Variation in seating depth

Where the cone of the seating stem touches the bullet is not the same as where you're measuring BTO, so they don't necessarily have any corelation to one another. Not much you can do about this, normally. Technically there are some options but they are kind of off in the weeds. Usually 'better bullets' or 'does it actually matter on your target?' are the go-to solutions here.

Also, I have seen instances where large variations in seating force resulted in measurable differences in seating depth - that *did* correlate with the seating force. Best to sort out the seating force issues first if that is the case.

Maybe. Helpful, I know

The seating stem is still going to be a smaller diameter than the bullet diameter, which is basically where the comparator is contacting. The reason you don't usually see manufacturers making the seater stem that big is the beveled cone area starts getting awful thin, and it starts belling out under heavy seating forces.

The other thing that sometimes happens is that the seating stem cone isn't deep enough, and bullets with long pointy tips - like VLDs, A-tips, or maybe some of the polymer tips - actually bottom out on the tip before they ever touch the ogive. Take the seating stem out, turn it over and drop a bullet into it. If it sits up relatively straight, that's probably *not* your issue. If it tips over to one side and you can see a gap around the rim of the seating cone... It likely *is* the problem.



Sometimes you can just order a new 'VLD' seating stem. Other times, you have to redneck diy it by taking a sacrificial bullet, coat the nose in something like JB bore paste, chuck it in a drill and lap the seater stem to fit. Or order a better seating die.

UndFrm said:

What could be the possible reasons for the variations in CBOL (ogive) length.
Considering that the total case length is the same, the projectile base to ogive length has been sorted to be the same, but yet after seating the bullet, there's variations in base to ogive length from cartridge to cartridge.

Using a Lee turrent press as single stage, but I do ensure that the cases are going as up in the die as possible, to remove any wiggle room there.

Thank you

Click to expand...

Various things will effect your CBTO (Cartridge Base to Ogive) measurement:

1. There are variations in bullet's BTO dimensions within a particular lot; more so with some brands of bullets than others, and always between lots of bullets. This is why some reloaders will sort their bullets to get better uniformity.

2. Variation in the neck tension results in variation in seating depth, which makes the CBTO measurement vary, of course. That variation in neck tension can come due to variations in neck wall thickness and/or variations in hardness of the neck walls due to work hardening the sizing process does to them.

3. Variation in the interference between the bullet and the interior of the neck wall (some may have more friction than others, even if the "neck tension" is the same). Depending on how case necks are cleaned and/or lubed, this can mitigate this kind of variance.

4. Play in the linkage of the press that's doing the seating can induce variance in seating depth.

5. Difference between where your comparator touches the ogive and where the bullet seating pin touches the ogive can vary, which will contribute to producing differences in CBTO.

Some of these things will have minimal effect and other can have great effect. All together, they can produce substantial differences. How little difference you want will dictate how you much attention you give any of these.