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”...
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 $$ Contributor
This has me debating whether I should buy an arbor press and die for seating. I always questioned their worth, that seems like a worthwhile reason.
depends on what you are trying to do.
one thing that i have found that effects consistent seating depth/neck tension and consistent shoulder bump is the changes in the brass over numerous firings. as it work hardens brass springback increases. many adjust their dies to compensate for this change. annealing, done right, puts the brass back to its original state and makes consistency of neck tension and shoulder bump much easier to accomplish.
no doubt using an arbor press and in line dies is the best way to get consistent bullet seating results but there are many other things like consistent neck thickness, neck concentricity, neck tension ..... that come into play here. when you get all the other variables right you can get great consistency with forster or whidden or some of the other threaded seating dies.
Ned Ludd
Silver $$ Contributor
Jody - If the issue you're having is with regard to consistent seating depth (i.e. CBTO) with variance in bullet base-to-ogive length (and therefore variance in effective internal case volume), you might think of it in terms of velocity variance during a seating depth test. How far in/out can you seat a bullet over a given charge weight before the change in effective case volume gives you a detectable change in pressure/velocity? It's probably a lot farther than you might think relative to bearing surface length variance. I typically start charge weight testing with the bullet seated in the middle of the range I will most likely test later when optimizing seating depth.
For example, for a tangent ogive bullet like the 185 Juggernaut, I'd start them at .015" off the lands. After identifying and selecting an optimized charge weight, I will generally start seating depth testing from .003" off the lands to .027" off the lands in .003" increments. Note that .015" off the lands is exactly in the middle of this range, meaning the bullet will only be moving .012", or half the total range, in either direction after the charge weight has been set. In my hands, this is not a sufficient change in internal volume to detectably alter velocity or push the load out of the charge window, and yet I would consider a .012" variance in bearing surface length in a given lot of bullets unusually high.
I think you'll find that slight differences in bullet BTO will not have a measurable effect on pressure/velocity. However, if it's a real concern, you can always sort bullets BTO.
For example, for a tangent ogive bullet like the 185 Juggernaut, I'd start them at .015" off the lands. After identifying and selecting an optimized charge weight, I will generally start seating depth testing from .003" off the lands to .027" off the lands in .003" increments. Note that .015" off the lands is exactly in the middle of this range, meaning the bullet will only be moving .012", or half the total range, in either direction after the charge weight has been set. In my hands, this is not a sufficient change in internal volume to detectably alter velocity or push the load out of the charge window, and yet I would consider a .012" variance in bearing surface length in a given lot of bullets unusually high.
I think you'll find that slight differences in bullet BTO will not have a measurable effect on pressure/velocity. However, if it's a real concern, you can always sort bullets BTO.
Take a bullet in a sized but unpowdered, unprimed case; seat the bullet at least 0.100 longer than your expected COAL (base to tip).
Carefully chamber the cartridge, smoothly close the bolt, smoothly open the bolt and very carefully extract the cartridge. Do this at least 3 times until you get 3 consecutive measurements within 0.002. Measure either CBTO or CBTT or both. This tells you the COAL or CBTO that touches the lands. Subtract the desired jump distance and start loading ammo.
Carefully chamber the cartridge, smoothly close the bolt, smoothly open the bolt and very carefully extract the cartridge. Do this at least 3 times until you get 3 consecutive measurements within 0.002. Measure either CBTO or CBTT or both. This tells you the COAL or CBTO that touches the lands. Subtract the desired jump distance and start loading ammo.
Are you describing the method I saw in Alex wheeler’s vid? Without the ejector and firing pin? If not, am I doing anything between bolt closures or will this bullet move in a sized neck?
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From Texas10.....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?[/QUOTE]
Texas10 is right on the money with neck tension variation. Using a screw in die some times a tight neck will not fully seat a bullet with a consistent stroke of the press. If you want the consistent seating depth, try using an expander mandrel (like a a k&m) not the one in your sizing die along with an arbor press and a Wilson seater. You will see a noticeable difference.
Darrin
Texas10 is right on the money with neck tension variation. Using a screw in die some times a tight neck will not fully seat a bullet with a consistent stroke of the press. If you want the consistent seating depth, try using an expander mandrel (like a a k&m) not the one in your sizing die along with an arbor press and a Wilson seater. You will see a noticeable difference.
Darrin
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