Seating depth tuning

[Keith Glasscock]

Hi everyone,

I am not new to reloading for competition, but reading the posts by several members here got me to experimenting with seating depth in far smaller increments than I had in the past.

I took my current F/TR long range load and messed with the seating depth. Using left over rounds that had been set at .011 jump, I shot one group. I then seated a few at .014 jump and gave it another go. Pushing each set in another .003, I moved five full increments before I ran out of left over loads.

Here is my question: What kind of behavior would you normally expect out of the experiment? Should I be seeing groups slowly go tighter, then slowly loosen up, or more like powder nodes - a scatter followed by significant improvement over a small window?

TIA,
Keith

 

[steve4102]

Scroll down to "From the Lab".

http://www.barnesbullets.com/resources/newsletters/september-2007-barnes-bullet-n/

 

[Shynloco]

Since I load strictly for accuracy and target shooting and reading and hearing the suggestions of several well accomplished target shooters, I conduct similar and "small incremental" bullet seating depth experiments with each and every bullet I test in each particular rifle I reload for. Now there is what I'd call a basic principle that may not always apply, but it's were I start from and that principle was taught to me by and old timer whose been reloading his own ammunition for the better part of 50 years. And that is that "boat tail bullets "GENERALLY" like jump, while flat based bullets "generally" like jam. But another general principle I've learned and respect is that each particular rifle is different and that what works in one, MAY NOT work in another. Hence with the boattails once I see consistency with a powder (and using a SAMMI spec for bullet seating (OAL) I'll start off and determine jam and then start working my way out by .005 and shoot 5 shots groups and watch how the groups change by moving .005 further. Once I note the change for the better, that when I only move by .002 to find that "sweet spot". FB"S get the same treatment but I usually find that "sweet spot" quick because of they're liking and proximity to jam to begin with.

Sounds to me like you are on the right track and can say those small increments will get you where you want to be.

Alex

 

[lpreddick]

prior to about a year ago i found accuracy in my target rifles by selecting a powder and it's weight matched with a bullet that was considered a good match THEN load a series starting .010 thou off the lands. using the same load i would then seat deeper by .005 and usually find a very accurate depth that shot tiney holes. chris long's article on the "shock wave theory" was my reference and i believe he has explained why seating depth produces these results. my problem was after finding the perfect powder, charge, bullet and seating depth i would load 20-40 rounds and after a month or so shooting these WOULD NOT produce the same results! the neck tension on the bullet changed and this will affect pressures and accuracy...many long discussions on this site re neck tension. fine a good load at a specific depth and shoot soon! due to this variation i am now seeking consistent accuracy by seating bullets into the lands and then varying powder charges until the tiney holes appear. my impression is that these rounds shoot the tiney holes several months later consistently. oh, by the way, chris long seats his bullets into the lands exclusively.

 

[Jay Christopherson]

Assuming you've already done your powder test and have loaded all your rounds to the node you found (and are just testing seating depth changes), I typically see a "bell curve" type effect, testing at .010" increments @ 200 yards. That is, groups will tighten up as you approach the best seating depth and loosen the farther you get from it. I've found it to be pretty clear at 200 yards. I usually test about an .080" window and can generally find a "node" where the load is fairly tolerant to seating depth within a .020" range. If you test much less than .010" increments, I've found it pretty difficult to easily see the effect, but YMMV.

One thing I have found is that you sometimes find a "screamer" group in the middle of a window where the other groups are terrible. I made the mistake of trying to load to that seating depth and spent all of my time desperately trying to stay exactly on it. Over a multi-day match, that can be a challenge. I say, look for the window that gives you some range to work with.

 

[BoydAllen]

I have always gotten my best accuracy in the range from a light touch to somewhere into the rifling. I am curious, have you tried seating in this range? If you have not, is there a particular reason? When making this sort of change (from jumping) you will need to reestablish your powder charge weight by testing. Of course if you are shooting a semiautomatic, I understand the reasoning behind jumping.

 

[Keith Glasscock]

I have always gotten my best accuracy in the range from a light touch to somewhere into the rifling. I am curious, have you tried seating in this range? If you have not, is there a particular reason? When making this sort of change (from jumping) you will need to reestablish your powder charge weight by testing. Of course if you are shooting a semiautomatic, I understand the reasoning behind jumping.

