Difference in CBTO lengths w/diff. weight bullets

[ilmonster]

I finally got a Hornady OAL gauge and threaded cartridge. In figuring out the distance to the lands on my R700 in 6.5CM using a Hornady 140 ELD-M, it was 2.176". I then tested different CBTO lengths starting .010" off the lands. Went fine.

Now I am using the gauge to determine the CBTO of Hornady 123 ELD-M. It is averaging 2.194". That's almost .020" more than the same measurement for the 140 ELD's. Does this make sense? At 2.194", it is still shorter than the length of the P-Mag, so they will fit my magazine (the R700 is in a KRG Bravo stock so I can use DBM's). Just wanted to make sure I'm not missing something. Thanks!

 

[TheOfficeT-Rex]

It's expected for different bullets to have a different CBTO to the lands, because the ogive shape and position are not the same. You're not missing anything, keep going with your loading.

 

[ilmonster]

That's what I thought. I know that two bullets of different weights from the same manufacturer will have slightly different shapes. I just wasn't sure if it could add up to .020". I took five or six measurements from 2.193 to 2.196 with three at 2.194". I will probably again start at .010" off the lands and five more each .010" off from the last, ending .060" off the lands.

 

[Chargerguy]

I found using this method much more repeatable. My Hornady OAL tool now sits and collects dust. YMMV.

 

[Shootin Stuff]

It's expected for different bullets to have a different CBTO to the lands, because the ogive shape and position are not the same. You're not missing anything, keep going with your loading.

^^^^^ Wot he sed.

Your comparator does not sit ON the ogive/bearing surface junction, just somewhere near it and different ogive shapes give different CBTO readings.

Also, if you measure CBTO with several comparators you will likely get differing numbers as well. All comparators are very slightly dimensionally different and once you have one, keep using the same one.

 

[Chargerguy]

Good point and very true. I have 2 Hornady .284 comparators and they both read significantly different. I made sure to mark them and record which one I used for CBTO measurements. This is also why sharing CBTO measurements with someone else, with different measuring devices, even if the same brand, is kind of pointless. And that’s where COAL should be used.

 

[ilmonster]

One other question. When I tried a first loading for the 123 ELDM using Hornady's 10th edition manual and their suggested 2.71" COAL, I measured a CBTO of 2.096" when the COAL was 2.71". My rifles max. CBTO is 2.194" which even when setting the bullet back .060" off the lands still give a CBTO of 2.134" - a lot longer than manual calculates out to. Maybe I'll start with the bullet .020" off the lands and end .070" off? Thoughts?

 

[Shootin Stuff]

One other question. When I tried a first loading for the 123 ELDM using Hornady's 10th edition manual and their suggested 2.71" COAL, I measured a CBTO of 2.096". My rifles max. CBTO is 2.194" which even when setting the bullet back .060" off the lands still give a CBTO of 2.134" - a lot longer than manual calculates out to. Maybe I'll start with the bullet .020" off the lands and end .070" off? Thoughts?

If I understand you correctly and the longer than book length will still feed from your mags then yes, it is ok to load longer than what the book says.

 

[ilmonster]

Yes, that length will fit in my P-Mags.

 

[Shootin Stuff]

Yes, that length will fit in my P-Mags.

Then you should be good to go.

 

[ilmonster]

thanks for the help guys! I'm going out to the range today and shoot some 5 round groups with the test rounds.

 

[Shootin Stuff]

Make sure to post groups.

 

[ilmonster]

Here are the groups (5 shots each at 100 yds) from yesterday... Looks like group #5 which is .030" off the lands might be it (2.194" as the max CBTO touching lands). Of course, if I feel like more work, I could now try .025", .030" and .035", or just load up some .030" and try and work on technique.

 

[Shootin Stuff]

Are you sure you didn’t pull the flyer on #5, that would be a very good group otherwise.

Personally I would load up a bunch seated long and take your seating press and clamp to the range with you. Try re seating to small increments either side of .030 and see how wide the window of consistency is.

Once you are happy then re seat the rest to the longer side of your window (throat erosion) and spend the remainder of the day banging steel on fundamentals and doping out those new eldm’s way out there.

Of course if it is a windy day and not worth shooting groups then just re seat them all to .030 when you get there and go work on fundamentals and wind calls.

 

[ilmonster]

Thanks all for the responses. The flyer on group #5 could certainly be due to my inconsistency. It wasn't a called flyer, but I'd chalk it up to me. I'd like to load some up at the range as suggested, but my press is a full size Lee Classic Cast. Not exactly portable. I'll get some more loaded before the weekend and hopefully get back out there to shoot some groups.

 

[ilmonster]

Well, I tried two additional groups today, one at .025" and one at .035", and neither grouped as well as .030" off the lands. I'll be sticking with that. Now, time to work on the shooter.

 

[Shootin Stuff]

Sounds like you’ve found your number, was your new .030 group as tight as the first?