[Unclenick]

Not necessarily. They usually give a COL to stay in SAAMI spec and keep a good hold on the bullet. Your throat can be quite different from another fellow's and best accuracy seating depth in it can be quite different from what it is in that fellow's gun.


Wyoredman

I think what Mr. Stecker at Berger wrote is a bit confusing in its wording, but here is how I understand it:

When Mr. Stecker says a meaningful change is 0.002" to 0.005", I believe he means that is the limit of useful resolution of seating depth adjustments. In other words, that's the kind of round-to-round variation in ogive distance off the lands you normally find after seating bullets, so attempting to make adjustments any smaller for any purpose whatsoever is pointless.

I don’t think Stecker meant you would necessarily see a statistically significant change in precision (group size) from making an adjustment as small as 0.002” to 0.005”, but even if you can, he seems to think it’s beside the point. He says they’ve found seating depth sweet spots are at least 0.030" wide, so all you really need is to test at seating depth increments narrow enough not to skip over a sweet spot that wide. That's a 0.030” step.

I’m not sure how generally true the above is. It applies to their VLD long secant ogive bullets, obviously from their testing, but I’m not convinced a tangent ogive or a short ogive can be counted on to behave the same in all guns. I’ve seen 0.010” make a difference, but that was in guns already shooting cloverleafs, and as I’ve explained elsewhere, small improvements tend to be masked when applied to large groups.

It seems reasonable to me that once you locate a sweet spot you should use smaller increments of seating depth either side of it to find where it’s ends lay, so you can center your loads in the middle of it. That will give you some tolerance slop if you have length consistency issues. In some guns it may also yield a bit of further improvement, but no guarantees there. The bottom line, to my mind is, if you think you have a gun that would benefit from a really small change in seating depth, it still makes sense to use the Berger approach to narrow the search before investing in ammunition to do a load workup for each small change.

If you follow Berger's target recommendation to start by working a load up for best accuracy with a bullet jammed 0.010" into the lands, that’s a logical starting point. That’s because a load that’s safe there should be safe throughout your test range. It will see its highest pressure jammed into the lands, and as you seat it a little deeper in search of a sweet spot, the pressures with that same charge will tend to get lower until you get deeper into the case than you likely need to go. The Berger test range should not take you south of that point. The old Dr. Lloyd Brownell study has an example of a round nose bullet's peak pressures in .30-06 getting steadily lower as it is backed away from the lands until it is 0.250" deeper than touching the lands. In his case there was only about 10% more pressure when touching the lands than at minimum with a constant charge weight, so it's also an example of the bullet shape and chamber shape affecting the whole phenomenon.

Bottom line, if you first work up a best precision load with the bullet jammed 0.010” forward of the contact point, then that same charge will be safe through your trials seating deeper up to Berger's recommended 0.150" maximum distance off the lands. That makes it a good alternative to just using a standard minimum load from a manual.

But you still have another issue, and that is that as the pressure drops the velocity drops and the barrel time increases, and that can move you off a sweet spot for powder charge or move you onto one. Thus you wind up crediting seating depth with a change that could have been made just with powder charge adjustment alone. So, what's a fellow to do to try to get these two variables segregated?

The obvious thing is also the least practical. You seat the load deeper in four steps, as recommended, then go through a whole powder charge accuracy load work up for each step to see which does best. Then if you make a fine adjustment, maybe have to go through the process with the change yet again. I'm no more thrilled with that prospect than you likely are. It's a lot of work, but also, if you can't wrap your work up in a single day, you complicate life with different conditions and other factors you don't really need in the way.

A better idea seems to me to be to take advantage of your chronograph. Create an accuracy load with the bullet jammed in the lands, and get the mean velocity for that load. Now shoot at least three rounds at each of the other depths and see if you can see one of them tightening. Take the tightest one, or if there is no clear winner just pick the middle and adjust the charge up to match the velocity of the tuned load that was in the lands. Assuming that’s a bigger charge, take the difference and multiply it by the square root of 2 (1.4 is close enough) and add the result to the original tuned charge. This should come very close to matching the barrel time of the tuned charge. It should give you a comparison of the two seating depths alone.

So, let’s make an example. You load a .308 in a Winchester case, with the 175 grain MatchKing jammed 0.010” into the lands and find 41.4 grains of IMR4064 gives you best accuracy at a velocity of 2560 fps. You next seat a bullet 0.160” deeper, and find you now need 42.8 grains of 4064 to get to 2560 fps, matching the velocity of the first load. The difference is 1.5 grains. You multiply 1.5 grains by 1.4, giving you 2.1 grains, and add that to 41.4 grains. You now have 43.5 grains. This will be a little faster than the original velocity, but it should be about right to produce the same barrel time as the original load, so it should already be pretty well tuned. You can do the same at the other seating depths, and it should come out close enough that you get a pretty true comparison of the effect of the seating depths themselves. You can re-tune the final load to be sure, but you won’t likely find you need to go more than a couple of 0.7% steps either side of it.