How exactly is seating depth changes causing the tightening of group size?

[BoydAllen]

Really...about all we can say with certainty is that," When I do this, this happens." Every thing else is supposition based on a guess as to what is happening. "Cut and try", and take notes. Once you have made enough seating depth changes, in a systematic and organized manner, and have notes as to what happened in each case, you will start to get a feel for this area of reloading. Typically, if a shooter has a long time between range sessions, does not load at the range, at least for these types of experiments, and does not take good notes as he works, he will stumble around in his disorganization, and only accidentally discover a good load....or not. Sadly, this is pretty typical of a lot of shooters.

 

[Steve Blair]

does not load at the range

[br]
Boyd, [br]
Loading at the range is not necessary but your other points are correct. Range loading potentially reduces the total time required but makes other aspects problematic. [br]
Are you measuring and/or correcting TIR at the range for each cartridge?
Are you using a scale accurate to better than a single kernel?
Are your loads 50 grains or more of relatively coarse powder? [br]
How many rounds do you fire in a match? High Power shooters, all divisions, may shoot 80 rounds per day with no cleaning opportunity. If the load won't shoot in a dirty barrel, I need to know that. So, I don't clean during load development. Again, the requirements of BR are different and are completely valid for that discipline but do not necessarily apply to other disciplines.

 

[jlow]

Here is something to consider and ponder – so far one theory is that seating the bullet at a different depth affects the distance and thus when it exits the crown, and you can argue it is the case volume or a combo of both. I think looking with strain gauge/pressure trace would be interesting and useful.

How about if we keep everything the same but bump the shoulder the same distance? Now, here the bullet seating depth is the same, the OAL of the round as measured via the ogive should remain the same but since the shoulder is bumped differently, the round when seated in the chamber as far forward as possible will in fact change the distance from the bullet ogive to the crown in the same manner as seating the bullet differently. Yes, I know that there as to be room or the bolt will push the shoulder in but what if there was enough room? Remember we are only talking about 1 thousands. I also realize that this will also change the internal volume of the case (as will seating the bullet differently) but likely to a greater degree since the diameter at the shoulder is greater than at the neck. Just thinking out loud.

Regardless of whether you agree this is a good test or not, it points out one thing which is if you keep everything the same but say just seat the bullet 1 thousands longer and you are seeing a big difference, and you can argue whether it is the position or the case volume that is changed, bumping the shoulders the same distance should and will likely will have as much if not more of an effect…. See Boyd why I was so anal about my should bumping in the other thread? LOL!

Yea, I am also pretty anal about my shooting even though I don’t reload at the range. I keep a spreadsheet of every shooting even with powder, bullet weight OAL, MV data, scans of every target I shoot with OnTarget analysis, and images of all the casings so that they can be reviewed later…

 

[mikecr]

I think that if that guy hadn't written that article stating his timing theory, I could have written an article saying that it was distance to the muzzle or some such other thing and everyone would now be saying its distance... With about as much proof...

Maybe, and maybe we're all wrong.
All I know is that it would be a serious advancement(someday) for us to hold a formula for best seating.

The problem is testing resources, and anecdotal thinking.
It would easily cost $100K+ and significant time to approach it scientifically.
Then:
Somebody shoots a record group with bullets loaded upside down, and the headlines imply precedence, even though he didn't manage much of any performance -before -or after that single event, or with other cartridges & conditions.
Next thing ya know all the backyard snipers are rationalizing baseless merits of it.. Gunwriters are making up BS about it(as usual)..
Hornady merchandises 'Sasquach Max' ammo,, and cats start chasing dogs!

I'm against it
I'm more for reasoning out across the board realities.

 

[BoydAllen]

I shoot short range and for that a single grain of powder doesn't mean squat. My dies are good enough so that my ammo does not need straightening. Different distances, different loading situations. If you can't make adjustments for conditions as they change, for short range, with the most common powder, you will be out of tune at some point, and if you guessed wrong about the weather, all of the time.

 

[mikecr]

Over-bumping does change static capacity, moreso than seating, and this does affect velocity in cartridges that aren't run up into an extreme pressure node.
But I don't see how this would simplify or isolate the affects of seating other than to mess up your relative seating distance w/resp to lands. It's throwing more variables into the problem at hand.

For example, if you bumped back an extra thou something like a 260, seated OTL,, this would reduce capacity, increasing load density with a given charge, raising MV a touch. The bullet may or may not end up 1thou closer to the lands on firing.
I predict this wouldn't affect grouping much(like 5thou of ~seating change can), and if it did make an offending difference(which is possible), what would it really mean?

 

[BoydAllen]

One thing that I forgot to mention is that I have a scale wind box that has the trickler inside with the scale, with the handle protruding out through the side of the box, so that I can throw coarse powders a little light, and trickle, just like I wold at home. The scale it tuned, and for the rifles that it is used for (not thousand yard competition) it is plenty accurate enough. The box design could be adapted for an electronic scale although the tables that are usually available for loading at the range, are usually not solid enough to put a good electronic scale on, with best results.

