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

[mikecr]

nonliberal's take on it makes sense, if seating truly amounts to tuning into/out of nodes.
IMO, seating would then have to be taken as a very serious part in load development, that is a coarse setting within, or out of reach of, a good node.

Let's say it's true so far;
If your powder range could not put a bullet(in head start) fully into a node well(not too fast, or too slow), then you wouldn't reach the very best in potential. You'd get only as good as you ended up. Right?
And maybe this is why results of seating adjustments, and load development in general, seem such an abstract in the shooting community.
Many reloaders assume seating depths, or otherwise assign little value to the possibles in it.
Just consider how long it's been completely assumed that VLDs must be jammed to shoot..
Or that seating is a final FINE adjustment..

Well I've noticed that I can pick any seating depth, and find a best powder load -with that.
But then, by starting over with another 'chosen' seating depth, my new best powder load would be either better or worse. Well this seems un-node-like..
In both bouts of testing, follow-up seating adjustment affects were not at all fine enough to be of much use. This is where I concluded that seating is more coarse of an adjustment than powder(way way more coarse). That I was going about load development backwards..

I started using upper-mid-pressure loads for both fireforming and seating checks at the same time, first coarse(~30thou), then finer(5thou).
Jam testing was handled seperately, matching velocities across a chrono, and considering results of it overall.
And with this, later incremental load development has gone much better. Ladders are no longer a holy mess.
Maybe, (I think) this approach is putting me very easily within reach of good nodes(instead of leaving me wondering if I ever really got into one at all).
I can adjust powder to the kernel when it's all said & done. And this is a final FINE adjustment.

 

[BoydAllen]

In the short range benchrest game, .005 is not considered a fine seating depth adjustment. About the most that I will move on a fine adjustment is .002. Knowledgeable shooters have said that they can see a difference on the target from that amount of variation within a group. Anyone that preloads is probably dealing with the restrictions inherent in long range competition, or shoots short rang score. In the short range group game, well over 90% of the competitors load between matches, for good reason, the competition is very stiff, and staying on the razor edge of tune an important part of being competitive. The rules of different kinds of shooting contests create different realities that I do not minimize, or discount. They are just, and will remain, different. In the past, when I was only tuning up loads for casual bench shooting, or variminting, it has been my impression (based on experience) that once a likely powder charge had been found, that working with seating depth was much more productive, than playing with a tenth of a grain of powder charge one way or the other. Different shooters can achieve good results using different procedures for working up loads, and both be successful. I will say that once I have a good seating depth at a given powder charge, I tend to make powder adjustments to stay in tune as ambient conditions change through the day.

 

[jlow]

I would agree with BoydAllen, 0.005 play in seating is pretty coarse. I usually can get within 0.001-0.002 inch.

I think perhaps this is the reason why group tightness (which relates more to when the bullet goes through the crown i.e. node) and MV SDEV (which relates to when it goes through the crown, neck tension uniformity, and powder weight) can become disassociated?

 

[nonliberal]

In my eyes a rather simplistic way to look at it is like a bracket race, but the finish line is the end of the barrel not the Target. So you have (in my case) 21" to get the bullet to exit the barrel on the node (bracket). You can use more or less horsepower to get to the finish line in the bracket, but if the race is only a hundred feet,starting position will make a more dramatic change.

That's why you can pick a seating depth and then find a powder and charge to match, your adding or taking away horsepower to stay in the bracket, but the physical starting position on a 21" bracket race can help you find find your tune when you can't make enough change with the powder alone.

An interesting side note on seating depth is you can also pick a powder charge that works, then with shims or a micro top change your seating depth by .005 between each group and you will see 3-4 distinct nodes. The group will grow more and more then get smaller as you go until you hit the next node just like working up in powder. I actually view seating depth as an equal partner to powder charge instead of secondary.

