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

[Steve Blair]

what's OBT ?

[br]
Optimal Barrel Time, Chris Long's shock wave theory.
http://www.the-long-family.com/optimal%20barrel%20time.htm

 

[BoydAllen]

A little seating depth story:
Years back, a friend and I, who had practiced together, were at a three day match, his first. In practice, he had been a little sharper on the wind than I, but during the first two days of the match, I had shot better, and he was not up to what I had seen in practice. About half way through the second afternoon, we were discussing what the problem might have been. When I asked him about his seating depth, he told me that he had not changed it, that it was the same as he had done so well with in practice. I suggested that he load up some that were .003 longer, and some more that were .003 shorter, and try them on his sighter target after shooting his next record group. (These days I would have told him to load up enough of each to test and then shoot his record.) Taking part of my suggestion, he loaded some long, but not any shorter, and the test looked better, so he made the change for the next match, and used it the following day, and although he didn't win any wood, he did a lot better, better than I.

A few years later, a friend was shooting an unlimited match, and his rail was just a little out of tune, I suggested a slight lengthening of seating depth, and they sucked in to satisfyingly small, competitive clusters.

At the time that I made these suggestions, I thought that what started out as a good depth might have come off the mark a bit, due to a slight amount of throat wear. At least that was my theory of the moment.

 

[jlow]

Interesting story! My guess is that these were seated close if not jammed? Throat erosion is certainly possible, but my other thought is could this be due to differences in temperature which may affect MV, thus the timing of the exit through the crown?

 

[BoydAllen]

Jam is a specific length for a given barrel, bullet and neck tension, that is the longest that a bullet can be seated, without being pushed back as the round is chambered. It is only "jammed" when seated to that length. The bullets were seated longer than touching....or if you prefer, into the rifling, but not jammed. The matches were in the same range of temperatures that we practiced in. In short range group competition the usual is five matches in the morning, and five after lunch. In warm weather, in Visalia, CA, the span of temperatures and humiditys, encountered within any given day can be considerable. The sub par performance was throughout the day, as was the superior. First rule, if it's not working, don't just try harder, change something. The good thing about loading between individual matches is that you can.

 

[300 RUM]

I know many will disagree with this but here it is any way. I just recently started to play around with the "OCW" stuff. I went thru all the proper procedures found the OCW and started to play with seating depths. WELL! my beginning loads were .010 off the lands, turned it to .005 jamb (shot aprox .300 groups). I decided the load a bit hot for my taste so I changed the load to something a bit lighter (I just picked a number 1.5gr lower then the original OCW load) loaded five each of .020, .015, .010, .005 jump, 5 right on the lands, 5 @ .005, .010 jamb shot them all. The results: the .010 jump and the .005 jamb shot equally well (both back at the .300 group size or maybe even a bit better)

What I'm getting at here is that the lower load did not perform well at all initially. My thought is no need for OCW, measure your powder charge with do-diligence and play with seating depth it is going to be the ticket to ride more then powder charge will be.

 

[jlow]

Jam is a specific length for a given barrel, bullet and neck tension, that is the longest that a bullet can be seated, without being pushed back as the round is chambered. It is only "jammed" when seated to that length. The bullets were seated longer than touching....or if you prefer, into the rifling, but not jammed. The matches were in the same range of temperatures that we practiced in. In short range group competition the usual is five matches in the morning, and five after lunch. In warm weather, in Visalia, CA, the span of temperatures and humiditys, encountered within any given day can be considerable. The sub par performance was throughout the day, as was the superior. First rule, if it's not working, don't just try harder, change something. The good thing about loading between individual matches is that you can.

Thanks Boyd! Yes in your situation, not temperature related. Still, the motive for my question is to ask whether a change in temperature (either environmental or just sitting in the chamber too long) affect the optimal seating depth of a round that has been tuned for a different temperature? Comes back to the question of would an optimal seating depth change as it relates to environmental conditions.