 

[jlow]

Over-bumping does change static capacity, moreso than seating, and this does affect velocity in cartridges that aren't run up into an extreme pressure node.
But I don't see how this would simplify or isolate the affects of seating other than to mess up your relative seating distance w/resp to lands. It's throwing more variables into the problem at hand.

For example, if you bumped back an extra thou something like a 260, seated OTL,, this would reduce capacity, increasing load density with a given charge, raising MV a touch. The bullet may or may not end up 1thou closer to the lands on firing.
I predict this wouldn't affect grouping much(like 5thou of ~seating change can), and if it did make an offending difference(which is possible), what would it really mean?

It’s obviously true that one cannot change a single parameter without affect another, so the interpretation of the data is complicated.

However, I don’t understand why bumping the shoulders back an extra thousand would not move the bullet one thousands further away from the crown? After all, when the firing pin hits the round, it is going to move the round as far forward in the chamber as the shoulders will allow, is this not true?

However, I think what I proposed would suggest at least equal importance (if not more) of being able to consistently bump the shoulders. One cannot avoid this conclusion as shoulder location would not only affect distance of the bullet to the crown but it should have a greater effect on case volume…..?

 

[Steve Blair]

Over-bumping does change static capacity, moreso than seating, and this does affect velocity in cartridges that aren't run up into an extreme pressure node.
But I don't see how this would simplify or isolate the affects of seating other than to mess up your relative seating distance w/resp to lands. It's throwing more variables into the problem at hand.

For example, if you bumped back an extra thou something like a 260, seated OTL,, this would reduce capacity, increasing load density with a given charge, raising MV a touch. The bullet may or may not end up 1thou closer to the lands on firing.
I predict this wouldn't affect grouping much(like 5thou of ~seating change can), and if it did make an offending difference(which is possible), what would it really mean?

It’s obviously true that one cannot change a single parameter without affect another, so the interpretation of the data is complicated.

However, I don’t understand why bumping the shoulders back an extra thousand would not move the bullet one thousands further away from the crown? After all, when the firing pin hits the round, it is going to move the round as far forward in the chamber as the shoulders will allow, is this not true?

However, I think what I proposed would suggest at least equal importance (if not more) of being able to consistently bump the shoulders. One cannot avoid this conclusion as shoulder location would not only affect distance of the bullet to the crown but it should have a greater effect on case volume…..?

[br]
Moving the shoulder back does not necessarily change the overall case length. If you push the shoulder back .005", the case length may remain the same and the neck length increase by .005". If the shoulder and neck moved together, it would be necessary to either compress or buckle the case body. The brass has to go somewhere. When the case is adequately lubricated, the die pushes the shoulder back without significantly affecting the case length. [br]
I prefer to keep my case shoulder within .0010-.0015" to the chamber shoulder and adjust other parameters to achieve results.

 

[jlow]

Over-bumping does change static capacity, moreso than seating, and this does affect velocity in cartridges that aren't run up into an extreme pressure node.
But I don't see how this would simplify or isolate the affects of seating other than to mess up your relative seating distance w/resp to lands. It's throwing more variables into the problem at hand.

For example, if you bumped back an extra thou something like a 260, seated OTL,, this would reduce capacity, increasing load density with a given charge, raising MV a touch. The bullet may or may not end up 1thou closer to the lands on firing.
I predict this wouldn't affect grouping much(like 5thou of ~seating change can), and if it did make an offending difference(which is possible), what would it really mean?

It’s obviously true that one cannot change a single parameter without affect another, so the interpretation of the data is complicated.

However, I don’t understand why bumping the shoulders back an extra thousand would not move the bullet one thousands further away from the crown? After all, when the firing pin hits the round, it is going to move the round as far forward in the chamber as the shoulders will allow, is this not true?

However, I think what I proposed would suggest at least equal importance (if not more) of being able to consistently bump the shoulders. One cannot avoid this conclusion as shoulder location would not only affect distance of the bullet to the crown but it should have a greater effect on case volume…..?

[br]
Moving the shoulder back does not necessarily change the overall case length. If you push the shoulder back .005", the case length may remain the same and the neck length increase by .005". If the shoulder and neck moved together, it would be necessary to either compress or buckle the case body. The brass has to go somewhere. When the case is adequately lubricated, the die pushes the shoulder back without significantly affecting the case length. [br]
I prefer to keep my case shoulder within .0010-.0015" to the chamber shoulder and adjust other parameters to achieve results.

Sleepygator, I think you mis read my OP on this topic. You are absolutely right that bumping the shoulders back does not change overall case length and I said exactly the same thing. The key here is if you bump the shoulders back by 5 thousands, a bullet seated in the case is going to move 5 thousands forward in the chamber (assuming that we are not jammed already) because the case is indexed in the chamber as to how far forward it can move by the shoulders?

