Seating depth testing for .308 Win

[TQB]

Hi,



Saturday, I went to the range to test different seating depths.

As you might recall I am testing for a Sako TRG 22 in .308 Win. I used Lapua Scenar -L 175 gn with 42gn of N140, CCI BR-2 primers and Lapua brass. This combo gave me an ES of 10, a SD of 4 and an average velocity of 2673 fps.

The grouping at 100m was this.



I proceeded to load 5 shot groups of each seating depth with 0.003’’ increments. The rifle was cleaned previously, and 5 shots of factory ammo were fired as fouling shots. The first two 5 shot groups were at rifling touch point, and the remaining were in 0.003’’ increments.

Wind was very low, and the altitude of the range is between 50 to 70m above sea level.

These were the results.



The recorded velocities were:


What are your thought on this test? Should I choose -0.024, -0.036 or further test -0.039 before making a decision?



Thank You,
Tiago

 

[PigButtons]

If it were me I would test it in .001" increments from .020" to .029" and see how wide the node is. The .021" and the .024" are so close to the same point of impact that it looks promising in that range. Then pick the best one especially if it is close to the center of the node.

 

[richardca99]

Your POI is almost unchanging in groups 21 through 23. At the seating depth corresponding to group 22, this leaves you margin for error on both sides of the OAL. I’d stay there…

 

[Laguna Freak]

What was the seating depth on this first target posted?

 

[TQB]

What was the seating depth on this first target posted?View attachment 1292952

It was touching the rifling - CBTOL 2.311''. The same as the first 2 groups from the bottom left.

 

[rijndael]

I also favored the bottom left groups but I avoid just touching the lands.

I'd rather jam or get off the lands because it seems to be less impacted by throat erosion. I like loads that work for a long period of time.

 

[Laguna Freak]

I think all of these guys are offering excellent feedback. My concern is PoI on your initial target 20 is substantially different than targets 21 and 22 at the same seating depth.

Otherwise vertical dispersion is pretty consistently tight through -0.012” on target 16. It looks to be approaching that same PoI again at -0.036”. To me, -0.009” is sticking out like a sore thumb saying do a powder charge ladder test.

So, I like -0.006” and -0.012”. I would shoot 0.001” both sides of -0.006” and -0.012” to see if there is at least one 0.003” wide node.

If either or both of those results are satisfying, I would do a 300 yd powder charge ladder in 0.1 gr intervals, with the best of those two seating depth nodes.

I have experienced sub-half moa loads at 100 that do not hold together down range.

Reasonably, a .003” wide seating depth node with a 0.2 to 0.3 gr powder charge node that yields +/-half moa at 300 yd is a likely path to long range accuracy nirvana.

That said, I would also keep -0.036” and beyond in mind if I became concerned about throat erosion.

You didn’t say it but I suspect you had low sun angle when you started which resulted in chronograph failure to report all of target 21, and may have reported in error on some of the succeeding targets until sun angle got higher.

 

[Ned Ludd]

Is there any chance the group at -.009" was anomalous; i.e. shooter error? If so, it looks like you might have three decent seating depth windows. One is at "touching", which would normally not be my first choice, although it looks to be working very consistently for you. Just be aware if you choose "touching", you may need to alter seating depth regularly to maintain "touching" as the lands erode. If the group at -.009" was caused by something other than seating depth, then you might have a nice window from -.006" to -.012". For that choice, I'd seat them at -.006" to provide the the most headroom before land erosion necessitated further seating depth adjustment. Likewise, if the groups at -.027" and/or -.033" were due to something other than seating depth, then there may be a nice wide window from about -.024" all the way to -.036". This is the unfortunate part about seating depth testing; with just one group per seating depth, even a single misplaced shot can make the test much more difficult to interpret. I'd personally probably repeat the entire test, possibly using 3-shot groups to save a few loaded rounds. Because you already have one target with 5-shot groups, repeating the test with 3-shot groups will likely be sufficient to confirm/deny what look to be the potential optimal windows from the first test.

Something else I find curious is the your ES/SD values across the entire range of seating depths. It's not uncommon to observe reasonably small differences in velocity across such a test, but there is an 85 fps spread in average velocity across this test range. Any idea what might have caused that? It seems larger than I would have expected. If you want to toss the ES/SD values into the decision-making mix, it would tend to rule out looking at -.006" to -.012" further as those groups had among the highest ES/SD values within the test. Small sample size (n = 5) may be some part of that, but it seems like something more may be going on there.

Seating rounds right at "touching" can sometimes be problematic due to aberrant velocity ES/SD, in addition to having to "chase the lands" to maintain the bullet/land seating depth relationship. Otherwise, I'd say your CBTO 2.311" rounds look to be the most consistent overall. They do not appear to exhibit abnormal ES/SD values, and the groups are consistently tight.

 

[Kracken McGraw]

Great Posts!
I think I would start off at .005 off and just enjoy the ride for a long time.

 

[TQB]

Something else I find curious is the your ES/SD values across the entire range of seating depths. It's not uncommon to observe reasonably small differences in velocity across such a test, but there is an 85 fps spread in average velocity across this test range. Any idea what might have caused that? It seems larger than I would have expected. If you want to toss the ES/SD values into the decision-making mix, it would tend to rule out looking at -.006" to -.012" further as those groups had among the highest ES/SD values within the test. Small sample size (n = 5) may be some part of that, but it seems like something more may be going on there.

I will repeat some of these seating depths and do some in their vicinity.

Some of the larger groups might be attributed to barrel mirage.

The ES/SD variations might be due to brass with different number of firings. I noticed that at least 2 bullets were significantly to seat, but I was tired and did not segregate them for foulers…. I will anneal the brass (AMP if I can ask a friend to do me that favour or flame if I can’t). By the way, my chronograph is a caldwell, therefore, some variations might be attributed to that as well.