec, but it will shoot 1.25" groups with peepsights at 100m. when seated to the factory spec im lucky to get 2.5"
measuring OAL should only be done with the exact bullet you are using to determine the jump, and seating in the first charged case. once the die is set they will all be the same, but may have variances in COL... the base to ogive will be identical.