Finding the right seating depth

[Tim Singleton]

I've commented on a couple posts recently about load development and have talked about how important it is to find the best seating depth for the bullet and barrel you are using.
This is not a method I came up with and struggled myself until I bought Tony Boyers book and learned this method. So I think it is worth sharing and talking about as much load development questions as come up regularly.

I chose the attached target not because they are good groups. Actually the opposite this barrel is just an average performer. So it is really clear as it comes into the seating depth it likes for this bullet.

On the same piece of heavy paper. I use old Manila file folders. Make a graph with 3 charge weights across the top from light to medium to upper end.
After finding the jam length for this bullet by loading longer and longer until the bullet is pushed back in the case. Check this several times measuring base to ogive of the dummy round. After this is determined go down the page with 4 different seating depths going from .005 off jam in .003 increments. In some cases you can move in .005 increments depending on the bullet type. Some bullets like the 6mm -105 and 107 like to be further out
Fire three shot groups at each powder weight and seating depth. 9x out of 10 the seating depth can be found in less than 50 shots. Then confirm with 5 shot groups

It's hard to believe a barrel could shoot that well at .009 off and scatter out at .012 off only .003 difference. I chose .009 off for the seating depth for this barrel and bullet
You are looking for a depth that shoots well from the light to upper powder charge weight

 

[DeltaBravo]

Tim,

I do almost the same thing. I start at "touch" for the new barrel and start about 1 grain less in powder from the previously successful load. Then I look for powder charge first, .3 grains at at time, then seating depth. After I've found the right seating depth, I'll check on neck tension with a smaller bushing (.262 vs .263) and see what happens. I choose the best, then stick with it. My new barrel is shooting as well as the previous one at the same powder and seating depth settings. I love my Shilen barrels!

Dennis

 

[Brians356]

I like .006" jump better, at 29.0 and 29.5. I would fine tune charge in .1 grain increments in that range at .006" jump, then fine tune jump again in .002" increments.

 

[Ghansen]

I have to ask what the conditions were at the time of your test? In doing the same thing as you in relatively calm wind conditions I have had clearly defined direction and been able to attribute some horizontal to an upshift in a light breeze. You can create a chart with the same structure using group size and the numbers also tell a tale.

 

[Tim Singleton]

I have to ask what the conditions were at the time of your test? In doing the same thing as you in relatively calm wind conditions I have had clearly defined direction and been able to attribute some horizontal to an upshift in a light breeze. You can create a chart with the same structure using group size and the numbers also tell a tale.

I usually measure them if I have a barrel that shoots well enough at different seating to require measuring. This one did not. I try to qualify barrels after lunch time around here and at most matches that is when the wind picks up. I don't hold for the wind. I do try to get all 3 off in as close as I can to the same condition. Don't just shoot with no regard for the wind. But I want to determine seating depth in real match conditions

Edited to remove any language referencing wind sensitivity

 

[bsekf]

Very interesting! Did you shoot "round robin"? I am always amazed how seating depth effects size but also placement on the target. I agree with the 3 shots initially........5 won't get any smaller.

Bill

 

[skiutah02]

Just out of curiosity, what does seating depth have to do with "wind resistance"? Even with a very tight grouping, bullets are still subject to the same effects of wind.

In any case, based on your target, I'd also have been tempted to go with .006". Looks like the performance was very close to that at .009" off, and it would give you more room to work with as the lands erode. As you're going with .009" off, do you plan to test in finer increments across the .006" to .012" off range? Having chosen your seating depth (.009" off) so close to another slightly shorter seating depth that obviously performs much more poorly (.012" off), it seems like you might want to know in a finer increment exactly how much farther off than .009" you can go before the groups open up. Having a </= .003" seating depth window seems to be cutting things extremely close.

^ this is very good advice, IMHO.

 

[BenPerfected]

The best tuned gun with the smallest group says to me that the tune of the gun/ammo combination is providing a stable bullet and it is less effected by the wind. An unstable bullet has more surface area (wobble) that is more impacted by the wind.
Ben

 

[Brians356]

Just out of curiosity, what does seating depth have to do with "wind resistance"?

Just out of curiosity, where did you find the phrase "wind resistance" in this thread? I sure can't.

As an aside (but I think maybe this is what the OP was referring to) when testing loads I concentrate on holding good vertical when squeezing off shots, and ignore crosswind, or any horizontal wiggling that wind might cause by wind buffeting me. And when measuring groups for comparison, especially in breezy conditions, I usually consider vertical dispersion and ignore horizontal spread. Especially when looking for nodes ala "EC Method".