 

[bozo699]

I read this thread and have many things to comment on but lately I haven't had the energy to comment much at all on the forum and tonight is no different so I will only say one thing, if you think seating depth doesn't change your velocity your chronograph is broken!!!! because it definitely does! that's all!
Wayne.

 

[mikecr]

My chrono isn't broken.
With many cartridges seating depth adjustments, even coarse adjustments, affect velocity little if any until you near the lands or donuts.

Boyd, the 5thou 'finer' adjustments, before incremental powder changes, are just to get near a node.
It's somewhat similar to Berger's recommended testing.
I'm not gonna burn out a barrel with over a hundred increments in seating.

Later, when I'm sure my load is good, I then tweak seating back & forth in 1thou increments to find my seating window center. That's it.
But all this does is shape the grouping until leaving my window(where it goes back to no-man's land). I don't think it's a tweak to tune.
'Near a node' for me is where I can easily go above and below a node with fine powder adjustments, and normal/alt powder changes from there make no difference to where this seating should be. So I log it & never mess with it again, unless changing bullets.

MY neck tension(-1thou ~length) affects this seating window width.
And primer striking affects everything at once. This needs to be looked at early also.

 

[Bradley Walker]

I think it is most interesting that all the guys shooting highpower end up with almost the exact same loads... Within a few tenths of powder.... MOSTLY SEATED AT MAGAZINE DEPTH...

I think seating depth is a factor but I'm not sure it's a factor for bullet of good design. Certain bullet shapes seem to be very forgiving to seating depth (which is good) and others are not (which is bad). I gravitate toward forgiveness over all out performance.

I stand by the same assertion I wrote about on my website back in the 1990's... Most "normal" rifle bullets shoot best at .025" off the lands. There are exceptions, but if I was to say there is a guideline, I would say "tune at .025" first, then try others". Chances are your initial tune will be the good one.

 

[BoydAllen]

I suppose that we need to define what we mean by shooting well. If you compare the best of highpower rifles to the best bench rifles, while each is better than the other for for their intended uses, there is a great difference in what is considered accurate. I expect to be able to work up loads for my factory varmint rifles that do better than half inch for five shots. From my bench rifles, I expect accuracy such that if they are tuned to their max, conditions are ideal, and I am up to it, I can shoot groups in the low twos, with an occasional one. Sometimes I think that it is not so much that we disagree, but that we are talking apples and oranges.

 

[nonliberal]

I suppose that we need to define what we mean by shooting well. If you compare the best of highpower rifles to the best bench rifles, while each is better than the other for for their intended uses, there is a great difference in what is considered accurate. I expect to be able to work up loads for my factory varmint rifles that do better than half inch for five shots. From my bench rifles, I expect accuracy such that if they are tuned to their max, conditions are ideal, and I am up to it, I can shoot groups in the low twos, with an occasional one. Sometimes I think that it is not so much that we disagree, but that we are talking apples and oranges.

I agree, there is a big difference in the time I spend working on a load for a hunting gun or a bench gun. For a varmint rifle I definitely do not go through the trouble of tuning the seating depth to perfection. It would be a waste of time and money. For a bench gun I feel it's essential.

 

[Link]

Gooday Gentlemen

Let me start by saying I really enjoy reading posts like this. However I rarely post. I am new to reloading but I do have several observations or questions to this post. Let me ask about jam, if the bullet is jammed into the lands it takes more pressure to start it moving down the barrel. Wouldn't more jam give the primer more of a chance to give the powder a more complete burn, acting like more powder or a bigger bang? Would this give the bullet a faster speed in the bore causing it to exit the crown sooner? This would effect the harmonics timing of the barrel ?

I would think neck tension would do the same thing? I have read that max achievable neck tension is about .0035. So maybe not so much?

thanks Link

;D

 

[necchi]

(based on experience) that once a likely powder charge had been found, that working with seating depth was much more productive, than playing with a tenth of a grain of powder charge one way or the other.

This has been my experiance also,,

 

[amamnn]

I have to think that Boyd meant to say .1 or .2 inch at 100 yards or a .1 or .2 MOA agg at both 100 and 200 yards for the bench rifles?

