Reloading some twice-fired brass this weekend for a couple of different rifles. I measured case length of about a dozen pieces and all were less than recommended trim-to length, so I didn’t trim anything. All were pretty dang consistent to the datum line, although not so much with overall case length. I loaded all to same CBTO (+/- .0005”), However.... had to do a lot of tinkering on each round with my Forster ultra micrometer seating die to obtain consistency with CBTO. Seems to me I don’t have consistent seating depths. Is one more important than the other? Seems in order to have consistent seating depth AND consistent CBTO, case length would have to be consistent first, right? Should I trim brass that is already less than recommended trim-to length?
CBTO vs seating depth
- Thread starter Jody Simcox
- Start date
Firstly, your case length has NO BEARING on CBTO, it is measured from case head to ogive of the bullet.
Secondly, unless bullets are from the same lot, they seldom measure EXACTLY the same from base to datum.
There are exemptions to this rule, however, Sierra match bullets often vary little if at all.
Cheers.
Secondly, unless bullets are from the same lot, they seldom measure EXACTLY the same from base to datum.
There are exemptions to this rule, however, Sierra match bullets often vary little if at all.
Cheers.
I think I got you question, but tell me if I'm wrong. You have consistent cartridge base to ogive(CBTO) coming out of your seating die, but you have different bullet base to ogive(BBTO) measurements on the bullets you load. So you know that your combustion volume is different even though you CBTO is the same.
I've had between lots of bullets a difference .054 in BBTO and I loaded them to the same CBTO. They seem to shoot the same. But I wonder if I was a more skilled shooter would I see a difference?
Pull your seating stem and see how well it matches up with the profile of the bullet. You may just need to use a different seating stem, one that fits better and pushes on the bullet more consistently. What is the un-tinkered with deviation in CBTO you are experiencing?
Case length (neck length) differences will indeed affect neck tension. How well are you holding case length from base to shoulder (datum) and how much are you setting back the shoulder when you FL size? Or are you neck sizing only the twice fired brass?
Case length (neck length) differences will indeed affect neck tension. How well are you holding case length from base to shoulder (datum) and how much are you setting back the shoulder when you FL size? Or are you neck sizing only the twice fired brass?
You guys are with me.... CBTO’s are now the same across all loaded rounds, but as I’m seating bullets, sometimes I had to dial down a little further than other times to get the correct CBTO, thus, I’m thinking some bullets are pushed down further into some cases than others (as much as .003-.008”). I always back off my seater die .010” between seatings, then work my way down to desired CBTO.
FL sizing annealed Lapua brass, shoulders getting pushed back.002”... Forster dies, 90gr Accubonds... I will check seating stem against bullet profile - thanks. Untinkered CBTO varies.003-.008”...
Remember that the seater contacts the bullets in a different location then the comparator.
Bullets can vary slightly. This has been covered on several threads as neck tension vary somewhat my seating depth will as well.
J
That's alot
Sorry... still very early in my reloading education and I don’t know all the acronyms. I just had a question I didn’t know the answer to. Wasn’t sure which was more important, the amount of bullet in the case neck or the CBTO, or if it really matters in my case. After all, these are basically just hunting rounds for Mississippi whitetails... I just really enjoy loading and showing my boys what the gun can do if we will try to do ours. Thanks
Normally, establishing tested best CBTO is most important to accuracy. Has your desired CBTO tested as best?
It would also help here if you knew whether or not you're actually seating bullet bearing deeper in necks. After all, if your basis for a notion here is a die setting -that means nothing.
It would also help here if you knew whether or not you're actually seating bullet bearing deeper in necks. After all, if your basis for a notion here is a die setting -that means nothing.
Neck tension definitely plays into the seating depth equation. I have found it extremely difficult to bump down a bullet a few thou. Neck tension combined with springiness in the shoulder conspire against small amounts of movement. You turn the micrometer down a couple of thou, nothing happens. You turn it more and something happens, but not what you expect. And the next one is WAY low in my experience.
One more thing, and maybe I missed your input on this, is your powder load compressed? IF so, seating depth is a crap shoot. Even if you get it right on, the bullet may slip out a bit after seating.
fatelvis
Silver $$ Contributor
This has me debating whether I should buy an arbor press and die for seating. I always questioned their worth, that seems like a worthwhile reason.
depends on what you are trying to do.
one thing that i have found that effects consistent seating depth/neck tension and consistent shoulder bump is the changes in the brass over numerous firings. as it work hardens brass springback increases. many adjust their dies to compensate for this change. annealing, done right, puts the brass back to its original state and makes consistency of neck tension and shoulder bump much easier to accomplish.
no doubt using an arbor press and in line dies is the best way to get consistent bullet seating results but there are many other things like consistent neck thickness, neck concentricity, neck tension ..... that come into play here. when you get all the other variables right you can get great consistency with forster or whidden or some of the other threaded seating dies.
IMO
Wilson makes the best tools bar non, starting with case gauges/ shoulder bump gauges and trimmers to in line seaters.
If Wilson doesn't make it you probably don't need it.
Shoot Small
J
Richard has a great outlook on this and has helped me several times. For me getting my brass in order has really paid dividends.
One thing that a guy can do is use a sharpie when seating- color any bullet and case that feels a little different then you can see if that's a flier or chambers different 'if so remove the brass for examination.
I had three cases I neck brushed out the Carbon this weekend and sure enough those three leaked out of a 1.75 group at 550
Not enough neck tension.
YRMV
J
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