Consistent CBTO?

[marchx]

I produced yesterday a new batch of ammo. Lapua brass fired 2 times, CCI BR4 primers, VVN 140 powder, 105gr BT Target. Brass sized using Forster bushing bump die, neck expanded using carbide 21st century mandrel.

I measured the CBTO: out of 30 , only 15 reloads have cbto 1.806-1.808 (ideally they should have 1.807). The rest has CBTO of 1.804-1805. How do you get consistent CBTO? By consistent I mean 1 thau of a difference.

 

[Straightshooter1]

I produced yesterday a new batch of ammo. Lapua brass fired 2 times, CCI BR4 primers, VVN 140 powder, 105gr BT Target. Brass sized using Forster bushing bump die, neck expanded using carbide 21st century mandrel.

I measured the CBTO: out of 30 , only 15 reloads have cbto 1.806-1.808 (ideally they should have 1.807). The rest has CBTO of 1.804-1805. How do you get consistent CBTO? By consistent I mean 1 thau of a difference.

Annealing . . . like after every firing. Then there's Dwell Time . . . when sizing, let the case sit in the die 5-7 seconds before removing to get consistent case measurement to shoulder datum. Consistent case trim length helps.

Also, making sure bullets are uniform in their BTO measurement and then making sure your getting consistent seating pressure.

 

[KMart]

Your seating stroke on the press must be the same each time. Seating quickly and bottoming out at the bottom of the stroke, then a smooth slower stroke on the next one barely bottoming out the next, will give you variation.
Measure your CBTO each time you seat.
Learn your measuring tool and make sure you are reading it correctly. You can get a couple of thousands variation just from a poor reading habits.

 

[Bill Norris]

I developed this technique for my seating. [When seating always use same speed and pressure to press handle thru the entire stroke and pause at bottom of stroke for dwell time. NEVER allow the press handle to bounce at bottom of stroke.]

 

[mmcu]

In addition to the above comments, be sure to sort your bullets.

 

[Bill Norris]

In addition to the above comments, be sure to sort your bullets.

Some bullets will vary as much as .003 difference when going from one lot to the next. Some will vary as much within the same box.

 

[DngBat7]

I would also try to get your hands on the bob green comparator. It would help these numbers. When using Berger’s, I get up to .001/.002 difference from bullet to bullet, from seating stem ogive to the larger ogive where it contacts the lands. This difference will show up in your cbto measurements

 

[gme]

some variation in CBTO is the result of the difference in where the seating stem in your die touches the bullet vs where your measuring device touches the bullet, it's also important that the tip of the bullet never bottoms out in the seating stem.

 

[marchx]

Thank you for all the comments.On the annealing question- I annealed my last batch of brass (induction, 2.6s for each case) and I had even more flyers than on the non-annealed batch. So, temporarily I lost my faith in annealing.
I use Forster Co-ax and my technique is simple- I lead the press handle with the same speed and force up to the bottom. I don't really see how can I improve my seating technique. On the measurement error- yes I think this is a major factor. I measure CBTO 3 times and check my zero on caliper in between. I use the hornady bullet comparator. I do not however measure ogive in Berger bullets.

 

[Straightshooter1]

some variation in CBTO is the result of the difference in where the seating stem in your die touches the bullet vs where your measuring device touches the bullet, it's also important that the tip of the bullet never bottoms out in the seating stem.

That's a good point, and why I now use a comparator insert that has the same diameter of my seating die to sort my bullets (if they happen to need sorting, like if the lot I have that shows more than a .002 spread). Quality bullets with consistent BTO's really helps.

 

[DngBat7]

some variation in CBTO is the result of the difference in where the seating stem in your die touches the bullet vs where your measuring device touches the bullet, it's also important that the tip of the bullet never bottoms out in the seating stem.

This is exactly what Bob Greens comparator measures for. GME explained this much better than me. You can seperate bullets into lots. Cbto will be much more accurate.

 

[marchx]

I would also try to get your hands on the bob green comparator. It would help these numbers. When using Berger’s, I get up to .001/.002 difference from bullet to bullet, from seating stem ogive to the larger ogive where it contacts the lands. This difference will show up in your cbto measurements

It's unlikely I can get this. Even if I could customs+VAT+shipping would make the price unbearable.

 

[Coyotefurharvester]

When I use a micrometer seater, they are all the same. First seating stroke, about .005 long of final cbto, measure, dial, finish seating. Otherwise you are at the standard variance of the lot of the bullets you are working with.

 

[marchx]

When I use a micrometer seater, they are all the same. First seating stroke, about .005 long of final cbto, measure, dial, finish seating. Otherwise you are at the standard variance of the lot of the bullets you are working with.

nicely done. I will try that.

 

[Skeptic123]

I would think your variances are due to neck tension differences and/or your seating technique. An arbor press seater will give you a much better feel for what is happening both re: neck tension and your technique

 

[comagutsa]

Sorry but I don't believe not annealing will cause this issue..
My cases I haven't been annealed for 12 reloads now on my 6BRX and I get 001 ± what I'm targeting for at most but most are spot on
I sort my Berger's from base to ogive in 001 lots and use a Wilson seating die in my K&N arbor press with indicator.

 

[chop house]

Some bullets will vary as much as .003 difference when going from one lot to the next. Some will vary as much within the same box.

i won't mention names, but i have seen a lot more than 0.003 in some pretty big name (NON-custom) brands. both between lots AND in single boxes.

i sorted some mass produced bullets and was still getting cbto variation. spent a half hour before i realized my problem was actually high primers. sheesh. i have a caliper mandrel from a member here that would have masked this problem. i was not using at the time, and in the end, since i was shooting bolt gun, y measurements served to flag my primer tool rather than my seater, stem, or neck tension.

 

[chop house]

by the way, some time ago @BoydAllen addressed this issue, and pointed out some top shooters 'bulk' load 'long' then measure cbto and touch up seating in batches to get spot on.

 

[mikecr]

Yeah, you never should have gotten past the first bad CBTO. Should have caught it, and addressed it, right there. Also, you need to choose: annealed, or un-annealed, load develop with what you choose.

There are lot to lot differences in bullet ogive radius, and as mentioned a Bob Green Comparator catches this.
BTO means nothing about anything.
Typically, the biggest influence to resultant CBTO is seating forces. While seating forces are excessive, or vary, seated CBTO will fight your intent.

With a light nylon brush through necks (for fresh carbon layer), and rational interference fit of seating bullets, you get lower more consistent seating forces. Then the seater plug wedges onto bullet nose more consistently, pushing the bullet more consistently to same point.

Seating is like shoulder bumping in that you're best to sneak into what you want. Never assume it,, always measure every single one right there & then.

 

[marchx]

I assume that the differences in neck tension/inconsistencies in brass itself AND ogive variation of bullets is enough to produce inconsistencies in seating depth.
What can we do without arbor press and wilson seating die?