 

[Steve Blair]

Sleepygator, I think you mis read my OP on this topic. You are absolutely right that bumping the shoulders back does not change overall case length and I said exactly the same thing. The key here is if you bump the shoulders back by 5 thousands, a bullet seated in the case is going to move 5 thousands forward in the chamber (assuming that we are not jammed already) because the case is indexed in the chamber as to how far forward it can move by the shoulders?

[br]
Generally, I would say yes, but it seems to be erratic. I bought a .30-'06 Springfield at a gun show in 1979, loaded some new brass and took it to the range. The seventh or eighth round had a head separation. It turned out that I could chamber a no-go gauge and the rifle had excessive headspace. I sectioned all the fired cases on a mill and three had no sign of separation or thinning, the others looked about to go. Given that it was new Winchester brass and had no previous issues, it seems that the cartridges were in different chamber positions when fired. All were loaded to book OAL and fed from the magazine. So, I think moving the shoulder back would not guarantee chamber position. If the shoulder was moved back a large amount, the extractor would probably limit how far it moved when the striker hit.

 

[jlow]

Sleepygator – I am sure you are correct that using the shoulder as index of the position of the round is not a good idea, I only mean that one could run an experiment using this method i.e. one could tilt the barrel down so that the round is already seated forward on the shoulders before hand and not rely on the firing pin. Only an experiment and not a reloading SOP like bullet seating depths.

The other reason it is interesting to me is because as you say, it is unreliable but could make a big difference, as such, it would suggest one could have and be responsible for significant variation in load behavior if the shoulder are not bumped consistently and not only slightly.

 

[necchi]

How about if we keep everything the same but bump the shoulder the same distance?

In this manner your changing how the case reacts in the chamber,,
Yes the bullet that is seated the same in relation would be .001 different, but your introducing another variable.
My ideas have always been to limit the variables and work one at a time searching for better results.

Nothing wrong with changing shoulder bump and observing the change in seating,,
but they are two different things.

P.s., Oop's posted prior to reading page 8, sorry mike but I agree with the multiple variables

 

[jlow]

Sigh! I think I took pains to point this out in my OP on this idea in Reply #102…. When I said “the same” it was referring only to dimensions. Changing only one parameter at a time in an experiment is always best practice but sometimes it is impossible in reloading…

 

[necchi]

Sorry, I had hoped by this point it would have been understood that seating does little to case volume,,
Changing case dimensions is a different issue as it applies to reactions in the chamber.

Tilting the gun to position the cartridge either against the breech or shoulder prior to ignition will do nothing when the nano second of the firing sequence is dissected.
Newtons 3rd Law of physics still applies with little regard to how quickly it happens.

Over thinking can lead to lead to a bit of confusion, while to some degree it is Rocket Science, alot of the application of that said science can be learned with Estes Rockets in the back yard.

 

[Bradley Walker]

So could someone tell me their depth tuning procedure?

 

[necchi]

That's simple too,
Shoot the rifle for group over a chronograph with the bullet seated at a mathematically incremental measured distance without changing any other variable.
Look at the chrono data'
Look at the groups on paper,
Choose the best.

The biggest deal with this method is that the actual shooting is the most beneficial to determine the best seating depth for that charge.

 

[Heavies]

So could someone tell me their depth tuning procedure?

The way I do it is...

Start with the bullet at hard jam
Work load to safe pressures and best accuracy, in the velocity that is sufficient.
When charge weight is found, I seat the bullet deeper by .005" fire 3 round groups.
Continue seating deeper by .005" until satisfied with the performance.
Choose 2 or more of the best seating depths and verify at longer range.

As the bullet seats deeper the group will form up into repeating patterns. As the groups tighten up there will be one or more areas where the group is very stable and consistent. It is good to test the depth at +.002 and -.002 to see if the outliers fall into the pattern. This way + or - .001" (which is very achievable with good match bullets) will not throw you out of tune.

 

[bozo699]

So could someone tell me their depth tuning procedure?

The way I do it is...

Start with the bullet at hard jam
Work load to safe pressures and best accuracy, in the velocity that is sufficient.
When charge weight is found, I seat the bullet deeper by .005" fire 3 round groups.
Continue seating deeper by .005" until satisfied with the performance.
Choose 2 or more of the best seating depths and verify at longer range.

As the bullet seats deeper the group will form up into repeating patterns. As the groups tighten up there will be one or more areas where the group is very stable and consistent. It is good to test the depth at +.002 and -.002 to see if the outliers fall into the pattern. This way + or - .001" (which is very achievable with good match bullets) will not throw you out of tune.

How is it possible to go deeper then Jam?.... Please explain.
Wayne.

 

[Bradley Walker]

When charge weight is found, I seat the bullet deeper by .005" fire 3 round groups.

"deeper" into the case... I assume.