 

[mikecr]

I've found also with some guns/loads there is a window where seating doesn't affect performance, and beyond that window changes a lot.
My 6.5WSSM likes 8thou off.
12thou, or 5thou, and the groups start getting ugly.
So I'm careful to qualify my ogives, seat exactly to the same measure(every one), and set my shoulder bumps close & consistent.
Can't afford to leave that window.

I could see where a gun leaving tune could use a little group reshaping, through seating or tension adjustment, during a match.

 

[necchi]

Comes back to the question of would an optimal seating depth change as it relates to environmental conditions.

In my experience No, a finished cartridge isn't going to grow or shrink with temp swings enough to be measured by anything but the finest tools, certainly not as much as even .001.
(I could be wrong, but I don't fret about length due to heat as questioned)
But, I was always taught "Don't cook'm", in other words the high temp of sunshine or a hot chamber does effect the powder reaction. I don't chamber a round in a hot gun until seconds before I'm ready to shoot.

 

[jlow]

Necchi, yes “don’t cook’m” is generally good advice unless you are intentionally trying to get that extra MV…

My question of temp relates mostly to environmental conditions outside of the chamber i.e. rounds sitting in cold/cool temps are still likely going to be running at lower chamber pressure when they are fired than warm/hot rounds(cooking them in the chamber would do the same thing) . That difference in chamber pressure should affect MV and I would think that would change when the bullet goes pass the crown. So if seating depth is adjusting for the same thing, it would seem logical that the environmental temp of the round might alter the optimal seating depth? Just theory and so the question.

 

[mikecr]

Well, beyond PB BR, I suggest that adjusting for temps is better controlled with powder adjustment.
Seating, once affecting, is not a small tweak.
Powder on the other hand can be adjusted to the kernel.

Capacity, and capacity variance does change things. But it's hard to imagine the tiny seating adjustments of a few thou in a neck would amount to affect -from the change in capacity.
I think it changes only the timing of the peak.

 

[BoydAllen]

+1

 

[necchi]

Yes the temp affects the way the powder burns and that effects MV, but it’s not linear.

We can’t say that for every temp degree increase, MV increases 10fps, powders all react differently with some being more tolerant and others being very erratic.

I’d really have to dig but one of the members did an extensive test with 3 powders in different temps over the chrony and laid out a nice graph, the spike in MV was dramatic as the temp increased.

Interesting theory and question jlow, I’m not as gifted as others when trying to explain things, I’ll have to let the brain wrap around that one for awhile.

Like mikecr is saying seating adjustments that make a difference in my groups are usually in the .010-.020 range or more for optimal,, not just a few .001

 

[Steve Blair]

Like mikecr is saying seating adjustments that make a difference in my groups are usually in the .010-.020 range or more for optimal,, not just a few .001

[br]
I think Mike was referring to a change in case capacity affecting pressure and velocity. I have certainly seen a seating depth change of several thousandths make a major group size change. It has also affected ES/SD, indicating a timing relationship to peak pressure.

 

[jlow]

Like mikecr is saying seating adjustments that make a difference in my groups are usually in the .010-.020 range or more for optimal,, not just a few .001

[br]
I think Mike was referring to a change in case capacity affecting pressure and velocity. I have certainly seen a seating depth change of several thousandths make a major group size change. It has also affected ES/SE, indicating a timing relationship to peak pressure.

Good call on the effect of seating depth on changes in ES/SE, so there must be something else going on other than the race to the crown, but the tricky thing here is that changes in seating depth does not have to affect only one thing and so case capacity, pressure, timing, race to the crown, could all contribute….

 

[bozo699]

Like mikecr is saying seating adjustments that make a difference in my groups are usually in the .010-.020 range or more for optimal,, not just a few .001

[br]
I think Mike was referring to a change in case capacity affecting pressure and velocity. I have certainly seen a seating depth change of several thousandths make a major group size change. It has also affected ES/SD, indicating a timing relationship to peak pressure.

