Charge weight tuning v seating depth tuning

[Curious]

Its seems that generally we tune a powder charge then we tune a seating depth, this seams the common route in testing like long range ladders or OCW style methods.

As I understand it powder charge has a direct influence on barrel harmonics, by varying it we see nodes move around or stabilise and we pick out the best to suit our needs. Then we move on to seating depth tuning again we see groups sizes and shapes differ and move around, again we pick what we feel is best and repeatable.

Generally this method works assuming we pick the right powder charge node to tune and the barrel likes the bullet/powder combo.

So what exactly is happening here guys?

With powder charge tuning I am lead to believe that Im timing my bullet exit point to a specific favorable point in the barrels harmonic cycle?

So what is varying the seating depth actually doing to make the overall tune better or worse in relation to the powder charge tune?

 

[Ledd Slinger]

Seating depth changes pressure. Although its more of a fine tuning. I believe it alters the pressure curve as well. Ive always thought of it as how quickly the bullet seals the bore with its bearing surface before faster moving gases are able to escape around it during ignition. A jam seating depth will usually create slightly higher pressures vs. a touch or jump.

 

[alinwa]

Here is one view;

An opinion.

With a bullet seated to a consistent spot in the leade/lands (many use a variation of 'jam') one can raise the powder charge incrementally while shooting groups along a horizontal line. If one has a sufficiently accurate setup one may see the groups print a sine wave across the paper. In other words, faster shots DO NOT just print higher on the paper, in fact group centers plateau or even descend with velocity. The over-riding up and down component of the barrel vibration is shown on the paper

Shoot the groups from left to right about an inch apart.

Then, by choosing a velocity "just over the top" or starting down the right side of a wave one can know he's in a "positive compensation" spot on the sine wave.

Now tune the vertical out using seating depth.

Changing seating depth also changes the wave-form and timing of vibrations......it's like the powder charge is the "coarse adjustment" and seating depth the "fine adjustment."

The reason seating depth changes the vibration pattern is a combination of pressure rises/bullet moves, dropping pressure/bullet stops on lands/pressure rises to engraving force/pressure drops once bullets starts but pressure is still rising more rapidly than it's falling for the first inch...etc

My point is that it's DIFFERENT depending on how far the bullet jumps or is jammed.

Generally one can jam an olde schoole 7 ogive into an olde school 1.5* leade only a thou or two before the bullet's just getting pushed back into the case. With this combination the entire "tuning curve" fits into only 6-8 thou of total adjustment.

Generally the longer the ogive and the shallower the leade angle this jam "into the lands" adjustment can be extended to 6-8 even 12thou beyond touching before pushback occurs. Often on the long ogive bullets a deep jamset shows a ring VS the old standard "square marks" of the lands.

Of course "jump" can be anything, provided one remembers that as you increase jump, you decrease initial capacity thus raising initial pressure. An example of this would be that case where one resets seating depth for "20 thou jump" and pressures out or pops a primer. Regardless what the loading manuals say about touching the lands producing a "pressure spike' it's equally common to show pressure from backing 'way off from the lands.

So, there are some thoughts, maybe provoke discussion

Results vary,

opinions vary and,

barrels/chambers/bullets/angles vary.

IMO the single biggest or most important rule is to keep good records of changes, hopefully leading to trends.

 

[alinwa]

oops, ledd beat me..... faster typer

 

[Ledd Slinger]

oops, ledd beat me..... faster typer

Yeah but your explanation is just a liiiiiiittle better than mine... OK, its a LOT better

 

[alinwa]

And, may I add, my description relates to PPC/BR sized cases....... When case capacity goes up, these parameters change. IME once one gets up over 60gr powder seating depth seems to have less effect. (This is ONLY MY CURRENT OPINION.)

 

[alinwa]

Yeah but your explanation is just a liiiiiiittle better than mine... OK, its a LOT better

Thank You for your kind words sir

 

[Ledd Slinger]

And, may I add, my description relates to PPC/BR sized cases....... When case capacity goes up, these parameters change. IME once one gets up over 60gr powder seating depth seems to have less effect. (This is ONLY MY CURRENT OPINION.)

Very true. Same goes for powder charges. In small cases where 0.1gr or.0.2gr can make a noticeable difference in the tune, it may not be very apparent or even measurable in a case like a 300 Ultra mag or 338 Lapua

 

[Tim Singleton]

IME
You do both powder and seating depth at the same time on the same piece of target paper so you can have all the data in front of you

 

[Alex Wheeler]

Seating depth does change pressure, very slightly. I have noticed seating depth can play as big a role as powder charge. Put the bullet at the wrong depth and no powder adjustment will fix it. I think barrel length for a given combo is important. To me seating depth is adjusting the distance the bullet is to the muzzle. Similar to what you can do with a tuner. I also notice seating depth has a similar effect on groups shape like a tuner. Almost like adjusting effective barrel length. Thats how I have come to look at it.

 

[urbanrifleman]

Seating depth does change pressure, very slightly. I have noticed seating depth can play as big a role as powder charge. Put the bullet at the wrong depth and no powder adjustment will fix it. I think barrel length for a given combo is important. To me seating depth is adjusting the distance the bullet is to the muzzle. Similar to what you can do with a tuner. I also notice seating depth has a similar effect on groups shape like a tuner. Almost like adjusting effective barrel length. Thats how I have come to look at it.

Wow. Totally get that... That might be the explanation exactly.

 

[urbanrifleman]

I was told there will be a seating depth where charge weight has a much less effect, and the effective accuracy charge window will be wide.

 

[alinwa]

IME
You do both powder and seating depth at the same time on the same piece of target paper so you can have all the data in front of you

I'm not schmart enough for that

For me it's ONE THING at a time else I get all cornfuzilated

 

[Tim Singleton]

I'm not schmart enough for that

For me it's ONE THING at a time else I get all cornfuzilated

i know you can do it Al

 

[jimmymac]

.011 from jam looks good on that 28 to 28.5 charge window.

 

[urbanrifleman]

i know you can do it Al

Is that a group? .011" middle?

 

[alinwa]

i know you can do it Al

HEY! I might could get useta' that

Here's one of mine

 

[alinwa]

this pic was 15-18yrs ago..... first shots thru a new experimental barrel/chambering. Obviously the criss-cross aiming points is kinda' dumb. I now draw the horizontal lines and stamp aiming circles with a self loading rubber stamp

 

[Tim Singleton]

Is that a group? .011" middle?

Just 3 shots
Those Barts bt's are pretty amazing

 

[mikecr]

Changing seating depth also changes the wave-form and timing of vibrations......it's like the powder charge is the "coarse adjustment" and seating depth the "fine adjustment."

Full seating testing(Berger recommended) shows totally opposite of this.
Seating is coarse, powder is fine.