Seat depth, what is really being tuned?

[Buster Hemlock]

When working up a load what do you feel adjusting seat depth is actually accomplishing? Do you think certain bullets shoot better at a particular distance off the lands/rifling or are we actually fine tuning the pressure/optimum barrel time for the bullet to be released from the muzzle?
If it were certain bullets like a certain amount of jump we would all be loading them the same off the lands but it seems to me guys find accuracy at different amounts of jump, some times by a considerable amount. How often do you find your self getting what seems to be the right seat depth, then finding your powder charge and then retuning your seat depth? If you end up retuning seat depth how much adjustment are you making when you come back to it? If anyone has done any work on this please send me some reference or a link.

 

[Remmy700]

IMHO every bullet has its favorite depth it likes to be concerning jump. It's a delicate balance with powder charge with seating depth especially those bullets that prefer a little jam cause now u introduce additional pressure not induced by just powder charge itself. I think also this has to do a little with out the barrel time as obviously a bullet closer to the lands will leave sooner after ignition than one seated deeper. I think for me seating depth is a bullet particular parameter and finding exactly where that bullet, and that depth with a specific charge plays well together while jiving with barrel harmonics. The ol" rabbit hole of reloading!!

 

[Buster Hemlock]

Definitely a rabbit hole. I've long wondered about this particular question and felt, especially recently, like it's really a matter of fine tuning pressure and getting that ideal time for the bullet to be released. I have two rifles I shoot the same bullet in and have one kissing the lands and the other at .037" off. .037" is at least to me a considerable difference in jump. I've recently thought of setting up one of my rifles with a pressure trace piezo strain gauge (seeing I have a RSI unit sitting around) and seeing if I can get similar results as far as groups. Idea is taking a known accuracy load and then adjusting the charge to get a different seating depth to run at a comparable barrel time to see if it groups the same. Keep ideas coming, if I get motivated I'll do it and post results. For science and $h!t

 

[Wolf76]

I've observed that certain bullets like certain things. Barnes likes to jump. Berger likes it close. While I originally thought it might be related to the timing aspect, testing proved otherwise.
Example: I can take any 300 wm/ Berger load and it shoots better closer to the lands regardless. This disproves the timing theory.
Why does barnes recommend starting 50 thousandths off. Nosler does as well with the lrab.
This is the reason I start with seating depth for load development. Having a close to perfect seating depth takes care of one crucial detail for all loads.

 

[User4302021]

Powder charge tunes the floppy nodes. Seating depth the nodes that affect bore dilation.

http://www.varmintal.com/amode.htm

That's my theory anyway. I haven't heard a better one. As a point of confirmation, when doing an OCW test, the POI changes sometimes dramatically with charge weight but not with seating depth. However, seating depth dramatically changes group size.

 

[MagnumManiac]

The easiest and most widely accepted answer is barrel harmonics and where the bullet exits in the 'whip' motion of the barrel. The tuning is timing the bullet to exit at the same point in the whip/oscillation. Whether this is when the barrel is stationary between the transition of movement or the middle, top or bottom is yet to be observed.

Cheers.

 

[Wolf76]

The easiest and most widely accepted answer is barrel harmonics and where the bullet exits in the 'whip' motion of the barrel. The tuning is timing the bullet to exit at the same point in the whip/oscillation. Whether this is when the barrel is stationary between the transition of movement or the middle, top or bottom is yet to be observed.

Cheers.

Respectfully, the recommendation from bullet manufacturers for a certain amount of jump contradicts this statement (barnes is perhaps the best example).
And I've seen far too many guns that wouldn't shoot with bullets more than 40 thousandths off, regardless of charge.
I think powder charge effects the timing / oscillation 99.999%.
Very few things are 100%, so I'll concede there are outliers to the above statements.

 

[Buster Hemlock]

Wolf, I load Berger's really close too and typically find great accuracy at .020" and closer, often .010" to touching. But I wonder if I cut back on powder a few tenths and push the bullet in deeper to get the same barrel time will they shoot as well. Guess I'm just going to have to try it. I'm tracking on barrel whip, muzzle dilation and OBT. I've seen some high speed video showing muzzle whip and bullet release as well as some shot looking at the muzzle and showing the muzzle dilate and contract. I spent some time talking with Chris Long on this subject a few years back and he was also intrigued. I'm thinking on it more now because I have a rifle I've been single feeding because my current load doesn't fit mag length. I guess it's time to order a few strain gauges and head to the range and do some tests.

