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

I use the OCW method to find optimal powder weight followed up by adjusting seating depths to tighten up the groups. These are loads that starts out in the OCW method seated to mag length and later slightly longer (5-30 thousands) to tighten the groups. Please note, the final rounds are nowhere close to the lands.

I was always working under the assumption that the main effect of changing the seating depth in this type of setup was due to changes in case volume which in turn affected chamber pressure, MV, and finally the timing of when the bullet exit the barrel thus allowing the bullet to avoid exiting when the harmonics was at the crown.

However, the more I think about this, the less sure I am of this explanation. The reason being that with the variation in case volume as it comes from the factory, and I only sort brass by weight and not case volume, what is the chance that the case volume is consistent enough to see slight difference in case volume due to slight seating depth adjustments i.e. 1-5 thousands in seating depths.

The other reason is assuming that even if one sort by case volume, exactly how precise is this method? Seems like it is not likely to be precise or consistent enough to allow one to see the slight volume changes due to slight changes in bullet seating depths?

Would very much like to hear what the experts think.
 
You won't see seating adjustments with a chronograph, and yet you can cause clear velocity changes from powder without losing a good tune. Right? That's what an OCW load amounts to (tolerance).

I've never read a single notion about the causes/affects of seating depth that pass all tests.
This says to me that one idea depends on another, which depends on another, etc.....
It's an abstract that would take massive resources to isolate, define, and predict.
 
In my opinion, the changes result in the bullet arriving at the muzzle at different points in its vibration cycle. As you have discovered, the proper tool for optimizing this timing is called a target. ;) Two bullets starting at different distances from the muzzle, even if their velocities are the same, will arrive at slightly different points in the barrels vibration cycle. Yes, this is conjecture, but have you heard of a better theory?
 
I'm the least qualified of any to explain this subject. Having said that, in my mind the seating depth variation is usually about the relationship of the bullet to the lands. This factor, again, in my mind, has to do with ignition, powder burn and pressure peaking. My theory is that seating bullets into the lands originated with cases with very thin necks that didn't have enough neck tension to hold the bullet for a proper uniform ignition of the powder. I've seen brass with such low neck tension that just a primer without any powder will push the bullet well into the barrel. Brass with turned necks in the .0085 to .0075 range and bullets only seated a small amount into the neck probably allows the powder charge and bullet to be forced out of the case a small amount by the blast from the primer, before complete ignition can take place. (Did you ever notice in the high-speed pictures of primer ignition that the so-called BR primers all have smaller, broader flames. Supposedly to cause the least upset to the powder charge.) But if the bullet is into the lands .005 to .010, it will hold the bullet and powder until the powder burn is well started. My theory continues that the more neck tension the brass has, the less need to seat into the lands to get proper ignition and, therefore, less need to tinker with seating depth.

In my shooting of varmint and target rifles I have found that once you are .010 off the lands any further seating depth has little or no effect.

That's it! Great to theorize on things you really know nothing about.
 
ReedG said:
Great to theorize on things you really know nothing about.

It's one thing the internet facilitates really well. For good or ill.

I'm hoping this thread brings some clarity. I'll wait right over here...
 
A good friend, who has shot benchrest for a lot of years, and who has done more testing of theories than any one that I know of, told me that, based on his testing, there are tuning nodes that occur in seating depth, both into and off of the lands. As the seating depth is very gradually changed the load will go in and out of tune in a periodic manner. As far as thin necks go, I once did a test in which I seated about .060 off the lands, the group was in the low .2s, with necks that were between .008 and .009 thick. The neck tension, in terms of difference of sized and loaded neck diameter, was about 0025. there are a number of successful benchrest shooters that jump their bullets. At eh end of the last Cactus Classic (benchrest match) I am told that Gary Ocock took a survey of the shooters, by raised hands, that included bullet seating, and that a significant portion of the shooters seat there bullets off the lands. In the pursuit of accuracy for a friend's custom magnum hunting rifle, we loaded and tested at a variety of seating depths, mostly jumping the bullets, the accuracy changed as the seating depth was changed, in a manner that seemed to be consistent with the theory that I mentioned at the beginning of this post. Most everyone that has a theory on how all of this works, is working from anecdotal evidence (as am I) As far as I am concerned, this gives a certain amount of equality to all tested theories, with the caveat that some seem to come to a state of certainty with less actual testing than others.
 