 

[BenPerfected]

Just out of curiosity, what does seating depth have to do with "wind resistance"?

Just out of curiosity, where did you find the phrase "wind resistance" in this thread? I sure can't.

As an aside (but I think maybe this is what the OP was referring to) when testing loads I concentrate on holding good vertical when squeezing off shots, and ignore crosswind, or any horizontal wiggling that wind might cause by wind buffeting me. And when measuring groups for comparison, especially in breezy conditions, I usually consider vertical dispersion and ignore horizontal spread. Especially when looking for nodes ala "EC Method".

 

[red_mamba]

Shouldn't you choose .003 and 29.5 because it has virtually no height error. And horizontal spread could be wind?

 

[Brians356]

Shouldn't you choose .003 and 29.5 because it has virtually no height error. And horizontal spread could be wind?

Some of us are at least as interested in how the POI changes vertically between successive charge / jump changes as we are in the vertical spread of a single three-shot group. You might find Erik Cortina's thread interesting:

http://forum.accurateshooter.com/index.php?topic=3814361.0

 

[joshb]

Thanks for posting, Tim. Your post was very helpful. I experienced a hiccup with my seating, and your input came at the perfect time. Thanks again, Josh.

 

[Ghansen]

It's hard to believe a barrel could shoot that well at .009 off and scatter out at .012 off only .003 difference. I chose .009 off for the seating depth for this barrel and bullet
You are looking for a depth that shoots well from the light to upper powder charge weight

Absolutely what I have observed with my last test. Finding a tune for my latest LV barrel, it produced 2 expanded clover leaf 3 shot results with no other explanation than the seating depth was really not going to work with that charge. I also found that where optimum depth occurred the chart I looked at showed an intersecting result where the values diminished for the weight and the depth. Like a countdown to optimum. I am unsure though if I have best wind performance.

 

[Tim Singleton]

Just out of curiosity, what does seating depth have to do with "wind resistance"? Even with a very tight grouping, bullets are still subject to the same effects of wind.

In any case, based on your target, I'd also have been tempted to go with .006". Looks like the performance was very close to that at .009" off, and it would give you more room to work with as the lands erode. As you're going with .009" off, do you plan to test in finer increments across the .006" to .012" off range? Having chosen your seating depth (.009" off) so close to another slightly shorter seating depth that obviously performs much more poorly (.012" off), it seems like you might want to know in a finer increment exactly how much farther off than .009" you can go before the groups open up. Having a </= .003" seating depth window seems to be cutting things extremely close.

.006 off was giving me the two and one. With paper between on the low and middle charge. I can't explain why a certain seating depth is more wind resistant. I know it to be fact. Had I performed this test in the morning in a dead calm condition it would have been hard to see the differences. I've done it. The .012 off wouldn't have been as scattered. That depth is extremely wind sensitive
One reason for qualifying barrels in the wind it just clearly shows the wind sensitive loads
Again I'm no where near smart enough to explain why. But it clearly shows on the target. The jam and the .003-.006 have way to much horizontal. Then the .006 starts closing up then the .009 is better yet then the .012 is extremely wind sensitive.
No one has to agree but you should give it a try

 

[Tim Singleton]

Shouldn't you choose .003 and 29.5 because it has virtually no height error. And horizontal spread could be wind?

the barrel did shoot a tiny group at that one specific charge at that seating depth. But would give no forgiveness in the powder load. we need a seating depth that shoots well across a wide powder window. Then it's not so upset as temp changes thru the day

 

[tenring]

You may wish to read Eric Stecker's article on seating Berger VLD's:

http://bulletin.accurateshooter.com/2009/03/berger-tips-for-loading-vld-bullets/

 

[Tim Singleton]

You may wish to read Eric Stecker's article on seating Berger VLD's:

http://bulletin.accurateshooter.com/2009/03/berger-tips-for-loading-vld-bullets/

Read it.
Thanks
Not at all different than what I have said. I stated in the op if shooting 105-107 grain bullets you may need to continue further into the case and go in .005 increments. As some bullets preferred to be further out
The point of the post is to draw attention to the importance of seating depth. Not that this method is the only way by any means. But to simply make people think before they load 30 rounds up in slightly different powder charges at a seating that is just picked at random

 

[Tim Singleton]

Completely out of tune on the left. Very wind sensitive
Slight seating depth and load change in tune on right. Same bullet and half vertical
Same match. Virtually same conditions

 

[Ghansen]

So Tim, from your observations how does adjusting seating depth for wind affect groups in light to no wind?