A lot of people not used to BR scoring need the decimals and such in order to be clear on the group sizes mentioned.

 

[BoydAllen]

Yep, I forget that not every one speaks BR slang.

 

[mikecr]

But keep in mind that tiny/underbore BR cartridges are different in nearly every respect from hunting capacity cartridges.
These really are apples to oranges attributes that should be picked through to take away valid meaning.

That is, you can't tune a 260, or 300WSM the same as a 6PPC, or 30BR.
Completely different capacities, barrel lengths, bullet types & weight, powders, pressures, temperatures, scales of affects, and timing of every single part of it. There is also pressure nodes(regions) with BR cartridges, the big work-around,, that are not viable in larger cartridges. These alone change the meaning and prerequisites to competitive tuning over non-competitive tuning.

Even if both large capacity and small are normalized to competitive use, they are still completely different formats.
Tuning for 100-300yd is different than 600-1kyd.
And tuning for hunting use, where every shot actually counts(accuracy wise), is different than anything else.

 

[BoydAllen]

Good post. Something that I would add is that IMO the best way to quickly see what happens when you change something, is to load at the range, and shoot over a chronograph. These combined with the use of wind flags, a good bench, and rifle support system, allow for the most rapid and efficient tuning. In spite of the advantages to doing this, there is a lot of resistance.

 

[M14AMU]

And in the FINAL ANALYSIS....the target will tell you all you need to know!

 

[Steve Wilson]

I'm with Boyd on the importance of loading at the range. It's a 45 minute drive to my range, each way. Say you make up 3 different loads at home, 2 five shot groups of each of them. Then drive all that way, only to find out after the first 3 shots that one of your loads ain't worth beans. Now you are stuck with 8 cartridges that aren't good for anything but foulers. Worse would be that you get a sticky bolt on the first shot of that load. You don't want to shoot them because of the pressure. Your press is 45 minutes away, leaving you no good way to pull the bullets.

This exact thing happened to me on Tuesday when I was playing with seating depth, on a proven accurate load in my 6 BR. I made up 10 rounds with the bullet +.015 into the lands, 10 at .010 off (my proven load) and 10 at -.025 off. First shot with the +.015 length gave me a sticky bolt. DAMN, my press is at home. If it was with me, I would have just seated one of them a little shorter..... +.010 in and see how the bolt felt, kept going shorter until the bolt lift was fine. In short, I would have been able to work with the seating length right then.

 

[coues52]

Great post ! ~Danny

 

[Bradley Walker]

I'm with Boyd on the importance of loading at the range. It's a 45 minute drive to my range, each way. Say you make up 3 different loads at home, 2 five shot groups of each of them. Then drive all that way, only to find out after the first 3 shots that one of your loads ain't worth beans. Now you are stuck with 8 cartridges that aren't good for anything but foulers. Worse would be that you get a sticky bolt on the first shot of that load. You don't want to shoot them because of the pressure. Your press is 45 minutes away, leaving you no good way to pull the bullets.

This exact thing happened to me on Tuesday when I was playing with seating depth, on a proven accurate load in my 6 BR. I made up 10 rounds with the bullet +.015 into the lands, 10 at .010 off (my proven load) and 10 at -.025 off. First shot with the +.015 length gave me a sticky bolt. DAMN, my press is at home. If it was with me, I would have just seated one of them a little shorter..... +.010 in and see how the bolt felt, kept going shorter until the bolt lift was fine. In short, I would have been able to work with the seating length right then.

How did the .025" off shoot?

 

[DougMH]

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.

Personally, I don't care about the why of it. In my case...

1) Ladder test with bullet on the lands (OTL) for the smallest vertical I can get.
2) Fiddle with jump/jam to see if I can get vertical with that optimum load even smaller.

With the Berger bullets I use, it's been my experience that OTL has worked best in all cases... all rifles. That's made it easier for me.