naive newbie seating depth question...

[lurcher]

I'm fairly new to reloading discussion on another forum got me thinking......

If you are using a rifle where OAL is restricted to magazine length then the approach of final fining tuning a load by changing seating depth is lost if you want to feed fro the magazine....

If the argument (one that I read on another forum) is that adjusting seating depth merely fine tunes pressure/velocity then can the same job be achieved by subtle incremental increases in powder weight say 0.1g increments (assuming seating depth is uniform, bullet weight is uniform, brass has been weight sorted etc).

Is there more to seating depth than merely slight variations in velocity that could be achieved by other means (subtle accurate powder weight changes or even changing brand of primers) or is there more to it than that and if so what are these factors that come into play

 

[whiskey08]

I completely disagree with those theories, I just read of a test on The 300 wsm and pessure and velocity differences based on seating depths. There was no difference until u started to jam the bullet than pressure and velocity went up.

My thoughts on seating depths are they are one of the the most critical elements to accuracy, assuming u did a good job prepping your brass.

You can be at a disadvantage by not being able to touch the lands. That's what caused me to rebarrel a recent gun. My buds gun on the other hand shoots right under .5 moa loading to mag length.

Lastly seating depth is not used to tweak velocity. In ur case I would seat as long as possible so u only have to move in one direction to find sweet spot.

Depending on type of powder I worked towards a specific load. For example 4064 was more accurate the harder I pushed the bullet. Varget on the other hand didn't like to be to close to max. Gl

 

[lpreddick]

lurcher: i chased accuracy for many years by increasing powder charges, therby increasing m velocity and occ tightening a group. i am now a FIRM believer in improving accuracy by working with seating depth of the bullet in relations to the riflings(lands). there is a theory, shockwave and optimum barrel time, that suggest , probably true, that a wave goes down the barrel at the time ofpowder sublimation(explosion) and that it returns to the action, then to the muzzel and back several times before the bullet exits the barrel. this wave is envisioned as a donut going back and forth in the barrel and that timing it's arrival at the muzzel as the bullet arrives produces the one hole accuracy that we chase. the arrival time of the bullet is most easily achieved by seating depth in the case. 10 thous variation from the lands will add or subtract milliseconds from the bullet's arrival time, coinsiding with the donut't arrival. finding this depth is the challenge. many match grade bullets will respond nicely with only a few thous adjustment. some bullets, however, will not shoot. start with bullet in the lands , then back off 5-10 thous and repeat. a "sweet spot" often appears as if by majic.

if i can see you, i can touch you.BANG!

 

[BoydAllen]

Different amounts of jump will tune like different amounts into the rifling. Many times shooters try one depth off the lands, and if it does not produce accuracy, quit trying. I have heard of shooter that have done well with as much as .060 jump. For this testing, I would recommend loading at the range, trying two shots, over flags, until something promising is found. Don't make big changes, or you may go past a sweet spot. Tuning is an art that takes patience and a methodical approach. Often, I have run into shooters that do not want to be bothered with one aspect of another of a balanced program, preferring to ignore important details and just obsess over tenths of a grain of powder. They tend to be self limiting.

 

[whiskey08]

here's some info about velocity/pressure and seating depths

http://www.loaddata.com/articles/PDF/3-Inch%20300%20WSM.pdf

 

[HolyMeekrob]

I can't give you any specifics about the hows and whys of seating depth, so all I'll say is that I view it as super-fine tuning, because measuring to 0.1gr with inherently flawed scales can only take you so far.
An analogy would be that a course stone will only so good a job in sharpening an edge. To get it razor sharp you'll eventually have to break out that fine japanese whetstone.

 

[artbosco]

If the argument (one that I read on another forum) is that adjusting seating depth merely fine tunes pressure/velocity then can the same job be achieved by subtle incremental increases in powder weight say 0.1g increments (assuming seating depth is uniform, bullet weight is uniform, brass has been weight sorted etc).

Is there more to seating depth than merely slight variations in velocity that could be achieved by other means (subtle accurate powder weight changes or even changing brand of primers) or is there more to it than that and if so what are these factors that come into play

Fine tune with what's available to you. If .1 grain incremental changes are all you can accomplish because of the rifle's limitations, for example, the use of a magazine, make the most of it.

