varience measureing to ogive ?
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Ned Ludd
Silver $$ Contributor
What bullets are you using? A certain amount of seating depth variance as determined by cartridge base-to-ogive (CBTO) can be caused by variance in bullet external dimensions. Sorting bullets base-to-ogive with calipers into length groups, or sorting using a tool such as Bob Green's comparator will largely solve this problem.
Alternatively, a certain amount of seating depth variance is not unexpected, even when using length-sorted bullets. However, it should be more on the order of +/- .001" or maybe .002", not .009". I use a target CBTO range for loaded rounds of .0005". For example, I might want an "acceptable" CBTO range of 2.2000" to 2.1995" for a loaded round. I set the die micrometer such that most (70-90%) of the loaded rounds will measure exactly 2.2000" with a single pull of the press handle. The few that might be out of the "acceptable" range will actually be long by half a thousandth (i.e. 2.2005") as measured by the calipers. It usually only takes another pull of the press handle (i.e. an extra "tap") to bring them into the desired seating depth range. This usually prevents any rounds from ever being seated deeper than intended. However, it does require that you measure each loaded round, which I do.
Another potential source of seating depth variance is non-uniform neck tension. Are you annealing your brass every firing? It can make a difference. More often than not, you can "feel" rounds that end up seated longer than desired through the press handle; they seem to seat harder, with a bit more effort, suggesting greater than average neck tension. A seating die that measures seating force is one way to quantify this issue, but potentially comes at the price of having to buy additional equipment. I usually set aside rounds that "feel" like they seated noticeably harder than the others as foulers/sighters, even if they seat within my chosen acceptable CBTO range, as they are likely to have greater neck tension, which can affect velocity and precision.
The bottom line is that .009" CBTO variance is pretty high. Identifying the underlying cause is the most expedient way to solve this issue. However, even if you can't identify a specific cause, you can usually improve the seating depth variance using some version of the method(s) I described above. In a worse case scenario, you may actually have to re-adjust the die micrometer for rounds that end up longer than desired. That is a process I find to be a PITA, but again, necessary if you want consistent very seating depth - something I feel is essential for good precision. As I mentioned, you have to measure every loaded round to be sure. As long as none are shorter than the desired CBTO, you can give them another tap in the seating die, or even adjust the mic, if necessary, to get them where you want them. But in order to do that, you need to know the CBTO measurement. It does take a little more time, but having measured every seated round for years, it is now habit for me to use the calipers as I seat each bullet.
Alternatively, a certain amount of seating depth variance is not unexpected, even when using length-sorted bullets. However, it should be more on the order of +/- .001" or maybe .002", not .009". I use a target CBTO range for loaded rounds of .0005". For example, I might want an "acceptable" CBTO range of 2.2000" to 2.1995" for a loaded round. I set the die micrometer such that most (70-90%) of the loaded rounds will measure exactly 2.2000" with a single pull of the press handle. The few that might be out of the "acceptable" range will actually be long by half a thousandth (i.e. 2.2005") as measured by the calipers. It usually only takes another pull of the press handle (i.e. an extra "tap") to bring them into the desired seating depth range. This usually prevents any rounds from ever being seated deeper than intended. However, it does require that you measure each loaded round, which I do.
Another potential source of seating depth variance is non-uniform neck tension. Are you annealing your brass every firing? It can make a difference. More often than not, you can "feel" rounds that end up seated longer than desired through the press handle; they seem to seat harder, with a bit more effort, suggesting greater than average neck tension. A seating die that measures seating force is one way to quantify this issue, but potentially comes at the price of having to buy additional equipment. I usually set aside rounds that "feel" like they seated noticeably harder than the others as foulers/sighters, even if they seat within my chosen acceptable CBTO range, as they are likely to have greater neck tension, which can affect velocity and precision.
The bottom line is that .009" CBTO variance is pretty high. Identifying the underlying cause is the most expedient way to solve this issue. However, even if you can't identify a specific cause, you can usually improve the seating depth variance using some version of the method(s) I described above. In a worse case scenario, you may actually have to re-adjust the die micrometer for rounds that end up longer than desired. That is a process I find to be a PITA, but again, necessary if you want consistent very seating depth - something I feel is essential for good precision. As I mentioned, you have to measure every loaded round to be sure. As long as none are shorter than the desired CBTO, you can give them another tap in the seating die, or even adjust the mic, if necessary, to get them where you want them. But in order to do that, you need to know the CBTO measurement. It does take a little more time, but having measured every seated round for years, it is now habit for me to use the calipers as I seat each bullet.
Jarhead6870
Gold $$ Contributor
As mentioned above, neck tension could be the culprit, however it can be related to other minor steps in the process. If you clean your brass using SS pins or an ultrasonic cleaner, you may be getting the necks too clean causing friction and inconsistent seating because of metal to metal contact. Using a dry lubricant will help solve this problem. If you are using a compressed load, this too can cause depth issues; if this is the case, using a drop funnel can help
A cheap and quick method to measure or test your other results is using a split case. Resize a case, then cut 4 slots into the shoulder using a Dremmel cut-off wheel. Insert the bullet in leaving it stick out more than normal. (it should be snug but loose enough that you can turn it with your fingers).= Very carefully chamber the round and extract it not allowing it to bump anything. You should be able to get readings within about .001" after a few tries. Pull the bullet out, squeeze the neck with your fingers to tighten it and reseat the bullet again. Measure it 8-10 times. No matter what method you use, the readings should be consistent.
RegionRat
Gold $$ Contributor
Poor chamfer (can lead to galling)?
Poor sizing (can make seating forces variable)?
Annealing (work hardening can cause seating force variations)?
Galling caused by cleaning process (tribological variations cause immediate seating force changes)?
