Seat depth, what is really being tuned?

[DUSTY NOGGIN]

i believe it has alot to do with , the freebore to full rifling transition angle , some chamberings are at 1 degree , some others at 3 degrees , some eroded to whatever degree , i think the momentum to overcome that swage as smoothly as possible is what allows all other barrel timing harmonic exit factors to apply , i believe there is a slowing until that swaging time is completed , i think more jump helps decreases any variance in that swage timing

different shapes of bullets MAY be better suited to different chamberings ... whether the bullet be a tangent, secant, round nose or whatever shape at the bearing surface to the ogive , the way it conflicts with that "transition to rifling angle" at the point of swage/firing is what i think allows the barrel timing/harmonics less variation

example , a vld shooting through a 3degree "freebore to full rifling transition angle" may have to overcome more of that angle all at once compared to a 1 degree transition with the same bullet

but then, you have the bullet core and jacket composition that changes that ease of swage ..as well

i think since most of us like to shoot the longer shaped bullets , if you have the option to pick a chamber with the 1 degree ... it may be the better option

if anyone has , constructive criticism on this theory .. or sees something i may be overlooking , chime in and straighten a twisted brain out

 

[Poppadoer]

Looks like everyone has done their homework on this matter.Great feedback.Load em up and shoot.

 

[rogerstv]

I would love to see a physicist delve deep into this subject. Physics and math can explain almost everything in this world.

 

[coop2564]

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.

If it was solely timing and harmonics. Seating depth wouldn't matter, you could just pick your Coal and use powder charge to accomplish timing and done.

 

[Mikecr]

If it was solely timing and harmonics. Seating depth wouldn't matter, you could just pick your Coal and use powder charge to accomplish timing and done.

You're right. Seating for timing fails tests.

Seating at a bad depth also produces vastly greater dispersion than same timing as adjusted by powder.
Seating is so coarse to results that I do not see it as a tuning adjustment at all.
It merely acts to contribute to, or alleviate, a less than ideal condition.
Similar to swapping primers(also not tuning).

 

[tony d willIiams]

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.

Good answer. I attempt to have my bullet leaving the barrel at the apex pause time. I believe it gives more room for error in judgement and temperature changes.

 

[dsculley]

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?

 

[Mikecr]

Some have said that Berger's don't like jump.

There never has been basis behind predictions of best seating.
Just the murmurs of a mob, that never actually tested it -until Berger recommended doing so(and told them how to do it).

 

[cohunt]

I have not seen the Bryan Litz method for testing seating depth. I currently use the Berger method. How does the Litz method work?

one in the same--you realize Litz is the ballistician for berger bullets, right?

 

[misterc01]

You kind of answered your own question regarding the whys. My experience shows that there is an optimum seating depth for each bullet and powder charge in my rifles as far as accuracy. One of mine is giving me great accuracy with Hornady factory A-max Black with a specific bullet weight. The same is not true for the ELD Match. I likes to be seated a little further out. I Do NOT seat to the lands, to alleviate potential pressure issues.

 

[Randy Tidwell]

Why does barnes recommend starting 50 thousandths off. Nosler does as well with the lrab.

The first time I tried the all copper Barnes out of a 270 that was a pretty consistent 3/4" Ruger No1, I couldn't believe how poorly they shot. I'm talking 2+ inch groups.

Next day I called Barnes and had a lengthy conversation with one of their techs.

He explained to me that because their copper is tougher than other manufactures, a pressure spike is caused by seating the bullets close to the lands. When fired, the bullet stalls causing the pressure to spike. With the bullets seated .050-.060" from the lands, the bullet gets a running start which allows the pressure spike to smooth out.

Sure enough, took the same load with the bullets seated .060" off the lands, right back to a sub 1" gun. Little fine tuning later and I was in the 1/2" range.

This is the way I understand it. I do know that we get very good accuracy from Barnes with the seating depth at least .050" from the lands, no closer.

JMHO

 

[aushunter1]

Don't know & I don't have to understand it to know it absolutely works which is the important thing to me!
This is just a few days ago, im doing LD for a Browning Blr 300wsm, my starting point for any reloading is 10thou when I get a charge that shows promise I the start changing the CBTO. Sometimes its more jump, sometimes its less.
This rifle likes 5thou over 15thou so Im sticking to that, I was even amazed that the 5thou groups were all good between 3gr powder charges!!

 

[Poppadoer]

Great information out there.

 

[YZ-80]

I can tell you what it does for me. Improves my groups, on average about .5 to .75 MOA. I added an OAL gage and bullet comparators to my reloading repertoire about 8 years ago and never looked back. For me, it was pretty basic. The instructions said to seat (CBTO) between .010 and .030 to the lands so I compromised and made it .020 and that's my starting point in just about every application. Sometimes I end up at around .015 but usually don't go much closer and see no need to. I think closer is better (but not too close). Other than that, I concentrate on uniformity in all other areas and seating depth is pretty much a "set it and forget it" proposition until I start chasing the throat. Works for me!