Mikecr – yes, no difference in MV, as stated in the OP, just tightening of group size.

BoydAllen – yes, I agree about the effect of tuning when the bullet arrive at the muzzle, just trying to figure out how slight seating depths affects this time of arrival. The easy one would be as I stated, case volume -> chamber pressure-> MV. However, although I don’t have a better theory, I am not totally convince due to what I have already stated – again I think my case volume is already too sloppy for me to be able to adjust them that precisely with seating depth – thus my conundrum.

ReedG – I understand the effects of seating close to or jamming into the lands, that to me is believable but as stated in the OP, this is the “other” situation where the bullet is nowhere close to the lands. Two different situations and two different questions and two different answers.

Killshot – my thoughts exactly.

Again, to clarify, I do believe and have first hand experience seeing how seating depths can tighten group size. Just trying to figure out how it works….
 
Lets not forget the pressure wave theory ;D

In the end it does'nt matter why it works so long as one is willing to test and verify.
 
I think tuning when the bullet arrives at the muzzle is part of the pressure wave theory?

As to whether it matter how it works, coming from a science background, I would say understanding how something works is always important. I see people “cook book” things all the time, it works fine until something goes wrong and when that happens, they are totally lost because they cannot work through the problem since they don’t understand what is happening and what they are doing in the first place.

Understanding how it works also have the added bonus that it works allows you to figure out how to fine tune and make it better. This cannot happen if you work with a “black box”.
 
jlow said:
I think tuning when the bullet arrives at the muzzle is part of the pressure wave theory?
Go
As to whether it matter how it works, coming from a science background, I would say understanding how something works is always important. I see people “cook book” things all the time, it works fine until something goes wrong and when that happens, they are totally lost because they cannot work through the problem since they don’t understand what is happening and what they are doing in the first place.

Understanding how it works also have the added bonus that it works allows you to figure out how to fine tune and make it better. This cannot happen if you work with a “black box”.

Very good point.

however, I think Varmint Al is the only one who truly knows what's happening, in regards to this subject anyway...
 
queen_stick said:
jlow said:
I think tuning when the bullet arrives at the muzzle is part of the pressure wave theory?
Go
As to whether it matter how it works, coming from a science background, I would say understanding how something works is always important. I see people “cook book” things all the time, it works fine until something goes wrong and when that happens, they are totally lost because they cannot work through the problem since they don’t understand what is happening and what they are doing in the first place.

Understanding how it works also have the added bonus that it works allows you to figure out how to fine tune and make it better. This cannot happen if you work with a “black box”.

Very good point.

however, I think Varmint Al is the only one who truly knows what's happening, in regards to this subject anyway...

We need Varmint Al!
 
jlow said:
queen_stick said:
jlow said:
I think tuning when the bullet arrives at the muzzle is part of the pressure wave theory?
Go
As to whether it matter how it works, coming from a science background, I would say understanding how something works is always important. I see people “cook book” things all the time, it works fine until something goes wrong and when that happens, they are totally lost because they cannot work through the problem since they don’t understand what is happening and what they are doing in the first place.

Understanding how it works also have the added bonus that it works allows you to figure out how to fine tune and make it better. This cannot happen if you work with a “black box”.

Very good point.

however, I think Varmint Al is the only one who truly knows what's happening, in regards to this subject anyway...

We need Varmint Al!

Agreed!

It would be awesome if he chimed in on the forum once in a while!
 