I have a friend who wins many NBRSA [National Benchrest Shooters Association] matches, with a bolt action rifle, and does not use bushings, but does however use a tuner on the end of his barrel.

He told me that he doesn't use bushings [for adjusting neck tension], because he wanted to eliminate one of the variables. He does establish a predetermined tension by reaming his F/L sizing die's neck area to the size that he feels will highly benefit his bullet, powder, barrel, primer, brass, seating depth combination.

He is one of few, who gets the most out of his barrel tuner which he designed. The tuner, you might say, took the place of the bushings. He has determined how he wants to fine tune and has done very well with his choice.

Others may be limited in what can be accomplished. You do the best with what you have to work with.

 

[lurcher]

Thanks guy's very intriguing. Another thing I remember reading somewhere was that there is often more than one sweet spot as a bullet is seated in increments further into the case and then tested. Is this likely or is there generally just one sweetspot.

As a throat wears and shooters start "chasing the lands" is the original gap still generally the sweet spot or can this change?

Also is there a "rough" maximum distance from lands to ogive beyond which depth tuning is a lost cause? I'm guessing that the reduction in case volume caused by increasing depth is not really contributing nearly as much to the effect compared to variation in the ogive to lands distance?

For instance in rifles such as weatherby's which have a deliberately long throat which I have read cannot be reached in some cases is seating depth tuning still an option?

Are there relationships between seating depths and other factors for instance if you increase or decrease neck tension and blow out grouping, can you readjust with seating depth?

 

[artbosco]

Another thing I remember reading somewhere was that there is often more than one sweet spot as a bullet is seated in increments further into the case and then tested. Is this likely or is there generally just one sweetspot.

The "sweet spot" that I'm familiar with is more often called a "Node" and is a powder charge, in grains, where the barrel is in tune. That's where vertical is eliminated. For instance, with a 6PPC, using N133 powder, you find one around 29.0 to 29.2 grains and another around 29.8 to 30.0 grains. You fine tune, with bullet seating depth, neck tension, and .1 grain powder increments.

As the throat wears, say after 300 to 400 rounds, I set my bullet out a little so that it maintains the same depth [for me a square mark on the bullet] as the original tune.

Once I find a tune with the Krieger barrel that I use, all subsequent Krieger barrels have the same number of grooves, same length and same twist. All cut with the same reamer [the one I own], by the same gunsmith.

All reloading is done with the same brass, same primers, same bullets, by the same bullet maker, and same powder. The only thing that changes at some time will be powder lot, which will necessitate a change of a few tenths of a grain in powder, accomplished in .1 increments. More often than not, the same seating depth and neck tension works.

I once combined a .1 grain change in powder with a .001" inch change in tension that had bullets going through the same hole when I followed my wind flags correctly.

If I blew out a group with a change in neck tension, I'd attempt to bring it back in, with a subsequent change in neck tension.

 

[BoydAllen]

A Benchrest shooter that I respect, and who has always been an experimenter tells me that there are seating depth nodes, both into the rifling and jumping. The problem that most guys have in making this sort of determination is that they have limiting factors other than the load. They may not think that they need flags, because they "only shoot when the wind is not blowing", or they may shoot off of lousy rest setup, or are trying to use a shooting style that there rifle is unsuited for. The list goes on. This is a weakest link thing. Unless you get all the ducks in a row, it can be like trying to measure thousandths of an inch with a tape measure.

 

[whiskey08]

I dont know if you personaly blew out a group with neck, tension? Thats one of those things at the bottom of my list to worry about. They're are enough other variables to worry about.

The discipline you shoot should have a part in how much neck tension you use. Some shooters use so light of a tension that if they have to extract a round sometimes the round stays in the lands.

If you a hunter than obviously you need enough tension that will keep the bullets from moving around during a hunting trip.

For the most part I think that the lighter the better. For simplicity sake, lets use 10" lbs vs 20"lbs of tension, and lets say you have 10% variance between cases. 10% of 10lbs is lots less than 10% of 20lbs. So with that theory in mind you just minimized as much as possible one more variable.

So I think that instead of tweaking powder, seating depth, primers, and now neck tension, just go with the lightest tension your discipline will allow and worry about seating depth and powder charge. That should be enough to keep you busy for a couple of years