Seating depth test

0.003" ? Ridiculous increments. You'll be spending all your time testing if you go that small. I'd do more like 30 thou increments. And LARGE sample sizes. How did you determine your charge weight?

Rickyd1 said:

Hi all, I’m building a load for my 6.5 creedmoor. I have selected the charge to be 43.7 n555. Gives a low e/s and s/d. I’m now going to do different seating depths on increments of 3thou. I have started at jam - 20 thou. How many steps should I do?

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What bullet?

Here’s some light reading that might help. I have not done this, but maybe someone else here has.

Rickyd1 said:

Hi all, I’m building a load for my 6.5 creedmoor. I have selected the charge to be 43.7 n555. Gives a low e/s and s/d. I’m now going to do different seating depths on increments of 3thou. I have started at jam - 20 thou. How many steps should I do?

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I would run 10 3 shot groups and see what you get. Anywhere that looks promising can then be tested with a larger sample set.

The component companies thank you for doing seating depth test in 0.003 increments. Don’t forget to account for the throat eroding that much over 200 or so rounds

I wouldn't do .003 jump testing. By the time you "found" what you liked, your throat would have eroded from other testing to the point where you were no longer jumping from the same spot.

I do like Mark's test in the link posted above. I personally do .015", and then occasionally will refine a little more if I have a barrel that seems more picky about seating depth on a particular bullet. I've tested windows in seating depth before, and found that it requires you to really stay on top of measuring and playing with your dies...all for a very minimal gain. I'm much more of a fan these days of trying another bullet until I get something that isn't so picky.

The .003 testing is much more popular with those trying to compete in Benchrest or F-Class. The beanbag and barricade crowd definitely isn't going to benefit at all from such small increments.

Hickswr said:

The component companies thank you for doing seating depth test in 0.003 increments. Don’t forget to account for the throat eroding that much over 200 or so rounds

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Exactly. That PRB article is interesting. From reading it, it seems to me that there may be a seating depth for each different bullet where it’s most forgiving and that might not change with different guns? If that’s so, it would be awesome if it was published data for each different bullet as it would save us all a lot of time and money. Of course, the component companies wouldn’t want that…….

I like to do 10 shot groups. The incremental amount depends on which bullet. For instance, the Berger VLD's are known to like to be very close, very little jump. I use 0.003" jumps and have even done a set of 0.001" jumps with notable changes in group size. The Gamekings seem to prefer more jump so the increment is 0.010" and then testing in-between when I think i have found the spot. Once I do find a good enough depth, I don't chase the erosion. If accuracy really got real bad, I might, but I'd be more inclined to get a new barrel.

There are going to be folks on here saying that seating depth does not matter.

There will be those that try to tell you some statistics they do not understand means you have to shoot a proper sample size.

Here is the TLDR version from the link that @KYAggie posted above.


Here is an observation of my own in Mark's charted data. Notice how if you pick any one rifle and follow the line while watching the vertical scale, some guns are much tighter than others and stay that way for several steps of depths. Other guns jump up and down worse than the stock markets and it would be hard to pick a durable zone.

Based on the sum total of the chart, you might say that all of these guns shot pretty well and that you could just jump 0.020" and not worry about it at all. If anything, the context of a hunting or PRS rig where the ammo is all pre-loaded is one thing, a BR match where a relay is 5 targets of 5 shots plus sighters, but they can tune in real time between relays is very different than pre-loading.

There is nothing wrong with giving seating depth a try, other than the bbl life and resources spent on it. If the OP's rifle is for hunting or low volume shooting, then I would suggest he starts at jump 0.005" and takes it in steps of 0.005" to go seven or eight steps and see if it makes any real difference for him at all.

Here is an example of a match bbl in 6 Dasher with a Berger 105 Hybrid where the graph represents the vertical at 600 yards. Not counting the first two steps that were rediculously lucky where all three shots were touching at 600 yards, there are two places where the seating depth seems to be quiet. One zone is roughly 0.035" - 0.045" and the other at 0.060" to 0.080".

Keep in mind this is low sample testing of only three runs. Hard to say if these results would keep or if the verticals would spread out with more samples. The gun shoots just about anything into under 0.5 MOA at 600 for 20 shot strings. Bartlein bbl. chambered by Gary Eliseo.

mmmm....data

I find myself in the camp that doesn't think seating depth really matters. IMO, what matters is they all come out the same (all within a couple thou of each other when measured base-to-ogive).

I've tried many different methods and shot lots of rounds trying to discern if it makes a difference, for a long time believing the more jump the better (.080"-.100" off the lands), and then just as much time trying the other way (0.020" off or less)... In the end, the only thing I determined was that I'd be an idiot if I thought the monkey pulling the trigger was reliably consistent enough shot-to-shot to be sure my variations in seating depth were solely and irrefutably responsible for what I was seeing on targets.

Now I don't even measure or really care where my lands are, I just load all my rounds to the shortest length I can while keeping the bullet's bearing surface above the case's neck/shoulder junction (which is actually another thing I'm also not sure even matters), and my rounds always shoot (and always fit in my mags), every time, with every different barrel.

My average 100 yard group is .1 MIL/0.3MOA more or less on command... and whether I print a smaller or larger one probaly has more to do with how I slept the night before than my seating depth.

Over the years I have slowly come to the point where I think that seating depth is something that only benchrest shooters should care about. It is my opinion that rest of us simply don't have the gear, the skills, or the need to chase this particular dragon. I'd rather spend the money that it would take to perform real valid seating depth tests on each new barrel on better bullets, brass, and measuring tools.