100% spot on!!! that is why my earlier statement that seating depth DOES change velocity, ES/SD which at 1K will be vertical dispersion on your target! For 1000 yard br shooting, I can't speak for any other discipline or range of fire but for 1K BR all the record holders I know find OCW and finish fine tuning with seating depth. A new shooter at our rang has found with the ne Hybrids that .001 makes a difference, new gun, new shooter and already he has figured this out, made the 4" club and has shot two 100 scores out of the last three matches! Seating depth DOES MATTER at least at this one distance and discipline although I can't believe this is the only discipline and distance that this PROVEN method would work in. If you all have doubts that seating depth makes a difference is all the better for me, especially if you shoot at Deep Creek, I need all the help I can get
Wayne.

 

[Bradley Walker]

Bradley,
I don't believe that there is only one factor. Yes, bullet shape is one. There are others. Are you aware of tuning nodes for powder charge weight? A reliable source tells me that his testing has shown that this is the case with seating depth as well. Fellows who play with tuners will tell you that sweet spots repeat at intervals. IMO what these all have in common is that they either effect the timing of when the bullet exits the muzzle, relative to where the muzzle is in its vibration cycle, or they change the cycle frequency to be a better match with a given load. None of this contradicts the idea that different bullet shapes have different seating depth preferences, or that you can use what you already know about where a bullet, or a similar bullet's preferences as an educated starting point for finding peak tune. I do that all the time...and it works.

I believe in tuning nodes, and I am sure that seating depth does have a little effect on velocity (which would change the tune), but I think its very interesting that you will see the SAME bullets shoot the best SEATED TO THE SAME JUMP OR ITL SETTINGS ALMOST UNIVERSALLY!!!

In other words there are trends within the particular type of bullet design in standard guns across the board... At least some kind of bell curve for sure.

 

[Bradley Walker]

What I'm getting at here is that the lower load did not perform well at all initially. My thought is no need for OCW, measure your powder charge with do-diligence and play with seating depth it is going to be the ticket to ride more then powder charge will be.

OCW only tunes to one node... There are two or three OCW in most rifles for a given recipe... You dropped to the next lower node. You might not have hit it and used the seating depth to change the velocity slightly.

 

[mikecr]

I'm pretty sure a lot of us have witnessed the horrible credibility behind 'trends' in bullet seating(e.g. VLD loar)..

I also assign little value to OCW testing, given that it pretty much dismisses seating as though an afterthought tweak.
This, even while seating holds the single largest adjustment to grouping performance, and anyone can test it to see just that.
That's not to suggest charge is less important, that OCW is bad, but if your seating ain't right, you could be a long way from potential OCW or not.

 

[queen_stick]

I'm pretty sure a lot of us have witnessed the horrible credibility behind 'trends' in bullet seating(e.g. VLD loar)..

I also assign little value to OCW testing, given that it pretty much dismisses seating as though an afterthought tweak.
This, even while seating holds the single largest adjustment to grouping performance, and anyone can test it to see just that.
That's not to suggest charge is less important, that OCW is bad, but if your seating ain't right, you could be a long way from potential OCW or not.

good point Mike, I have to agree with that.

However, you have to find some sort of starting point; whether it's charge, velocity, barrel timing, or seating depth. OCW might not account for seating depth, but it should give you a good starting point, and from there you can tweak other variables, like seating depth. While I think this discussion is great for weighing pros and cons of each method, I think it's also leaning towards the old discussion of what came first... the chicken or the egg I mean no disrespect in any way with this post, but at some point, you just have to load and shoot! Concentrate on wind... that's what gets most of us...

 

[Bradley Walker]

I'm pretty sure a lot of us have witnessed the horrible credibility behind 'trends' in bullet seating(e.g. VLD loar)..

I would assert that "VLD Lore" is the very small majority of seating depth data, and in no way is relevant as a measure of the whole.

The VAST majority of bullet seating depth "lore" is for "typical" style bullets.

Which all shoot very well at similar seating depths or even standard or magazine length.