Compressed load (directly causes seating force variations and bullet tilts)?
Poor seating caused by press stroke or stiffness (do you cam over or how do you describe your sizing and seating press stroke)?
I probably missed a few other common causes, but think about these up front.
What would your comments be on your brass prep (have you changed anything recently)?
How do you feel about your sizing process (what type of neck sizing are you using)?
Is the sizing die, or seating die, or anything else new in the process?
Was your process ever checked before and found to be good, or is this the first time you have checked?
Please describe the numbers and stats, i.e., I seated 20 and found 10 way out, or I seated 100 and found 2 way out.
Poor sizing (can make seating forces variable)?
Annealing (work hardening can cause seating force variations)?
Galling caused by cleaning process (tribological variations cause immediate seating force changes)?
Compressed load (directly causes seating force variations and bullet tilts)?
Poor seating caused by press stroke or stiffness (do you cam over or how do you describe your sizing and seating press stroke)?
I probably missed a few other common causes, but think about these up front.
What would your comments be on your brass prep (have you changed anything recently)?
How do you feel about your sizing process (what type of neck sizing are you using)?
Is the sizing die, or seating die, or anything else new in the process?
Was your process ever checked before and found to be good, or is this the first time you have checked?
Please describe the numbers and stats, i.e., I seated 20 and found 10 way out, or I seated 100 and found 2 way out.
I may have screwed up. I was thinking the distance from the lands to the ogive/bearing surface interface. These bullets were a couple from the same lot but measure quite different.
I use a micrometer die too but also check every round with the tool shown below and try to keep them to within .001". My Lee press can vary the seating distance depending on how hard it is bottomed out. I try to seat it easily and measure, then a bit harder to move the bullet a thousandth or two deeper if necessary.
The bullets above were measured from both ends by this tool whicl is .010" smaller than the bullet diameter.
Dusty Stevens
Shiner
You got bullet nose variance woes. Your seating die pushes in a different spot on the ogive than youre measuring.
rwj
Gold $$ Contributor
The target will tell you. If you can’t shoot/see the difference, why worry?
B23
Gold $$ Contributor
True, but where the bullet contacts the lands is more near where a bullet comparator measures, isn't it?
Ned Ludd
Silver $$ Contributor
The simplest answer to this question is that you want it to be substantially less than whatever increment you use in a seating depth test. For example, I generally test seating depth in .003" increments, so I try to keep CBTO variance to .001" or less. If the variance is greater than one increment in a seating depth test, then you really have no idea what seating depths you're actually testing.
Bear in mind that if you have a nice wide seating depth window, or if your accuracy/precision requirements aren't that severe, you might not notice .009" CBTO variance very much. I've had lots of .308 Win FGMM 175 Match ammo that had around .008" to .009" CBTO variance and it shot extremely well in one of my rifles. Nonetheless, one of the advantages of preparing handloads is that we can minimize the total variance for quite a number of variables that may additively contribute to poor precision. For that reason alone, I'd try to rectify the .009" CBTO variance, starting by measuring bullets BTO and OAL to see whether sorting into length groups will largely solve your problem. If so, doing that and tweaking your press micrometer settings may be all that is required, which would be a relatively painless fix.
Dusty Stevens
Shiner
MikeMcCasland
Team Texas F-T/R
69gr SMK would make me think an AR15. You seating these to 2.26 and compressing powder with these loads?
If I'm using 'comp' seaters with compressed loads, I'll get some variation. I've never seen .009, but I've seen more than I normally would.
If I'm using 'comp' seaters with compressed loads, I'll get some variation. I've never seen .009, but I've seen more than I normally would.
I am heading to range tomorrow so I went and checked all my loaded ammo. I had 75 rounds of .223 77gn SMKs and BTO measurements were plus or minus .004, after reseating they were plus or minus .001. The thirty rounds of 6.0 CM's using 107 SMK's were mostly plus or minus .001 with a couple way out there at + .009 so I reseated those also. I have played with seating enough to know a few thousandths can make a big difference on paper so I just added a new step to my range pre checks. Thanks OP I had never checked my rounds that were loaded a week or so before the matches, now I will
My rifle is a Tikka T3x 223 rem
BountyHunter
Gold $$ Contributor
First measure the bullets for variance and then seat. IF the bullets are all the same, then it is not the bullets causing the variance. However, I have seen cases of bullets in one box varying by .017.
If I were shooting a bolt gun, it would seem to me that the critical measurement on the bullet would be the bearing surface length. That is what interfaces with your barrel and is responsible for pressure and velocity variations. Sorting your bullets to a uniform bearing surface length and then pointing the bullet would be all I would be interested in. The slight difference in bullet wt. doesn't seem to make any difference on the target.
Set the neck tension up light enough to hold a bullet, and seat the bullet 20-30 thousands long. Let the rifle seat the bullets as the bolt closes. Seating depth and OAL can't get any more consistent than that.
As your barrel throat wears, you never have to chase the changing dimension as your loaded rounds will always be seated uniformly.
Once I have established a known good load, if a barrel won't shoot well using the above procedure, it is not a good barrel and I move on to another. So far, the above has worked in four different calibers and at least three to four barrels on each rifle.
Bob
Set the neck tension up light enough to hold a bullet, and seat the bullet 20-30 thousands long. Let the rifle seat the bullets as the bolt closes. Seating depth and OAL can't get any more consistent than that.
As your barrel throat wears, you never have to chase the changing dimension as your loaded rounds will always be seated uniformly.
Once I have established a known good load, if a barrel won't shoot well using the above procedure, it is not a good barrel and I move on to another. So far, the above has worked in four different calibers and at least three to four barrels on each rifle.
Bob
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