CBTO variance issue

I'm perplexed, and could use some of the brain power here.

I randomly chose 5 bullets from a box of .308 SMK 175's. To my surprise, they all measured identical on the comparator.
But when I loaded these 5 bullets, I got 3 different CBTO measurements, a variance of .005 shortest to longest.
So my question is what else can cause this?
I'm using a Forster Ultra Seater die, once fired LC brass trimmed to 2.005, RCBS Rock Chucker press. I checked to see that the die was not loose.
The only thing I can come up with would be some variance in the brass as it sits in the shell holder. Would this be a viable explanation? Could something else be causing it?

Were you using the same comparator insert for both measurements?
Another thought might be primer not seated the same

I think the seater stem isnt cut for the smk's. Call RCBS and I bet they tell you to send a couple bullets and $30 and they will send you a new seater stem that has been built special for your bullets.

hmm interested to know what the outcome it....

Yes, the same comparator insert was used. Primer seating is consistent, but wouldn't matter as the primer does not come in contact with the shell holder.

RCBS won't help me with a Forster die. But it's good point. But still, I would think that the seater stem would hit the bullets in the same place each time.

Still perplexed.

I would contact the die maker and talk to them. I bet they will help you figure it out.

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Originally Posted by Texas Solo

I'm perplexed, and could use some of the brain power here.

I randomly chose 5 bullets from a box of .308 SMK 175's. To my surprise, they all measured identical on the comparator.
But when I loaded these 5 bullets, I got 3 different CBTO measurements, a variance of .005 shortest to longest.
So my question is what else can cause this?
I'm using a Forster Ultra Seater die, once fired LC brass trimmed to 2.005, RCBS Rock Chucker press. I checked to see that the die was not loose.
The only thing I can come up with would be some variance in the brass as it sits in the shell holder. Would this be a viable explanation? Could something else be causing it?

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It is a seating problem, mostly caused by the die. Sometime trimming the meplats on the bullets will straighten out this problem, as sometimes them ragged ends get caught up in the seating stem, and cause irregularities. Another indicator of seating depth change is running them on a bullet run out gauge. If the needle starts out at a different place the bullets is seated in a different spot.

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Originally Posted by seanhagerty

I would contact the die maker and talk to them. I bet they will help you figure it out.

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Just sent an e-mail to Forster. I'm not saying the problem is in the die, but maybe they can help figure it out.
My Forster Ultra seater die for my .223, loading Berger 80gr VLD's, is spot on to the 1/1000th. I don't even check them anymore.

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Originally Posted by 82boy

It is a seating problem, mostly caused by the die. Sometime trimming the meplats on the bullets will straighten out this problem, as sometimes them ragged ends get caught up in the seating stem, and cause irregularities. Another indicator of seating depth change is running them on a bullet run out gauge. If the needle starts out at a different place the bullets is seated in a different spot.

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I get your point, well received. However, The Forster die uses a seater plug which seats off the ogive, not the meplat (tip of bullet). One reason why I went with the Forster die.

I don't have a run out gauge, but I believe it would show as you describe, which would only serve to confirm that my lengths vary. I completely agree that it's a seating issue, as the bullets were exactly the same. I don't see how brass could cause the issue. I'm just trying to discover if there's something else that could cause this before I blame the die. Process of elimination.

Waiting to hear from Forster and welcoming further ideas.

Does your neck tension vary much? I have Forster Micrometer seaters too, and it seems that if the bullet is hard to seat, as in a lot of neck tension, that my lengths to ogive will vary slightly. Enough to show up on the runout gauge as 82 pointed out. I've often wondered about this too. Could it be the seater stem flexing slightly under load? That's all that makes sense to me. FWIW, my Redding Micrometer seater does the same thing in my .308. I wonder if the micrometer seaters have thinner stems, making them more prone to flexing? Total guess there.

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Originally Posted by barrel-nut

Does your neck tension vary much? I have Forster Micrometer seaters too, and it seems that if the bullet is hard to seat, as in a lot of neck tension, that my lengths to ogive will vary slightly. Enough to show up on the runout gauge as 82 pointed out. I've often wondered about this too. Could it be the seater stem flexing slightly under load? That's all that makes sense to me. FWIW, my Redding Micrometer seater does the same thing in my .308. I wonder if the micrometer seaters have thinner stems, making them more prone to flexing? Total guess there.

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Excellant thought, thanks. I just checked the neck tension on the 5 rounds, they're all the same at .002". But, that's AFTER playing with the die to get the CBTO the same on all of them. I'll have to load some more and check the neck tensions BEFORE adjusting the lengths to determine if there's an issue there. Thanks for the thought!

FWIW...I just re-read the set up instructions for the die. It says to seat them long, measure the CBTO, then re-seat as necessary. That's what I've been doing, but measuring EVERY round....twice....is a PITA.

Lube left inside the neck of cases can change the seating force and possibly cause a variance
Hope you get it figured out and report back
Jack

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Originally Posted by Texas Solo

I get your point, well received. However, The Forster die uses a seater plug which seats off the ogive, not the meplat (tip of bullet). One reason why I went with the Forster die.

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Well it sounds good, but trust me look a bit closer. My self and A friend both use Forster competition micrometer dies. I never had a problem as I trimmed my meplats, but my friend did, once he stated trimming meplats the problem went away. Yes it is suppose to grab the bullet by the ogive, but it done, and the tip will interfere with the seater plug, as he found out, and then he reworked his seater plug and problem went away.

One last thought where is the powder level in the case ? It could be that the powder is compressing as the bullet is seated, and the powder is compression or not compressing as much causing the issue.

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Originally Posted by 82boy

Well it sounds good, but trust me look a bit closer. My self and A friend both use Forster competition micrometer dies. I never had a problem as I trimmed my meplats, but my friend did, once he stated trimming meplats the problem went away. Yes it is suppose to grab the bullet by the ogive, but it done, and the tip will interfere with the seater plug, as he found out, and then he reworked his seater plug and problem went away.

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Good info, thanks. I'm waiting to hear from Forster, and I'm going to try loading some Nosler's and Hornady's. If the problem continues, I guess I'll try fixing the seater plug, as I'm too lazy to trim all the meplats :)

Not a compressed load....42.5 Varget

Just got this reply from Forster:

"Thanks for contacting us about this issue you seem to be having with the base to O-give measurement variation on you 308 loads. Your problem is very important to us.

The first thing to check would to make sure the bullet seating stem is coming in contact with the O-give of your bullets. Take that die apart at the knurled portion, separating the top of your die from the threaded sleeve. Then hold the bullets you are using up to the seating stem and make sure it is making contact all the way around the O-give of the bullet. If it is not we would have you send in the seating stem with some of the bullets you are using so we can polish it to fit better.


We try and make those stems be universal for all bullets, but with the vast amount that are out there, on the rarest of occasions we will see an instant that one wont match up as it should."

82Boy....looks like you nailed it. I'll take the die apart tonight and check it....running late for work :(