Seat depth, what is really being tuned?

First I'd like to say thanks for everyone's input, I'm just getting caught up. Secondly, My way of working up a load is picking a mid range load based on past experience, other shooters I know and trust or published data. Load up a rough seating depth test similar to Berger/Litz (I say rough because sometimes .005" can make a big difference). Find what seat depth it's seeming to like and load up my ladder test to identify what powder charges to further investigate. Once I get charge dialed in I circle back to seat depth. A lot of times I end up close to my initial seat depth but on a few occasions I've ended up moving .040" and once I ended up redoing the whole Berger/Litz test and nearly .140" from where I had previously thought was the sweet spot. Because of this I'm wondering if others have come back to seat depth after their initial assessment and found a different ideal seat depth? Playing with quickload just now for my newly rebarreled 300 WM and my current load data I'm seeing that moving the bullet .120" deeper into the cartridge causes a pressure change of about 3000 PSI which is similar to adding just over a grain of powder, similar barrel time and velocity by doing this. So I think where I may go with this is seat the bullet deeper, cut back on the charge by a little to maintain the same OBT and go see what happens. Now to try and keep things fair and even I'm going to check runout on each piece of brass and loaded round and only use rounds showing .002" TIR or better, and am leaning towards trying to keep the shorter ones at .001" TIR as larger runout will be amplified with the larger amount of jump. Thoughts on how to improve what I'm planning on testing? The current load shoots single digit SD's under 1/2" consistently, typically 1/4"-3/8" CTC so I'm not looking to improve group size or improve SD but maintain similar groups and SD's with a loaded round that will fit mag length so I'm not single feeding as this gun was built to be a 600 yard and in gun. I know I can switch bullets and accomplish this but I have a little time and like to tinker and this is the stuff that keeps me interested.
 
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Feel free to share what bullets your shooting and how much jump. If we get enough info I would compile it and see if any trends can be ascertained and then share it. If it's bullet dependent as far as how much jump they like then cartridge, powder, brass, primers, rifle shouldn't be much of a factor and some trends should arise. I shoot a lot of Bergers and have my data on them so maybe let's start there.
 
Perfect timing on this subject. I have been tuning a 300WSM with 200ELDX with RL-17. I can push it easily 2900 without ejector marks or bolt lift and even faster with some slight ejector marks. Played with jump right up to 0.010 and although it shot OK it really wasn't giving results that I was hoping for. So today I decided to try to match the hatch so to speak but matching up to the Hornady Precision Hunter load that is listed at 2820 and 2.860 which is SAAMI max length for the WSM. I bracketed the likely velocity of 2820 by 0.3 either side of the expected velocity to try to see if anything jumps out. So what do you think what happened? The load that averaged out at 2820 with ES of 12 and SD of 6 was basically through the same dang hole! I have spent a lot of time with loads anywhere 0.010 to 0.040 off the lands and obviously much longer than SAAMI. So here is a perfect example that a basic SAAMI dimension load was by far the most accurate out of my rifle with RL-17 and the 200ELDX.
 
dsculley said:
Some have said that Berger's don't like jump. In my 6.5x47L I found them best at 0.070" jump. In my 7mm RM I am jumping them 0.075". I am currently using RDF's in my 6.5x47 and jumping them 0.060".

I have not seen the Bryan Litz method for testing seating depth. I currently use the Berger method. How does the Litz method work?
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Absolutely, my 300wm shoots 185 Hybrid Targets best at about .095" jump, my 28 Nosler shoots 175gr Elite Hunter best at about .095" jump also.

Also, im shooting 105 RDF in a 6mm CM at .060" jump.
 
Being taught about reloading by a benchrest shooter many years ago, I always started at .010" off with all bullets. .050" was crazy talk. I had been shooting Barnes ttsx in a factory hunting rifle in 30-06 at .010" jump with 1.25 MOA type precision until I was training a newbie to load and he wanted to try the Barnes .050" recommendation. I though why not try it as well? Well I loaded a set at .010, .020, 030, 040 and .050 and wouldn't you know as I got further away they tightened up! .050" it is now for that particular set-up! Producing solid 3/4 MOA out to 400 yards. Not sure if this is an answer or not but someone may find something in there.
 
Buster Hemlock said:
How far off the lands were you to replicate the hornady precision hunter?
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Buster,
The 2.860 was 0.099 off the lands measured at the ogive which is not really a jump but more like a leap! Yes, that number is correct! I've had rifles that shot really well 0.045 of the lands but never this far. Factory accuracy versus reloader accuracy is the reason why we all push the bullets closer to take advantage of so called better accuracy potential. Once in a while you get another cold water shot on your head to open your eyes on accuracy potential being far off the lands.
 
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Got home at a decent time and got some ammo loaded for some initial pressure testing. Hopefully get out Saturday and do some shooting and then get back out Sunday to test the shorter seat depth loads. Another trip after that will be needed most likely to dial in the barrel time on the shorter rounds as the first attempt, hopefully Sunday, will be based on my best estimate using Quickload. I'll keep you gentlemen posted.
 
I've always wondered if part of the wide variation in the jump that different shooters find to be optimum is caused by the inconsistency in accurately measuring the location of the lands, by different people/methods.
 
Any tips on where to find the Brian Litz method for tuning jump? I'd rather not buy a book and my Google is failing me.
 
For what it's worth...
I've got a 7mm RMAG Red Hawk Rifle. 168gr Berger Hunting VLD's. COAL/CBTO definitely is a factor. My "proof?" Before I got into reloading I had a "pro" help me set it up. For the optimum load, we tested several different configurations on unfired Nosler brass. Saw large differences until we got the CBTO right. Now shoots 0.26MOA.

I offer the following: https://bergerbullets.com/effects-o...coal-and-cartridge-base-to-ogive-cbto-part-1/
 

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