 

[Wolf76]

Wolf, I load Berger's really close too and typically find great accuracy at .020" and closer, often .010" to touching. But I wonder if I cut back on powder a few tenths and push the bullet in deeper to get the same barrel time will they shoot as well. Guess I'm just going to have to try it. I'm tracking on barrel whip, muzzle dilation and OBT. I've seen some high speed video showing muzzle whip and bullet release as well as some shot looking at the muzzle and showing the muzzle dilate and contract. I spent some time talking with Chris Long on this subject a few years back and he was also intrigued. I'm thinking on it more now because I have a rifle I've been single feeding because my current load doesn't fit mag length. I guess it's time to order a few strain gauges and head to the range and do some tests.

Let us know what you find out. I'd guess you'd need slightly more powder to maintain the bullet timing, given it has to travel further.

We may find out that the lack of precision in our loads and/or gun is the issue. Maybe keeping the bullets close to the lands helps offset the other issues. Food for thought.

 

[Mikecr]

I have a notion about it that's impossible for me to prove.
Picture a ping-pong ball air pushed in a tube towards a smaller fitted tube. The ball is rarely ever, if ever, perfectly aligned. It rattles a bit at the mouth of the fitted tube -before full entry/engraving. That bit of rattle, or entry delay, is a variable which affects the air pressure behind it. For us, the air pressure is analogous but highly amplified as pressure from powder burn rate, which is itself very affected by slightest early variances of pressure.
Testing could reveal a running distance that allows the ball to enter the fitted tube most cleanly & consistently. It could be an abstract, with no predicting of it. We might refer to it as tuning.

It would seem easy to assume that pre-fitting the ball into the mouth of a fitted tube would eliminate entry delays. That must be better, right?
Well, it could be better in one regard while worsening another. Maybe the cartridge/powder combination does not like a ball stuck & stubborn to move, without a free running distance. Maybe it causes a powder column to hit too high of a peak pressure, and higher pressure burn rate, too soon, while there is a lot more powder to burn, hopefully closer to it's designed/intended burn rate.
N133 powder is designed for and rewarding to reach a high early peak pressure. H1000, not so much.

Anyway, right or wrong, it passes tests where others fail..

 

[TwoMore]

i have a factory 700 in 7rum, you can see with a bore scope the throat is heavy cut to one side, ( you can see a small amount of rifling in the freebore area on one side) the only way to get it to shoot decent is to jump it big. seems to shoot smaller bullets better

 

[Poppadoer]

I read an article on how to see where your lands are and the best jump for most bullets with calibers included.Google is a powerful tool.Just Google seating bullets from lands.

 

[blkdog]

I just finished the Brian Litz method of testing for COAL and I was amazed at how obvious the seating depth that the load preferred became. I did it with a 6.5 Creedmoor and a .204 Ruger. Both guns showed a definite preference for a given depth. From now on I will do his test with my other rifles.

 

[bill123]

I have a notion about it that's impossible for me to prove.
Picture a ping-pong ball air pushed in a tube towards a smaller fitted tube. The ball is rarely ever, if ever, perfectly aligned. It rattles a bit at the mouth of the fitted tube -before full entry/engraving. That bit of rattle, or entry delay, is a variable which affects the air pressure behind it. For us, the air pressure is analogous but highly amplified as pressure from powder burn rate, which is itself very affected by slightest early variances of pressure.
Testing could reveal a running distance that allows the ball to enter the fitted tube most cleanly & consistently. It could be an abstract, with no predicting of it. We might refer to it as tuning.

It would seem easy to assume that pre-fitting the ball into the mouth of a fitted tube would eliminate entry delays. That must be better, right?
Well, it could be better in one regard while worsening another. Maybe the cartridge/powder combination does not like a ball stuck & stubborn to move, without a free running distance. Maybe it causes a powder column to hit too high of a peak pressure, and higher pressure burn rate, too soon, while there is a lot more powder to burn, hopefully closer to it's designed/intended burn rate.
N133 powder is designed for and rewarding to reach a high early peak pressure. H1000, not so much.

Anyway, right or wrong, it passes tests where others fail..

This sounds similar to Chris Long's theory in his Optimum Barrel Time Paper here: http://www.the-long-family.com/OBT_paper.htm .

Scroll down to "A Theory to Fit #3" to see his explanation.

Also, If I understand his paper correctly, he says that barrel whip is not a factor.