OK but,,,,,,,, if it was all scientific only scientists could shoot well.
I've been at it long enough to see guns/methods that defy all normal scientific explanations ;)
Ever see a factory gun that could shoot 0's or low 1's with upwards of 200fps velocity variations for those five shots. I have.
Science will never explain that.

Its also very rare that I explain the hoops required to jump through also.
I wish you the best in seeking your definitive.
 
Speaking of Varmint Al, I've always found this page in his extensive library to be usefull to understanding.
http://www.varmintal.net/amode.htm

Read the the Posibility mode and Conclusion at least.

All this has to do with seating in those minute adjustments, getting the bullet to leave the barrel at the best time,,each time.
Very much like BoydAllen's first responce to the OP,, ;)

And your right, once you wrap yer brain around it, things get easier
 
I wouldn’t give scientist any undue credit. It takes A LOT more than being scientific to be a good shooter, there are things like shooting skill, a good rifle to name a few…. However, it is an inescapable fact that the reloader/shooter who makes that extra effort in understanding what he is doing and why something happens moves ahead and is more successful.

As for that factory gun example, being scientific in no way means that you have the ability to understand everything around you, if that was the case then the first scientist of this world would be able to explain everything and we would be done. Leaning is a stepwise process but being scientific just means that you are using that tool to make progress, and making progress is what I am trying to do here.

Necchi – thanks for the link very interesting. I do not dismiss the conclusions from Varmint Al or BoydAllen’s response, in fact I agree with it. My question is OK, I can see how this explains leaving the muzzle at the right time gives you tighter groups but the question still remains – how does seating a bullet to a certain depth makes this happen? I already pose what I originally believe was the reasoning but also explain why I have doubts in my own reasoning in the OP…..
 
What happens if you successively remove small sections of the barrel and recrown keeping the load constant? has this been shown to have a node type effect on groups also also?
 
It’s an interesting test but it does not quite answer my question although it does test the theory of how the node relates to when the bullet exits the barrel.

Theoretically, shortening the barrel should affect the node but I think it will be complicated by the recrown i.e. the crown will have to be exactly the same as it was before.
 
I have not tried cutting off a barrel to tune, probably because I find it so much easier to just tune by loading at the range. You can do a similar experiment by adding weight to the muzzle. As you add weight you slow the primary frequency of the barrel's vibration, and since tuning nodes are usually multiple and periodic, you should eventually come to one this way. assuming that your barrel is free floated and your action properly bedded.

While we are on nodes, I should mention that the word is used in two different ways. One is to denote a place or places along the continuum of possible loads that produces better accuracy than loads above or below, or in the case of seating depth longer or shorter.

The other use has to do with the physical vibration of the barrel. If we look at the various harmonics of vibration that produce physical waves along the barrel, the crossover points of these waves, where there is no amplitude are referred to as nodes.
 
jlow said:
It’s an interesting test but it does not quite answer my question although it does test the theory of how the node relates to when the bullet exits the barrel.

I think I understand what your saying... If brass volume can vary which would cause pressure variations and result in faster bullets out of some cases than others, why would small adjustments in seating depth make a difference?

I would think the most simple explanation would be the small variation in horsepower from case capacity variations isn't enough to overcome the physical "head start" that a bullet seated out farther has.

Like 2 runners that are very close in speed but one gets a head start. I will take a long time for the one in back to catch up and pass. In a barrel the bullet only has 18"-32" to make up the difference before it exits the barrel at a particular node.

Example: I have been able to shoot some pretty good groups just fire forming brass, but I can't use a bunch of different seating depths and get a group. One bullet seated long or short can usually be seen on target.
 
OK, thanks! You are absolutely correct in your understanding of my question.

So it seems your theory is that the difference in seating depth is simply the bullet has a “head start” – hmmmm! That seems to me to be the best theory yet! Yes, a one thousands difference in seating depth would mean the bullet would exit slightly earlier at the crown and that small difference could make a huge difference considering how fast the oscillation is travelling up and down the barrel….

I like it! I like it a lot!
 

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