I have tried touching the lands all the way in to .020 into them. To be honest, I've had less than stellar results with that approach and these bullet (200 hybrids).

The reason for the question is this: I got results that look a lot like powder charge variations. I get a scatter followed by a tighter group, another tighter group, a scatter, and a tighter group again.

My question is whether this is the normal result. If it is, I know what to do. If it is a fluke, I'll give t another try on another day.

Thanks,

 

[fredhorace77]

I don't know why but I've always found really good accuracy .030" out from what my hodgdons manual Sam. specs give. Of course this is just in factory rifles. when I adjust seating depth I work in .005" increments. and yes as u adjust setting depth you should see groups either open up or shrink up. once u go past the sweet spot they will open back up again. sometimes the difference in group size is hard to see sometimes its a big difference

 

[Keith Glasscock]

Here are the test groups - all 3 shots. seating depth is written below to each group (they came out rotated). These were shot prone with a bipod. Flags were not available as I no longer have access to a club.

 

[BoydAllen]

Believe what you see, it is real. A lot of fellows who have not done as careful of an investigation are not aware of these repeating patterns In fact, it took a heads up from a friend to clue me in to them. In the past, I would try some random jump, and seeing that it did not work well, conclude that the rifle or bullet or both did not work well with jump. On the other hand, you found it all by yourself. Good for you.

 

[Nomad47]

Hmmm, your rifle does not like jumps that end in "1". Just curious, why did you use a .003 change for all but one and a .004 change for that one?

 

[Nomad47]

I'm not sure where I got this little tidbit in my notes but here it is:
Adjusting seating depth
Increase COL if group forms a triangle
Decrease COL if group is 2 close & 1 apart

If you look at your .011 seat, they are two close and one apart, then the .021 seat is a triangle. So it looks like my notes have some merit in your case.

 

[Keith Glasscock]

Hmmm, your rifle does not like jumps that end in "1". Just curious, why did you use a .003 change for all but one and a .004 change for that one?

Simple answer - I fumbled the depth adjustment. I figured it wasn't worth going back to the seating die to fix all the remaining ones when I found it later.

 

[fredhorace77]

i would pick .014 and go with that

 

[gotcha]

I think I'd redo the entire test and see if it repeats. Your only talking 15 more rounds.

 

[lpreddick]

yes...a triangle. when i see one i feel like i'm almost there...the bullets are trying to one hole? i then try two things. 1: increase and decrease seating depth by .005. shoot and measure. 2: increase and decrease powder charge by .2-.3 gr depending on size of case, leaving seating depth the same. shoot and measure. if i'm not happy i try a powder with a faster or slower burn rate. if you shoot an inherently accurate cartridge (6 BR, 6 BRX, 22 BR, 6 PPC, 22 PPC etc) you'll be happy soon.

 

[savageshooter86]

I used Berger's method of seating depth tuning and it worked for my rifle(and I wasn't shooting Bergers) I used the method for target rifles, not a hunting rifle. Since you are using their bullets may give it a shot next time around and see what happens.

 

[Erik Cortina]

These are typical results. Windows are .003" apart, so don't move more than .003" at a time.

As far as boat tail bullets liking jump and flat base bullets liking jam or vise versa. What contacts the lands is the ogive, so boat tail vs. flat base would not dictate that at all.

 

[Jay Christopherson]

These are typical results. Windows are .003" apart, so don't move more than .003" at a time.

As far as boat tail bullets liking jump and flat base bullets liking jam or vise versa. What contacts the lands is the ogive, so boat tail vs. flat base would not dictate that at all.

I'd disagree with that. I think the windows are definitely wider than that based on the experiences I've had.

 

[Erik Cortina]

The Windows can go both ways... I've seen secant ogive bullets have a fairly wide 3-5 window. And I have witnessed tangent bullets be incredibly narrow. And I have seen the opposite of course.

I will start at kiss minus 2 and come out(away) 2-3 at a time for the COARSE adjustment. It all depends upon where you're at as to how much it effects the group. If you're say .002 off and you see a paint stripe at 1k... you might witness quite an improvement with just a .002 change. If you're .200 off and make a .002 change, you may not see much.

Tom

I'm always in the lands, so that's probably why .003" make such a big difference then.