Do you retest seating depth when changing powder??

Does optimal seating depth stay the same with different powders or do you do complete work up again including seating depth test when switching to different propellant ??

If it matters, I'm thinking of trying different powder in 308 with 215 bergers and currently I'm running slight jam.

 

The short answer is yes.
When you change a component, your in-barrel dynamics change.
Any change will reflect on the results downrange.
If you want to achieve peak accuracy, you must satisfy yourself that all the steps available to you have been done to ensure optimum results.
Besides, if you don't, you will always have a question mark, a doubt every time you depress the trigger.

Cheers,

Phil aka tazzman

 

Yes.............. A week ago I loaded some 70gr Berger VLDs (.223) which I had been shooting with IMR 8208 XBR but I used VV-140 for this lot. I also loaded some 69gr Sierra Match King Moly bullets, also normally shot with IMR 8208 XBR, behind some VV-133. I did a full work up for both of them, discovered the charge weight which shot best, and then went home to make 10 rounds at each of 5 different seating depths.

Sunday I tested the various seating depths. The SMKs shot best at .005" jump as they have been doing with the IMR. However, the Berger VLDs, which had been seated at .005" jump with the IMR, proved to be more precise at a jump of .010" with the Vhitavuori powder. In other words, one bullet remained the same but the other one liked a different seating depth.

 

I do not anymore. Testing showed me that best seating (in a coarse sense) doesn't really change. It is what it is.
Fine tweaking of seating still 'shapes' grouping, but this is only up to a few thou either way, and does not produce big swings like full testing, unless coming off/into lands, or bridging a powder node.

The trick is finding best seating to begin.
I use a version of Berger's recommended(full seating testing). In this it's critical that you do the testing nowhere near any powder node. Otherwise, your seating adjustments can take you into 2 significant/different changes at once -masking actual results.
Keep in mind also that if you NEED a high starting pressure provided by jamming, for your load to perform(with some underbore, like a 30br), then best seating determined off the lands(OTL) may not be beneficial overall. In this case, might as well seat well into lands(ITL), powder develop, and then tweak seating to shape grouping.

If you take a shaping window into account it explains results like Mozella posted above(this is common). His best seating off-node may have been 8thou OTL. But in-node, best grouping shape could certainly occur +/- 3thou.

Some folks also get the notion that seating is a fine adjustment, but that's actually furthest from truth. Full seating testing does way more to grouping than powder. They see fine adjustment, because typically they didn't do full testing. Most pull a seating depth out of their butts -for powder testing. Then when they go to seating adjustments from within a powder node, it don't take a whole lot to collapse that powder node. So they end up accepting butt pulled seating +/- a smidge for 'best' :

 

mikecr - what do you mean by the above - can you elaborate a bit more. I mean specifically even if you do use a node charge, don't you hold charge weight constant and only change one variable i.e. seating depth?

 

Thanks for help guys. Any other views/personal experiences?

 

Could you provide a link to the instructions? (Specifically the section about doing testing nowhere near any powder node - I don't find that in Brian Litz' essay on CBTO, which is apparently not what you refer to.)